Bufo Marinus)

THE AMERICAN JOURNAL OF ANATOMY 163:195-222 (1982)

Functional Morphology of Lingual Protrusion in Marine Toads (Bufo marinus)

CARL GANS AND GERARD C. GORNIAK Division of Biological Sciences, The University of Michigan, Ann Arbor, Michigan 48109

ABSTRACT Bufo marinus catches its prey by stiffening the intrinsic muscles of the tongue, rapidly flipping the tongue out of the mouth. High-speed cine- matography synchronized with computer-analyzed electromyograms (EMGs) shows that during the flip the tongue is supported by the M. genioglossus medialis and that this muscle stiffens into a rod when stimulated. Coincident stiffening of the transversely arranged M. genioglossus basalis provides a wedge under the anterior tip of this rod. Stiffening of the M. submentalis depresses the mandibular symphysis and brings the dentary tips together. The M. submentalis also acts on the wedge of the basalis to raise and rotate the rigid rod of the medialis over the symphysial attachment. The tip of this lingual rod carries along the pad and soft tissues of the tongue. The lingual pad, positioned in the posterodorsal portion of the resting tongue, rotates during eversion so that its dorsal surface impacts onto the prey object. Retraction starts by contraction of the elongate, parallel fibers of the M. hyoglossus; this retracts the medial sulcus of the pad and holds the prey by a suction cup-like effect. The extensibility of the buccal membranes allows the pad to be retracted first; it reaches the posterior portion of the buccal cavity before the still-rigid, backward rotating M. genioglossus has reached the level of the symphysis. Protraction of the hyoid facilitates the extension of the M. hyoglossus. The M. sternohyoideus only retracts the hyoid and stabilizes it when the tongue starts to pull posteriorly; it does not assist tongue protrusion. The Mm. petrohyoideus and omohyoideus show only incidental activity, and the M. depressor mandibulae participates in mouth opening but is not otherwise involved in the flip. Previous hypotheses of the flipping mechanism are reviewed and evaluated.

It is well known that many frogs catch their standard works as Noble (1931:201) only prey with a rapid flip of the tongue, that the briefly comment on the tongue mechanism, anterior end of this tongue is fixed and the and even Boker (1937231) only provides cur- posterior free in most species, and that the sory mention. emerging tongue rotates over the mandibular For a long time, the hydrostatic action the- symphysis. The system is curious in that the ory of Hartog (1901a,b), questioned by Gaupp tongue of most anurans is anchored anterior (1901) in the same year, appeared in most text- to the hyoid and lacks a hard-tissue skeleton books of introductory zoology. In 1961, a pop- that could induce and support this rapid pro- ular article (Gans) presented an alternative trusion. The mechanism generating rotation muscular protrusion mechanism, as several of the tongue, as well as the manner in which series of photographs provided by Mr. Mervin the tongue grips the prey, were long accorded F. Roberts, made the prevalent views ques- minimal attention, although the retrolingual tionable. A more formal functional analysis surface of the frog tongue has been used in the was started (Gans, 19621, but it soon became resolution of some basic questions in muscle clear that any convincing decision among the physiology (Biesladecki and Herzig, 1859; The present address of Dr. Gerard C. Gorniak is Department of Fischl and Kahn, 1928; Gelfan, 1930; Pratt and Biological Science, Florida State University, Tallahassee, FL 32306. Reid, 1930; Fulton and Lutz, 1940). Such Received August 17, 1981. Accepted November 23, 1981.

0002-9106/82/1633-0195$07.50 0 1982 ALAN R. LISS, INC. 196 C. GANS AND G.C. GORNIAK various conflicting hypotheses in the literature Three papers appeared in 1901 that were must be based upon electromyographical anal- cited often and widely. The first two, respec- ysis, the tools for which were then unavailable tively the original and translation of a brief to us. note by Hartog (1901a, 1901b), note (1) that Finally, it is possible here to offer a func- the lymphatic sublingual sinus extends into tional analysis of the lingual flip in Bufo mar- the tongue itself, and (2) that the tongue rises inus, a toad selected because of its large size, up and springs forward if this sinus is injected availability, and tongue architecture (Regal with colored cocoa butter “under pressure.” and Gans, 1977); our analysis is based on me- Hartog maintained that contraction of the dium-speed (100-400 fps) films, coupled with mylohyoid (M. intermandibularis posterior?) synchronized electromyograms. This report erects the tongue by shifting lymph from the concentrates on the actions and movements of posteroventral portion of the sinus to its in- muscles and other tissues associated with lin- tralingual portion; he does not discuss the or- gual protrusion and retraction. Almost all of igin of the lymph nor demonstrate that the these muscles participate in other roles as well, volume of the posterior spaces is adequate to and such actions are not dealt with here. Be- fill the tongue as they are emptied. cause of the widespread interest in the biology Apparently, Gaupp (1901) encountered the of frogs and their use in research and training protrusion problem in the course of his revision in the biomedical and animal sciences, we in- of the Ecker text (1882). He relied mainly on clude a brief historical summary of studies and histological sections rather than upon dissec- hypotheses of the mechanism. tions of the area in question, and rejected “a prior?’ the hydrostatic mechanism as being too HISTORICAL REVIEW slow. Gaupp claimed that (1) the anterior It seems best to ignore some early accounts (basal) part of the tongue is pulled forward by of the lingual flip, suggesting that the tongue the pars basalis of the M. genioglossus, (2) the is propelled by a jet of air passing from the posterior portion of the tongue passively fol- glottis (refs. in Dumeril and Bibron, 1841:8, lows the rotation of the anterior over the man- 127; also Magimel-Pellonnier, 1924). The first dibular symphysis, and (3) retraction is modern discussion regarding the functional achieved primarily by the M. hyoglossus. The anatomy of the frog tongue was furnished by importance of the paper lies in its detailed Dug& (18271, who later provided the first use- demonstration of the topography and relations ful descriptive and developmental account of of the several muscular and lymphatic vol- anuran osteology and myology (Duges, 1834). umes. Although Gaupp (1896:138)had earlier While his descriptions of lingual myology (of suggested that the submentalis (sous-menton- various European species) were quite gross, he nier of Duges) serves to raise the mandibular noted that the tongue was activated by the symphysis during ventilation, he did not take Mm. hyoglossus and genioglossus, assisted by further notice of Duges’ view that these ele- a complicated interaction with the muscles ments could be depressed by contraction of the depressing the lower jaw and advancing the submentalis. hyoid system. The key point, ignored for the In 1924, Magimel-Pelonnier published an next century, is given in his statement, “le elegant thesis on the amphibian tongue (not sous-mentonnier qui, rapprochant les cited again until 1977), reporting on the lin- branches de la machoire, non-seulement les gual anatomy and embryology of 45 species of affermit, mais en retrecit l’arc . . . recourbe en frogs. The study is important, because it doc- bas, ce qui favorise d’autant le mecanisme ci- uments the existence of morphological diver- dessus indiqu6.” sity and makes clear that various kinds of frogs The authors of the next 70 years (Klein, must differ in the way they protrude their ton- 1850; Fixsen, 1857; Wiedersheim, in Ecker, gues. The functional analysis, based upon di- 1882; Ferdinand, 1894) concerned themselves rect observation, cinematography of tongue primarily with the relative function of the Mm. movements (in various European species), hyoglossus and genioglossus and with the de- anatomy, and stimulation of the various mus- scription of the intralingual musculature. Fur- cles, divided the movement sequence of ad- thermore, Wiedersheim (in Ecker, 1882:l l)in- vanced frogs into three phases; but curiously terpreted the intermaxillary gland (of the it made no mention of the Hartog (1901a, b) upper jaw) as mucous and stated that its se- hypothesis. In Magimel-Pelonnier’s first phase cretion would be wiped off by the anteriorly the M. genioglossus is inactive, and the re- passing tongue. tracting portions of the M. hyoglossus are ac- TONGUE PROTRUSION IN MARINE TOADS 197 tive. The anterior fold of the tongue is then and of the mentomeckelian elements. He as- pulled toward the hyal base, so that the trans- cribed movement of this joint to contraction of verse connective tissues are stretched like the the M. geniohyoideus (rather than the M. sub- string of a bow. During the second phase, the mentalis) and illustrated the movement with M. hyoglossus suddenly becomes inactive, and a rather diagrammatic sketch. the genioglossus starts to fire, pulling the ton- In 1961, Severtzov (like Tatarinov, a student gue anteriorly into a flip. In advanced species, of Schmalhausen) presented a more extensive such as members of Bufo, the flip is facilitated paper in which he discussed the morphology by folded connective tissues located anterior and flipping mechanism of the tongue in eight to the tongue pad, and by a muscular hinge species of frogs. He postulated a combined hy- formed in part by the short fibers of the M. drostatic and muscular mechanism operating genioglossus and the basal and lingual sinuses. in three stages: (1)erection of the anterior por- The movable mentomeckelian bones of some tion of the tongue by contraction of the M. gen- species are mentioned as facilitating the ro- ioglossus; (2) straightening of the tongue by tational movement. During the third phase, “manometric” injection of lymph from the pos- the entire M. hyoglossus becomes active, pull- terior part of the lymphatic sinus into the ton- ing the lingual fibers around the prey and then gue, and simultaneous lowering of the sym- retracting the tongue and prey toward the en- physis by contraction of the Mm. trance of the esophagus. Different mechanisms geniohyoideus and intermandibularis (ante- are proposed for various primitive frogs and rior = M. submentalis); and (3) the actual the proteroglossine frogs. throwing of the tongue by action of the M. gen- In 1933, Gnanamuthu described the throat ioglossus. musculature and tongue of two Indian frogs In the same year, there appeared a popular and reacted to Holmes’ (1919, and later edi- report (Gans, 1961, also 1962), proposing (on tions) erroneous ascription of Hartog’s views the bases of anatomy, strobe photographs, and to Gaupp. The “extreme rapidity” of the tongue stimulation experiments) that the American flip allowed Gnanamuthu to make only a spec- bullfrog, Rana catesbeiana, flips the tongue by ulative interpretation. He concluded that (1) (1) contracting the doubly pinnate M. genio- all fibers within the tongue appertain either glossus, thus forming a rod, and (2) rotating to the M. genioglossus or the M. hyoglossus this rod by a mechanical couple, activated (i.e., there are no distinct intrinsic lingual fi- mainly by contraction of the M. submentalis. bers), (2) neither the intermaxillary gland nor Action of this muscle approximates the man- the lymph spaces are associated with tongue dibles anteromedially. As a result, the men- propulsion, (3) the hyoid musculature per se tomeckelian bones are depressed (medially), is not involved, and (4) the hyoglossus is nor- and the attached anterior end of the M. gen- mally in tension and relaxes when stimulated. ioglossus is lowered. Further, as the M. sub- This relaxation (of a muscle fitting the “tonic” mentalis straightens it pushes against the category) was claimed to provide a forward re- middle of the M. genioglossus. The soft tissues bound, that serves to flip the tongue, when of the lingual tip are assumed to be carried combined with the “upward and forward” force along and then to slide anteriorly by their in- produced by the M. genioglossus basalis. trinsic inertia as the rod-like M. genioglossus In a 1942 note, Barclay presented a recon- basalis rotates around its anterior end. The structed series from individual photographs of tongue is later retracted by action of the M. tongue protrusion in Bufo vulgaris. He argued hyoglossus. The photographic illustrations briefly for a muscular rather than hydrostatic show that the tongue does not touch the roof propulsive mechanism, in view of (1) the short of the mouth during the forward flip and con- time of protrusion (50 msec), (2) the absence firm Gnanamuthu’s suggestion that the inter- of obvious inflation, and (3) the “constriction” maxillary gland is not involved in the inges- visible in dorsal view when the tongue is in tion sequence. Independently, Francis (1961) the fully extended position. demonstrated that the intermaxillary gland In 1957, Tatarinov published a preliminary produces serous rather than sticky mucus se- report (grossly mistranslated in the U.S. ver- cretions and has a role in “tasting” the food sion of the journal) of a survey of the lingual once it is in the mouth. The diagrams of the mechanism of 18 species of frogs. He ascribed popular report have been reproduced in a num- the flipping to muscular contraction and ar- ber of texts, leading to the implication that the gued that the flip is accomplished mainly by mechanism described for Rana applies to “the a ventral movement of the symphysial junction frog” and is therefore universal among frogs. 198 C. GANS AND G.C. GORNIAK In 1977 Regal and Gans, using strobe movies drawn under a Wild dissecting microscope with of feeding in phaneroglossan frogs, called at- a camera lucida. Simultaneously, the lengths tention to the diversity of lingual myology in of the fibers were measured regionally using anurans and noted that this diversity had been a millimeter ruler and ocular micrometer. documented earlier by Magimel-Pelonnier Most feeding sequences were filmed at (1924). They expanded the Gans model and speeds between 140 and 200 frames per second, presented various functional and phylogenetic and some at 400 frames per second, with a High hypotheses, specifically desighed to call atten- Cam 16-mm high-speed cine camera (Sid Red- tion to the diversity of tongues and to the pos- lake Inc.). Illumination was provided by either sible use of the tongue as a phylogenetic char- a high-output strobe (Strobex 140) or conven- acteristic. tional movie flood lights. A signal from the The same year, Emerson (1977) proposed a output of a photocell mounted on the feeding different model on the basis of cinefluoroscopy compartment or directly from the camera shut- and muscle stimulation (but not electromyog- ter mechanism was recorded on a Honeywell raphy) in Bufo marinus. According to her re- tape recorder to allow correlation of move- port, anterior movement of the hyoid, rather ments with EMG. The camera was focused on than the genial musculature, flips the tongue. the feeding dish, and the field was large The model involves three phases. During phase enough to include a mirror placed at 45” to one one, the Mm. sternohyoideus, geniohyoideus, side. The food was dropped before the toad and and genioglossus fire, fixating the hyoid pos- filming started when it showed intention teriorly, opening the jaws and “storing poten- movements. tial energy” (p. 119) in the system. During For EMG, the toads were first anesthetized phase two, the tongue, which is then under by intraperitoneal injection (0.04 mg per g tension, and the hyoid are allowed to move body weight) of Tricaine Methanesulfonate when the M. sternohyoideus becomes inactive. (MS 222). Four twisted bipolar electrodes of The anterior movement of the released hyoid 0.076-mm teflon-coated stainless-steel wire then imparts “kinetic energy to the tongue to (Medwire Inc., Mount Vernon, New York) with protrude it out of the mouth” (p. 119). 1-mm bare ends were then implanted using 22- gauge hypodermic needles (intradermal tips). MATERIALS AND METHODS The needles were passed through a 1-cm skin The tongue flip was studied in 21 unanesth- incision ventral and posterior to the parotid etized and unrestrained toads (Bufo marinus; gland, and then subcutaneously to the oral cav- snout-vent length 10-15 cm; weight 264-693 ity; the position of each tip of the needle could g), using high-speed cinematography and syn- be monitored by palpation of the opened mouth chronized electromyography (EMG). Each toad of the toad. Brief reverse stimulation via the was maintained in an individual compartment electrodes during implantation helped to de- with a dish placed near its front. Toads were termine the location of the electrode tips; later, conditioned to take mealworms (Tenebrio sp.) the location was verified by routine autopsy. droppedain front of them while a strobe light The electrode wires were kinked several times was flashing or bright lights were turned on. as the needle was removed, and their free ends This report is based on 36 feeding sequences then soldered to a harness of earphone wire (from 17 toads) with good simultaneous cine that was tied to the pelvic girdle. Recording and EMG, another 76 feeding sequences with sessions always started more than 24 hours only EMG, and films of more than 100 control after electrode insertion to permit recovery feeding sequences in 5 unoperated animals. In from anesthesia. Signals from the four EMG three other animals the lingual and hyoid mus- electrodes were amplified through 26A2 Tek- cles were exposed under deep anesthesia, and tronix preamplifiers, monitored on a Brush 481 then stimulated prior to their removal; some strip chart recorder and a Tektronix 565 os- movements of hyoid and tongue were simul- cilloscope and stored on 1-inch magnetic tape taneously filmed. The removed muscles were by a Honeywell 5600 medium-band 14-channel blotted dry, weighed, fastened to tongue de- tape recorder. pressors and placed into 10% buffered For- The films were projected frame-by-frame malin. After fixation, the muscles were placed with a Lafayette analytical projector and the in 30% nitric acid for 2-4 days which dissolved positions of mandible and tongue sketched in the connective tissues. The nitric acid was then reference to the contour of the entire head, aspirated, the muscle was placed in 50% glyc- eyes, nostrils, tympanum and landmarks on erol, and the regional fiber directions were the feeding chamber. The control films re- TONGUE PROTRUSION IN MARINE TOADS 199 vealed no significant change in feeding behav- and two posterolateral processes (horns and ior as a result of electrode insertion. cornua), extending from each side (Fig. 2). Bufo Hard copy printouts of the electromyograms marinus lacks an anterior process of the cer- were first checked for the start and stop of atohyal, and the rounded alar plates point to- muscular activity and for changes in relative ward the ceratohyal projections. The cerato- firing amplitude. Subsequently, signals stored hyal first passes rostrally from the body of the on tape were analyzed on 2 modified Hewlett- hyoid, then curves sharply laterally and pos- Packard 21MX minicomputer. The output of teriorly. Each hypobranchial extends horizon- the photocell or camera shutter (reflecting the tally from the body of the hyoid but then curves instant that the film was exposed) provided dorsally where the caudal limit of the posterior temporal cues for EMG analysis. The computer process shows membranous connection with was programmed to count the number of EMG the larynx. spikes from one photocell signal to the next The floor of the buccal cavity is lined by a and to determine the mean spike amplitude for densely reinforced mucous membrane that that interval. The program could be started joins the dorsal edge of the mandibles just after a specified number of frames and noise medial to the tooth rows. The tongue is large, were eliminated. Bar graphs of the number of fleshy, and rounded, having a widely protrud- spikes, the spike amplitude, and the percent ing free posterior edge. The anterior portion of the spike numbers times the amplitude rel- (adjacent to the symphysis) is smooth and ex- ative to the maximum value of the product tensile and has a relatively thin mucous cover; during ingestion were plotted using an Tek- the posterior two-thirds including the tip have tronix 4051 graphic display unit (32Kmemory) a thickened papillose glandular mucosa that and an interactive Tektronix 4662 digital plot- may show a medial trough. The lingual tissue ter. The computer analysis provides informa- appears flabby in anesthetized specimens; ap- tion about muscular activity preceding the parently it is shaped by muscular action. condition observed in each picture. The values The muscles ofthe tongue attach to the lower available then sum the electrical events over jaw and to the hyoid (Figs. 1 and 2). Multiple the inter-picture interval with mean activity layers of muscles pass anteroposteriorly near assumed to have preceded the mechanical the midline of the buccal floor; but lateral to event by one-half this time. the M. geniohyoideus, the thin layer of the M. intermandibularis posterior and the more pos- GENERAL TOPOGRAPHIC ANATOMY teriorly placed M. interhyoideus are the only Action of the tongue involves only the tis- muscles separating buccal membranes from sues of the mandibles and hyoid. This brief the gular skin. The hyoid edges and horns, description of these skeletal elements serves underlying the mucous membranes of the oral only to orient the subsequent section on myol- floor, protrude into this free space. om. The architecture of the medial mandibular The composite mandibles are narrow and region is most clearly seen from a sagittal sec- rodlike, of oval cross-section and show an over- tion of a specimen preserved with the mouth lapping of anterior and posterior elements closed (Fig. 3). Anteriorly, a short, stout, trans- along their middle, so that each mandible may verse muscle (M. submentalis) crosses between be deformed by rotating it about its long axis. the mandibular tips at or below the level of the The mandibles are connected anteriorly by ventral mandibular edge. Superficial to it lies fairly rigid cartilages and connective tissues the dense medial aponeurosis of the thin, in the mentomeckelian region. Opening of the roughly transverse M. intermandibularis pos- mouth rotates the mandibles about the quad- terior. This sheet, and its posterior continua- ratoarticular joint. When the mouth is closed, tion as the M. interhyoideus, extends poste- the lower jaws appear relatively straight in riorly to the level of the jaw joint (attachment lateral view and lie in the plane of their an- and architecture of the posterior portion are terior and posterior articulations. In ventral affected by sexual dimorphism owing to the view, the mandibular arch is U-shaped and the formation of vocal pouches). Deep to the inter- posteriorly pointing free ends bear the artic- mandibular sheet lies the thin bipartite M. ulation to the quadrate. geniohyoideus passing longitudinally along Much of the intermandibular space (i.e., buc- the medial zone (Fig. 1B). It originates on the cal floor) is filled by the thin and wide carti- laryngeal fascia and posterior portion of the laginous hyoid (cf. Trewavas, 1933). Each side hyoid and inserts on muscles and skeleton of of the hyoid has four, i.e., two anterolateral the symphysial region. Except for the M. sub- 200 C. GANS AND G.C. GORNIAK TONGUE PROTRUSION IN MARINE TOADS 201

mentalis, all of these and the following muscles I are paired. I Dorsal to these longitudinal muscles lie the Mm. genioglossus and hyoglossus, muscles of the tongue proper. In the anterior region, a dense basal mass (M. genioglossus basalis; the “couche inferieure” of Magimel-Pellonier, 1924) forms a thick crescent (or pair of legs) of short, radially arranged fibers that crosses from the bony tip of one mandible to that of the other, posterior to the symphysial region and across the base of the tongue. A second, more dorsal and longer pair of fiber-sets (the M. genioglossus “medialis”; the “couche superieure” of Magimel-Pellonier, 1924) runs as paired rods along the central, anterior half of the tongue. Its fibers split into bundles that radiate medially and laterally from each rod toward the posterior lingual edge and insert at intervals deep to the mucous membrane of the lingual surface. The M. hyoglossus consists of an array of much longer fibers that origi- nates from the posterior horns of the hyoid and passes anteriorly as a rounded bundle ventral to the hyoid plate. The muscle enters the ven- tral surface of the tongue (adjacent to the thin retrolingual membrane). The body of the M. hyoglossus then splits, not into a meshwork of Larynx / 1 individual fibers, but into discrete defined fas- cicles that leave the muscle at intervals and Fig. 2. Bufo marinus. Dorsal view of the dissected lower run posterolaterally to insert on the dorsal lin- jaw to show distal portion of the M. hyoglossus with the tongue in situ (left) and flipped forward (right side). Note gual surface, mostly near the middle of the the relative shift of the hyoid plate. distal surface of the tongue. Where the fasci- cles traverse the body of the tongue, it appears filled with loose connective tissue, among which pass blood vessels and nerve trunks. base of the tongue into which it passes ante- Both sagittal and deep ventral dissections riorly to terminate in a blind diverticulum. disclose that the proximal portions of the M. (This arrangement explains Hartog’s (1901a) hyoglossus lie in a tunnel-shaped sinus (Fig. results. Strain of the membranous wall in- 3); this sinus reaches its greatest extent at the duced by filling the v-shaped space should in- crease the angle between its legs, and pres- surization of the contents would tend to straighten it further.) Ventral views show that the membranous walls of the lateral edges of Fig. 1. Bufo mrinus. Ventral view of skinned head to the sinus include the nervous and vascular show three levels of dissection. (A) Superficial level showing supplies that follow the sinus into the tongue. only the M. intermandibularis posterior (IMP) and its con- nective tissue sheet (MA) covering the M. submentalis (SM The ascending bundles are beautifully looped deep). (B) Deeper level with the M. intermandibularis pos- when the tongue is at rest, suggesting re- terior reflected and Mm. sternohyoideus (STH) and genioh- markable extensibility that is documented in yoideus lateralis (GHL) and medialis (GHM) removed on tongues fixed in the straightened position. the left-hand side. DM, M. depressor mandibulae; HG, M. Much of the posterior portion of the hyoid hyoglossus; HG E, muscular extension of M. hyoglossus to- ward vascular coil; OH, M. omohyoideus; PTH, M. petroh- lies dorsal to the pectoral girdle. A thin con- yoideus; SM, submentalis. C. Deepest view with all of these nective-tissue sheet extends along the ventral muscles removed and only the M. genioglossus remaining surface of the hyoid, between the ceratohyals. in position. The insert shows the M. genioglossus basalis Together with the fasciae of the Mm. inter- (GGB) in cross-section as an array of short fibers inserting on its two defining raphes. GGD, M. genioglossus distalis; mandibularis posterior and geniohyoideus, it GGM, M. genioglossus medialis. forms the ventral and lateral boundaries of the 202 C. GANS AND G.C. GORNIAK

GGB GGM GGD \ I I A

B

C

D

3 TONGUE PROTRUSION IN MARINE TOADS 203 connective-tissue tunnel within which the M. gions of the muscle (Table 1).The superficial hyoglossus lies. The roof of the tunnel is formed fibers of the posterior region are the longest, by the body of the hyoid. Both the anterior those of the middle region are intermediate horns of the hyoid and its processes attach to and those of the anterior part are the shortest. the reinforced mucous membrane that forms When the M. submentalis is stimulated, the the lateral floor of the oral cavity. Anteriorly, bulk of the muscle is pulled anteriorly against this sheet reflects dorsally along the lateral the symphysial region. Simultaneously it borders of the M. genioglossus and is contin- shifts dorsally and displaces the overlying ton- uous with the investing fascia of this muscle. gue. MYOLOGICAL DETAILS Mm. intermandibularis posterior and M. submentalis (SM) interhyoideus The compact, oval, and unpaired M. sub- The thin, flat M. intermandibularis posterior mentalis (M. intermandibularis anterior) lies (IMP) lies in a thin but strong connective-tis- at the ventral margin of the anterior tip of the sue aponeurosis that crosses the gular regions mandible (Figs. 1B and 3). Its fibers are con- between the ventral edges of the mandibular tinuous between attachment sites; those of the rami (Fig. 1A). The muscle is paired and its anterior third curve anteriorly, but most of the medial aponeurosis is narrow posteriorly, but fibers lie in an anteriorly concave curve. The widens anteriorly where it covers most of the M. submentalis is a distinct muscle and not ventral surface of the M. submentalis; no mus- part of the medially separated M. interman- cle fibers cross the midline. Where they ap- dibularis posterior. The anterolateral aspects proach the mandible, the fibers of the M. in- of the M. submentalis are covered with a tri- termandibularis posterior leave the angular tendinous sheet that provides attach- aponeurosis (which attaches to its ventrome- ment for its fibers. This triangular sheet then dial edge) to turn sharply dorsad and then in- inserts on the anteromedial tips of the man- sert near the dorsomedial edge of the mandi- dibles and the adjacent lateral edge of the men- ble. The fibers of the muscle are arranged in tomeckelian cartilages. flat bundles that change in their orientation The superficial fibers of the M. submentalis and length from back to front (Table 1).Thus are longer ventrally than dorsally in all re- the anterior fibers radiate anterolaterally from the midline, the next group runs nearly straight laterally, and the most posterior fibers pass posterolaterally. A discontinuity marks the transition between the anterior M. inter- Fig. 3. Bufo marinus. Median (A) and sagittal (B) sec- tions through the tongue in situ. (C) Section through the mandibularis posterior and the more posterior tongue while it is being lifted, with the stiffened rod rotating M. interhyoideus. over the symphysis. (D)Section after the rod has proceeded The fibers are longest in the posterior region, beyond the symphysis carrying the soft tissues with it. (E) but gradually decrease in length more ante- The rod at the end of its travel with the soft tissues being riorly; the shortest fibers lie near the sym- propelled further by their own momentum. Note that the lingual pad has rotated so that it now faces ventrally. Also physis. Unilateral stimulation of the fibers note the pattern of the radiating fibers of the Mm. geniog- moves the central aponeurosis toward the lossus distalis (dashed) and hyoglossus. GGB, M. genioglos- stimulated side. sus basalis; GGM, M. genioglossus medialis; GHM, M. gen- iohyoideus medialis; H, hyoid plate; HG, M. hyoglossus; M. geniohyoideus IMP, M. intermandibularis posterior; S, lymphatic sinus at base of tongue; SM, M. submentalis. A paired strap-like M. geniohyoideus lies on The diagrams document the mechanism of the propulsion each side of the midline. Each of these muscles of the tongue. Contraction of the M. submentalis changes is divided into a wide, short, lateral component the cross-section of this muscle from a horizontal to vertical oval (note cross indicating attachment level) thus increasing (GHL) and a narrower and longer medial por- its vertical dimension so that the muscle projects above the tion (GHM) that show some color differences symphysis. Its movement lifts the stiffened M. genioglossus (Fig. 1B).The two portions (which are not truly basalis, and this provides a second wedge transmitting the discrete) are closely attached and appear fused upward and forward momentum to the side of the rigid M. genioglossus medialis. Simultaneously, action of the Mm. midway along their length; but they separate geniohyoideus and submentalis depresses the mandibular anteriorly some distance posterior to the po- symphysis and, with this, the anterior fixed tip of the M. sition of the M. submentalis, and posteriorly genioglossus medialis. Thus the anterior tip of this muscle near the level at which the M. sternohyoideus is depressed and its middle is raised and protruded. The resulting couple causes the stiffened medialis to rotate out- inserts on the hyoid. The superficial fibers of ward carrying along the soft tissues. the medial portions of the two sides run 204 C. GANS AND G.C. GORNIAK

TABLE 1. Ranges of fiber lengths (in mm) of different regions of hyoid and lingual muscles in three Bufo marinus fwts: 693 g; 561 g; 429 g) Range of fiber lengths Muscle Bufo I Bufo I1 Bufo I11 M. intermandibularis 17-20(P) 18-20(P) 17-19(P) posterior 5-7 (A) 5-8 (A) 4-7 (A) M. submentalis 6-7 (PI 7-9 (PI 7-10(P) 4-5 (A) 46(A) 4-6 (A) M. geniohyoideus 25-27(L) 26-33(L) 2830(LI 36-37(M) 38-40(M) 3639(M] M. geniohyoideus' basalis 3-5 4-7 medialis 7-10(A, 9-ll(A, Ll L) 13-17(P, L) 13-16(P, L) 12-19(M) 12-17(M) 22-24(C) 19-2 1(C ) M. hyoglossus 28-37(L) 2840(L) 45-52(M) 50-54(M) M. sternohyoideus' sternal head ll-l5(D) 10-17(D) 19-35(S) 29-32(S) abdominal head 22-32 2330 M. omohyoideus* 16-17 16-19 M. petrohyoide~s~ anterior head 12(A); 17-20(P) middle head 19 posterior head 13

A, anterior; C, central; D, deep; L, lateral; M, medial; P, posterior; S, superficial Because of prolonged nitric acid treatment, the fibers of the M. genioglossus of Bufo I broke on contact and thus entire fibers could not be measured. The Mm. stenohyoideus and omohyoideus of Bufo I were not removed. The M. petrohyoideus of Bufo I and Bufo I1 were not removed. straight anteroposteriorly from origin to in- deep fibers of the medial head are slightly sertion. These fibers form the superficial por- longer than those of the superficial one, and tion of the medial edge along both the anterior all medial fibers are longer than the lateral and posterior separations. However, one group ones (Table 1). Stimulation of the M. genio- of deep fibers crosses from the posteromedial hyoideus pulls the hyoid anteriorly; when the head to join the anterolateral head. These deep hyoid is stabilized, action of the muscle deflects fibers form the medial edge of the posterior the anterior portion of the mandibular arch separation and the lateral edge of the anterior ventrally. one. Consequently, these are not two separate muscles, but a single muscle with multiple fas- M. genioglossus cicular subdivisions. The paired M. genioglossus is extremely The medial portion of the M. geniohyoideus complex. It originates from the posterodorsal originates from the fascia covering the ventral aspects of the mandibular tips and the men- surface of the laryngeal cartilages. It curves tomeckelians and extends from here into the dorsal to the M. submentalis to insert on the base and body of the tongue (Figs. lC, 3). dorsomedial surfaces of the mentomeckelian Chemical dissection indicates that the muscle cartilages. The lateral head originates along consists of two discrete portions, the basal the posterior horn of the hyoid and passes an- (GGB) and the medial-distal one (GGM, GGD, teriorly, lateral to the insertion of the Mm. Table 1).The former is so deeply tied into con- sternohyoideus and omohyoideus. Anteriorly, nective tissues that Magimel-Pellonier (1924) the lateral heads insert on the ventromedial commented that it could not be dissected in- aspects of the lower jaw; lateral to the men- tact. tomeckelian, some of the fibers terminate on The basal portion (M. genioglossus basalis) the dorsal surface of the M. submentalis. consists of two horizontal rods of muscle that Both the lateral and medial portions of the curve just dorsal to the M. geniohyoideus and M. geniohyoideus are parallel-fibered. The posterior to the mandibular symphysis. Each TONGUE PROTRUSION IN MARINE TOADS 205

rod is rounded and contains thick ventral and longest (Table 1).The radiating fibers increase dorsal raphes that run parallel to its long axis. in length proceeding posteriorly from the bas- The fibers of this basal portion are short, lie alis portion; the straight horizontal fibers of at right angles to the length of the rod and the “medialis” are shorter than the lateral and attach to both raphes. Cross-sections through medial radiating fibers. Stimulation of the bas- the rod show that the anteriormost fibers curve alis portion of the M. genioglossus results in first dorso-anteriorly from the ventral raphe ventroanterior movement that lifts the tongue and then dorsoposteriorly to reach the dorsal and rotates its base toward the symphysis. one; the posteriormost fibers first curve dorso- Stimulation of the rod-shaped portion of the posteriorly and then dorsoanteriorly toward medialis constricts this muscle to form a rigid the dorsal raphe. The more central fibers al- cylinder, while local application of stimuli most pass in a straight line from one raphe to shifts portions of the tongue. the other. Each rod is attached laterally to the dorsomedial edge of one mandibular tip and M.hyoglossus medially to the other rod; however, the entire The M. hyoglossus (HG) originates as two anterior surface of the arch of the basalis is proximal heads, one from each of the posterior connected to the skeletal arch. horns of the hyoid (Figs. lB, 2, and 3). The More dorsally the lingual mass shows a slen- fibers of the proximal heads follow the course der medial block of connective tissue, from of the posterior horns and merge at the ven- which arises the long-fibered longitudinal troposterior border of the hyoid corpus. Then “medialis” portion of the M. genioglossus. the muscle passes anteriorly along the ventral While the sides of this mass give off fascicles, surface of the hyoid but (at rest) swings dor- the main portion of the muscle is organized sally between its anterior horns to entend pos- into a pair of parallel rods that almost reaches teriorly into the body of the tongue. At this the tip of the tongue when this is contracted. level the muscle starts to break into distinct The muscle originates as a medial pair of mus- fascicles; small bundles leave its lateral edges cular rods from the medial connective tissue and dorsal surface to insert onto the dorsal raphe at the base of the tongue and from the surface of the tongue, i.e., adjacent and deep middorsal edge of the mentomeckelian carti- to the lingual pad. The main mass of the mus- lages. Small fascicles first leave the ventral cle continues posteriorly, but a medial sepa- and lateral sides of this mass just posterior to ration again becomes obvious. The two distal the basalis portion and run almost at right heads run along the midline on the ventral angles to the medial rod; a horizontal fascia1 surface of the tongue just deep to the retrolin- plane separates the dorsally curving posterior gual membrane. At intervals, small bundles fibers of the basalis from these horizontally of fibers radiate from the lateral aspect of each running fibers of the “medialis.” Further sets distal head to insert near the periphery of the of fiber bundles then radiate from the lateral tongue and along the sides of a poorly defined and medial aspects of each rod. The many lat- trough in the mucous membranes that lies on eral bundles run posterolaterally, and the few the midline and dorsal to the distal head of the medial ones nearly straight posteriorly along M. hyoglossus. However, the central fiber mass the midline; both insert into the lingual sur- of the distal heads continues to run posteriorly face where they interdigitate between the in- along the midline. Near the lingual tip, each sertions of the M. hyoglossus. One is tempted side of the hyoglossus breaks up into discrete to separate this muscular portion into a central fascicles. There is complete overlap of their in- “medialis” and a peripheral “distalis.” How- sertion sites, and both sets of radiating fibers ever, the medial and distal sections, while dif- insert in an overlapping pattern across the fering in location, represent the two ends of distal (posterior in situ) tip of the tongue. the same fibers, all of which originate from the The parallel fibers of the M. hyoglossus are same raphe of medial connective tissue. Near by far the longest of those in the buccal region the tip of the tongue, two large medial bundles (Table 1).The lateralmost fibers are the short- of fibers then break up, fanning out postero- est and the medialmost the longest; there is a laterally, and inserting into the distal pad of gradual increase in fiber length from the lat- the tongue. Even here bilaterality is strictly eral to medial portions of each head. Stimu- maintained. lation of the M. hyoglossus, when the tongue The fibers of the basalis are very much the is extended, produces a localized bulging of the shortest, and the radiating medial fibers in- central, lingual surface near the tip and re- serting into the distal pad of the tongue the traction of the tongue toward the mouth. 206 C. GANS AND G.C. GORNIAK

M. sternohyoideus short; however, the fibers of the remaining por- The M. sternohyoideus (STH) arises medi- tion of this head are long and of equivalent the two cylin- ally from the sternum proper and laterally lengths. The fibers of the first of from the external surface of the abdomen. The drical posterior heads are all the same length I but slightly shorter than the long fibers the two heads of origin pass dorsally and ante- of anterior head. The fibers of the posteriormost riorly and join each other dorsal to the cora- head are also all of the same length but are coid. The muscle then passes between the two the shortest of the three heads. Stimulation of heads of the M. geniohyoideus (Fig. 1B). The petrohyoideus of one side moves the bulk of the M. sternohyoideus inserts along the the M. hyoid dorsally and posterolaterally. lateral boundary of the body of the hyoid; how- ever, an anterior slip of the muscle attaches M. omohyoideus rostrally on the anterior horn (deep to the GHL The omohyoideus (OH) originates from in Fig. 1B). M. the dorsal anterior surface of the scapula (Fig. The fibers of both heads of the M. sterno- 1B). It inserts on the body of the hyoid just hyoideus are arranged in parallel. While the dorsal to the center of insertion of the M. ster- abdominal head is longer than the sternal one, nohyoideus. Its fibers are parallel, run anter- its fibers are divided transversely into an an- omedially and are of equivalent lengths. Stim- terior and posterior group by a tendinous in- ulation of the M. omohyoideus of one side scription that lies just posterior to the origin moves the hyoid posterolaterally. of the sternal head. The fibers of the anterior part of the abdominal head are generally M. depressor mandibulae longer than those of the posterior head. The The M. depressor mandibulae (DM) connects sternal head shows two longitudinal subdivi- the skull to the mandible (Fig. 1B). It origi- sions: (1) a ventral group of long fibers that nates from the ventral posterolateral portion runs medial and parallel to those of the ab- of the postorbital shelf, passes ventrolaterally dominal head and (2) a dorsal group of short and slightly posteriorly over the quadratoar- fibers that runs anterolaterally deep to the ticular joint and then attaches to the posterior abdominal head. Stimulation of the M. ster- edge of the angulosplenial portion of the man- nohyoideus of one side moves the hyoid pos- dible. teroventrally and slightly toward that side. MOVEMENTS M. petrohyoideus General The M. petrohyoideus (PTH) originates deep The feeding sequence of toads is here sub- to the M. sternocleidomastoideus (which is divided into four phases (1) preparatory, (2) deep to the M. depressor mandibulae) from the tongue protrusion, (3) tongue retraction, and ventral posterolateral portion of the postorbi- (4) mouth closing. “Preparatory” starts with tal shelf. The M. petrohyoidei can be divided forward movement of the toad and terminates into one anterior and two posterior portions just as the mouth starts to open. “Tongue pro- (Fig. 1B). The anterior portion (M. petrohyoid- trusion” involves rotation of the lingual base eus anterior) is thin and flat and arises deep about the symphysis (rotation phase), followed to the posterior portions. Its fibers course ven- by elongation of the lingual tip (elongation trally and anteromedially to insert on the ven- phase), and ends with impact of the tongue on tral surface of the body of the hyoid, deep to the prey. “Tongue retraction” ends when the the M. geniohyoideus and just lateral to the tongue has completed its rotation to the back fascia1 tunnel of the M. hyoglossus. The fibers of the buccal cavity, and “mouth closing” ends of the more anterior of the two cylindrical pos- as the mandible returns to the closed position terior heads pass ventroposteriorly and insert (Figs. 4, and 5). It is necessary to record move- on the posterior horn of the hyoid near its pos- ments of the upper and lower jaws relative to terior process. The fibers of the more posterior the ground and to each other, and of portions head also pass ventral, but more posteriorly of the tongue relative to the symphysis. Move- than those of the more anterior head, to insert ments of the snout relative to the ground in- at the tip of the posterior horn at the attach- dicate movements of the upper trunk about the ment of the M. hyoglossus. forefeet. The fibers of all three heads of the M. pe- The preparatory phase takes 180 * 39 msec trohyoideus are arranged in parallel. The most when the food object is about 2 to 2 ?hhead ventroanterior fibers of the anterior head are lengths from the mandibular symphysis. The TONGUE PROTRUSION IN MARINE TOADS 207

B

Fig. 4. Bufo marinus. Sketches of toads showing the position). (E) Tongue starts to retract and prey held by suc- process of tongue protrusion (left) and retraction (right). (A) tion-cup effect; (F) the lingual pad has crossed the symphysis Tongue at rest in floor of mouth; (B) the symphysis has been and is again depressed while the still formed rod extends depressed, the Mm. submentalis and genioglossi contracted beyond it; (G)the pad has moved further into the oral cavity and the formed rod is rotating carrying the soft tissues and the rod starts to melt; and (H)the rod has melted com- along; (C) the rod has rotated beyond the symphysis and the pletely, M. genioglossi lie posterior to the symphysis, and soft tissues are being accelerated; and (D) the mass has the mass of the tongue drops ventrally. Note that the pad been elongated and the soft tissues are impinging on the retracts before the rod. prey. Note the rotation of the pad (from dorsal to a ventral 208 C. GANS AND G.C. GORNIAK

4

SM 0

W 60 n 120 3 t-- 4 -I a. 2 GGB 0 a w 60 -Y 120 n 4 c/) z a GGM 0 W 2 60

U GHL o !2 60 3 Z 120 -Y GHM 0 c/) z 60 a 120 W 2 4

DM 0

60

120

TIME (7 msec/unit)

5 TONGUE PROTRUSION IN MARINE TOADS 209

mouth is open for 143 ? 22 msec. The initial If the prey is dropped immediately in front rapid opening takes only 15.68 * 3.17 msec. of the snout there may be no discernable fur- The actual protrusion of the tongue, from the ther preparation except that the eyes are al- time it lifts from the floor of the mouth until ways covered by the nictitating membrane full impact on an object, between 1to 1YZ head prior to the flip. The tongue is then flipped lengths from the mandibular symphysis, lasts without further movement of the head or for 36.8 & 3.5 msec. Closing, which follows trunk. Occasionally the snout is depressed after the tongue has returned to the mouth, slightly prior to the flip of the tongue, which takes 79.39 * 21.65 msec. then includes mainly the rotation but little extension. If prey is placed more than one, but The preparatory phase less than two, head lengths from the snout, the Movements of the prey provide the primary preparatory phase continues by extension of cues for the initiation of the preparatory phase the hindlimbs. This rotates the upper trunk (Ingle, 1976; Ewert et al., 19701, although over the extended forelimbs, so that the snout members of some populations of Bufo marinus and anterior trunk are moved forward and apparently feed on vegetation (Zug and Zug, then downward toward the prey at a constant 1979) and individuals have been noted to take rate. Simultaneously, movements of the buccal dog and cat food (Alexander, 1964; Clark, floor cease, and the nictitating membrane 1974). Our toads ignored all dead prey and starts to cover the eye signalling protrusion. tended to make preliminary visual discrimi- nation among prey items. When living prey The tongue protrusion phase stops moving, the toad stops its approach until Protrusion involves tightening of the lingual the prey again starts to move. Toads have def- mass near the mandibular symphysis, opening inite control of the direction and distance of of the mouth, rotating a stiffened lingual core the flip. Direction is mainly indicated by po- away from the buccal floor and around the sym- sitioning the head toward the prey. However, physis, shifting of the lingual soft tissues on there is some lateral control, as indicated by the head of the core past the symphysis, their toads that tilted their head prior to flips; and sliding along the core and impacting onto the a specimen that was blind in one eye always prey (Fig. 4). The thickened, dimpled posterior flipped the tongue asymmetrically. lingual pad impacts onto the prey; while the When prey is introduced some distance pad faces dorsad at rest, it faces ventrad as it away, a hungry toad assumes an alert position. impacts onto the prey. The protrusion starts The forelimbs are then extended, raising the as the mouth is opening, and the moving ton- head and upper trunk, the snout is pointed gue never touches the upper jaw. The force of toward the prey, the eyes are open, and the impact imparts pressure on the intrinsic lin- head is motionless. The toad may walk or hop gual glands, and some oftheir secretion is emp- toward prey in intermittent movements; the tied onto the contacted surface. oscillation cycles of the buccal floor continue. Movements of the tongue appear to precede opening of the mouth as the first film frames always show the lingual tissues bunched near the symphysis. The next event appears to de- pend upon the angle at which the tongue has to be projected. If the prey is some distance Fig. 5. Bufo marinus. Summary diagram to show EMGs away and the projection angle is close to hor- of some buccal muscles during the flip of the tongue. Num- izontal, projection starts immediately. Indeed bers on right side give the number of flips, followed by the it may proceed with the mouth only partially number of animals. Each bar of the graph shows the mean spike number (above the line) and the mean spike amplitude opened so that the rolling tongue barely clears in units of .08 millivolts each (below the line). The dashed the roof of the mouth. If the prey is closer to lines define the intervals of preparatory, protrusive, retrac- the snout, the mandible first drops through 20" tive, and closing movements by defining the start of opening to 30" before the tongue is activated. (SO), start of tongue protrusion (SP),start of retraction (SR) and end of retraction (ER). The sequences are aligned on The first stage of the flip is seen as a strong the start of opening. ventral deflection of the entire anterior quarter The muscles in order are the Mm. submentalis (SM), gen- of the lower jaw; this forms an initial sharp ioglossus basalis (GGB), genioglossus medialis (GGM), gen- bend in the floor of the mouth. This bend is iohyoideus lateralis (GHL), geniohyoideus medialis (GHM), and depressor mandibulae (DM). Note the differing rela- apparently permitted by rotation of the ante- tionships between mean spike number and amplitude dur- rior ossified zone of the mandible about the ing the phases of the tongue flip. posterior one at the unfused mid-mandibular 210 C. GANS AND G.C. GORNIAK articulation. Simultaneously the base of the posteriorly (Fig. 4). Lingual retraction appears tongue appears to rise from the floor of the to begin even before or within 10 msec of im- mouth, rotating anteriorly. In frontal view, the pact of tongue onto the ground and prey. (Sev- protruding mass appears wedge-shaped; the eral times, retraction was apparently triggered outward rotation forms the tongue into a nar- too early, the soft tissues were arrested during row leading edge as it crosses the level of the the outward rotation, and their extension re- symphysis. This edge extends the full height versed before they hit the surface. The soft tis- of the tongue during passage over the sym- sues first swung posteriorly and downward but physis and during the first stage of rotation then reversed direction, swinging anteriorly beyond the mouth. Lateral views show that the and upward to lie dorsal to the lingual base.) soft tissues, which at rest occupy the dorsal The extensible nature of the soft lingual tis- lingual surface, are trailing and that much of sues and the mucous membranes is the most the soft tissue still remains within the mouth. striking aspect of the retraction phase. The As the lower jaw drops further, the initial bend papillate surface of the tongue (Waller, 1849) of the symphysial portion becomes less acute generally seems to be pulled out of contact with relative to the posterior portion of the lower the ground by a pull on the medial aspects of jaw, as if this had caught up to the depressed the lingual surface. Anterior views of the ton- mandibular tips. gue show a deep depression of the medial sur- Within another 20 msec the ridge has con- face during retraction. This deformation ap- tinued over the symphysis. By less than 30 parently induces a suction effect on larger msec the basal portion has achieved its final prey, as noted by a film record in which the position; it then extends beyond the symphysis tongue was seen to adhere to a petri dish and along a more or less straight line toward the lift it off the substrate, the bond only being prey. During the last stages of straightening, broken when the dish hit the snout. The soft the soft tissues have begun to follow the ro- tissues connecting the pad-like lingual tips to tating base; and the tongue looks heart-shaped the lingual base, as well as the mucous mem- in frontal view. The anterior movement of branes connecting lingual tip to symphysis, these tissues continues along the axis formed appear to be remarkably extensile. The lingual by the basal muscles. The now anteriorly ex- pad and its attached prey will have proceeded tended basal portion remains fixed, and the far into the buccal cavity before the basal rod more dorsal part slides across the symphysis and the connective tissues of the lingual base and continues to slip along the basal mass that will have completed crossing the symphysis. points toward the prey. The separation be- The basal muscle mass, which has remained tween the anteriorly moving soft tissues and fairly rigid during the start of retraction, does the stationary basal portion reflects the angle not remain erect thereafter and rotate back- between the extended tongue and the ground. ward across the symphysis; rather, it appears The lower jaw generally continues to accel- to melt over the symphysial region. erate downward until the basal portion points Consequently, movements show hysteresis at the prey; in most flips the jaw reaches max- between the flip and the return. During the imal opening acceleration at about 25" of gape. flip the surface of the tongue moves almost Sometimes this movement involves the entire twice as fast as during the return. The basal head. The soft tissues of the lingual tip lag 7 rod moves out first accelerating in straight ro- msec behind the base; they travel anteriorly tation over the symphysis. The lingual pad on for 21 msec after they pass the tip of the man- the distal tip moves out last. Once the soft tis- dible. At the end of the protrusion phase, the sues have passed the symphysis, they unroll mouth gapes about 60". at a regular and relatively constant rate; their Whatever the distance between head and velocity never reaches that of the initial ro- prey, the lingual tip unrolls onto the surface; tation of the base. In contrast, the lingual pad but it stretches much more when the prey ob- returns most rapidly, while the basal rod fol- ject is a head width or farther from the sym- lows at less than half of its velocity first ro- physis. The inertia inherent in the moving tating and then deforming around the sym- mass can be seen to deform it around any ir- physis. regularities of the substratum. As the soft tissues pass the mentomeckelian region, the edge of the jaw again rotates ven- The tongue retraction phase trally, forming a prominent V-shaped notch at The mouth is open throughout the retraction the symphysis. The tip of the tongue first phase, and the snout may move upward and moves directly posteriorly for 14-21 msec, TONGUE PROTRUSION IN MARINE TOADS 21 1

4 w IMP 0

60

120 4

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STH o

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120 5 10 15 20 25 30 35 40 45 50 TIME (7 rnsec/unit)

Fig. 6. Bufo marinus. Summary diagram to show EMGs (SO), start of tongue protrusion (SP),start of retraction (SR) of some buccal muscles during the flip of the tongue. Num- and end of retraction (ER). The sequences are aligned on bers on right side give the number of flips, followed by the the start of opening. number of animals. Each bar of the graph shows the mean The muscles in order are the Mm. intermandibularis pos- spike number (above the line) and the mean spike amplitude terior (IMP), hyoglossus (HG), sternohyoideus (STH), pe- in units of .08 millivolts each (below the line). The dashed trohyoideus (FTH), and omohyoideus (OH). Note the differ- lines define the intervals of preparatory, protrusive, retrac- ing relationships between mean spike number and tive, and closing movements by defining the start of opening amplitude during the phases of the tongue flip. 212 C. GANS AND G.C. GORNIAK bunching onto the lingual base. The entire ton- number, and the product thereof were then gue starts to curl upward and posteriorly about graphed for each movement sequence. The 7 msec later. The tip of the tongue reaches spike amplitude and product gave the best cor- maximal backward acceleration as it ap- relations with mechanical events; indeed, for proaches the symphysis about 35-42 msec into certain muscles the spike number by itself was the retraction phase. By then, the mouth has remarkably uninformative, as it remained rel- opened lo" more than during the protrusion atively constant even though EMG amplitude phase, and backward acceleration at the tip of changed markedly (Fig. 6). The product is re- the tongue decreases. At the end of retraction, ferred to below, unless specific mention is made the base and tip of the tongue drop ventrally. of an alternate comparison (Fig. 7). The level Maximal protrusive acceleration of the lin- of activity during each subdivision is given as gual tip is twice the magnitude of retraction a percentage of the maximum value of the (when holding a prey object of 0.2 g mass). product shown during a movement sequence. The mouth closing phase (See Figs. 7 and 8 for absolute values). Comparisons among many toads indicate The mouth begins to close after the surface that the standard errors of the product of spike of the tongue has returned to a horizontal po- number and amplitude determined from EMGs sition near the back of the oral cavity. If the of, for instance, the M. submentalis and M. animal rotated its trunk over the forelimbs, geniohyoideus medialis are relatively low for the head and body will still continue to rotate the protrusion phase and show excellent agree- backward and upward throughout closing. ment from flip to flip (Fig. 9). In contrast, the Maximal closing acceleration of the mandibles firing patterns (and numerical values) of these occurs within 14 msec after the start of closing muscles differ markedly during the prepara- and decreases thereafter. In frontal view, the tory phase, during tongue retraction and dur- V-shaped ventral bend of the mentomeckelian ing closing of the mouth. region disappears suddenly, after the entire mass of the tongue has returned to its hori- The preparatory phase zontal position and the mandibles then are During most of the preparatory phase, while again parallel to that of the upper jaw. The the upper trunk rotates toward the prey, ac- oscillatory movements of the buccal floor begin tivity levels of the Mm. genioglossus basalis again once the mouth closing phase is com- and medialis, intermandibularis posterior, de- plete. pressor mandibulae and submentalis are low and variable; whereas that of the M. genio- ELECTROMYOGRAPHY hyoideus lateralis is high. General The activity level of the M. genioglossus bas- Electromyograms (EMGs) coincident with alis fluctuates between 0 and 40%, that of the the flip and retraction of the tongue are de- M. genioglossus medialis between 0 and lo%, scribed for the lingual, hyoid and mandibular that of the M. intermandibularis posterior be- muscles (Figs. 6-8). These test whether the tween 5 and 85%, that of the M. depressor muscles in question are actually active at par- mandibulae between 10 to 100% and that of ticular periods and whether the individual ac- the M. submentalis between 0 to 60%. How- tions, observed during stimulation, indeed oc- ever, the M. geniohyoideus lateralis, is very cur coincident with the movements recorded active, generally reaching between 60 and on film. Activity may be almost continuous 100%. The Mm. geniohyoideus medialis, hy- while the tongue is flipped and returned to the oglossus, and sternohyoideus may show occa- mouth. Consequently, the onset and cessation of muscular activity by themselves provide in- adequate iniormation about muscular activity during prey capture. However, several descrip- tions of the EMG differ markedly among the Fig. 7. Bufo mnrinus. Summary diagram to show EMGs subdivisions of the movement sequence; quan- of the same buccal muscles and flips shown in Figure 5. tification of the EMGs simplifies interphase Each bar of the graph shows the mean spike number times comparisons. mean amplitude as a percentage of the maximum value; the The EMG for each major muscle was deter- maximum value of cumulative voltage in units of .08 mil- livolt is given for each graph. Note that these cumulative mined during at least five tongue flips coinci- values correlate better with the phases of the lingual flip dent with cine; further intermuscle compari- than does either the mean spike number or mean amplitude sons were also made. EMG amplitude, spike separately. TONGUE PROTRUSION IN MARINE TOADS 213

SM

GGB

GGM

loo 1 223.6 c I GHL 50

0 loo 1 192.5 c

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DM

5 10 15 20 25 30 35 40 45 50

TIME (i’msec/unit)

7 214 C. GANS AND G.C. GORNIAK

ii !! ! W loo 94.7 r n i 3 I-- A .-.. [L !! I 2 !! a

a HG ~ W 50 ,A 2- 211'o I- lo0l0 !! K !! ! W !! m !! !! z !! 3 STH 50 --II ! Z .- loo 171'2 !! ! w I -v 0 u .- ! 1 !! ! [L ili a 0.34 !!SP ! loo 1 SO;:-- ISR IER LL !! ! ! 0 PTH 50 !! ! 1 !! ! w II I CJ ii i a 0 I- 100 Z 0.7 !! ! ! W !! ! ! - !!-! ! 0 !! ! ! K OH 50 !! ! ! W !! ! ! [L !! ! ! !! ! 0 i rnn I I. I I n 5 10 15 20 25 30 35 40 45 50

TIME (7rnsec/unitI Fig. 8. Bufo marinus. Summary diagram to show EMGs than do either spike number or mean amplitude separately. of the same buccal muscles and flips shown in Figure 6. The diagram shows clearly that the M. sternohyoideus Each bar of the graph shows the mean spike number and shows only minor activity until retraction. The relative val- mean amplitude as a percentage of the maximum value; the ues for the Mm. petrohyoideus and omohyoideus are even maximum value of cumulative voltage in units of .08 mil- more misleading as seen by the maximum values given for livolt is given for each graph. Note that these cumulative each graph. However, neither of these muscles makes any values correlate better with the phases of the lingual flip significant contribution to protrusion or retraction. TONGUE PROTRUSION IN MARINE TOADS 215

-. !! ! ! !! ! I II I SM

4 !! ! ! II-. I 4

GGB 0

4 !! ! II I 4l

GGM 0

I ii i 4 !I-- I !! ! ! 4 !! ! ! !! ! ! II I I GHL 0

4 !! ! ! II.- I 4 !! ! !! ! II I

GHM 0

!! ! ! 4 *I I 5 10 15 20 25 30 35 40 45 50 TIME (7 msec/unitl

Fig. 9. Bufo marinus. Coefficients of variation for spike Note that variability for the muscles involved in the flip number (above line) and amplitude (below line) of the EMGs appears to be least during protrusion and greater during for each of the intervals of a lingual flip. The values are for retraction, closing, and preparation. the same flip sequences and muscles as shown in Figure 5. 216 C. GANS AND G.C. GORNIAK sional bursts of low-level activity (generally the M. submentalis drops to below 40%, but below 20%). The M. petrohyoideus anterior is again increases to 60 to 70% as the lingual tip silent, whereas the M. omohyoideus shows in- reaches the prey. Activity of the M. genioglos- termittent low-level bursts of activity. sus basalis initially decreases to below 70%; it varies considerably thereafter, either in- The protrusive phase creasing to 75 and 100% or remaining near With the exception of the Mm. petrohyoideus 50%. Activity of the M. genioglossus medialis anterior, all the muscles from which we re- also varies considerably between 0 and 50%. corded are active during protrusion. However, Activity of the M. geniohyoideus lateralis is the level of activity changes from the start of between 40 to 70% as the lingual tip passes protrusion, when the basal rod of the tongue from the mouth and reaches the prey, whereas forms and rotates over the symphysis, to its activity of the M. geniohyoideus medialis in- end when the inertia of the soft tissues of the creases to 75 to 100%.Simultaneously, activity tongue stretches them toward the prey. of the M. hyoglossus first rises to between 50 The Mm. submentalis, genioglossus basalis and 75% but then decreases to below 40% as and medialis, and geniohyoideus lateralis, all the lingual tip reaches the prey. The M. inter- show high levels of activity as the base of the mandibularis still shows marked variability tongue is stiffened and the tips of the lower ranging between 5 and 60%; however, its ac- jaw are depressed. Activity of the M. submen- tivity generally ranges between 25 and 35% talis increases to a level of 80 to loo%, that of as the tip reaches the prey. Activity of the M. the M. genioglossus basalis increases to 80 to depressor mandibulae shows a level between loo%, that of the M. genioglossus medialis in- 10 and 50%. The M. sternohyoideus remains creases to 50 to loo%, and that of the M. gen- active below 30%, and the M. petrohyoideus iohyoideus lateralis decreases to 50 to 70%. anterior is silent. Simultaneously, activity of the M. geniohyoi- During the protrusive phase, the mean num- deus medialis increases to 50 to 60%, that of bers of spikes generally correlate positively the M. depressor mandibulae is between 90 with the mean spike amplitudes for the Mm. and loo%, and that of the M. intermandibu- submentalis, genioglossus basalis, geniohyoi- laris posterior varies between 15 and 50%. deus medialis and hyoglossus (Fig. 6). How- Activity of the M. sternohyoideus remains be- ever, for the Mm. geniohyoideus lateralis, de- low a 20% level, the M. omohyoideus may show pressor mandibulae, sternohyoideus, and an occasional burst of low-level activity, while intermandibularis posterior, changes in the that of the M. hyoglossus increases slightly to mean spike number show no obvious correla- 0 to 30%. tion with mean spike amplitude. As the stiff basal rod of the tongue rotates about the depressed symphysis, the M. genio- The retraction phase glossus basalis and medialis, as well as both All muscles, except the M. petrohyoideus the Mm. geniohyoideus lateralis and medialis, anterior, are active during the retraction show high levels of activity; whereas the ac- phase. However, their activity levels differ tivity of the M. submentalis decreases. The considerably. Mm. genioglossus basalis and genioglossus During the initial retraction of the soft tis- medialis, continue to fire at levels of 80 to 100% sues of the tongue, muscular activity of the M. and 50 to loo%, respectively. Activity of the hyoglossus increases to 90 to loo%, and the M. M. geniohyoideus medialis increases to 70 to genioglossus medialis fires at a level between 80%; while that of the M. submentalis de- 20 and 50%. As the soft tissue mass folds on creases to 50% where it remains steady, and itself and moves toward the mouth, the M. sub- that of the M. geniohyoideus lateralis de- mentalis is active between 50 and 75%, the M. creases to below 50%. Activity of the M. ster- genioglossus basalis between 20 and 40%, M. nohyoideus remains below 20%, while that of genioglossus medialis below 20%, and the M. the M. hyoglossus increases slightly to a level hyoglossus between 50 and 75%. Activity of of 30 to 40%. the M. geniohyoideus lateralis increases to As the basal lingual rod has rotated into a 70% and to 80%, but then decreases to below position parallel to the lower jaws and the soft 60%. Activity of the M. geniohyoideus medialis tissues are thrown toward the prey, activities first drops below 70% as the tongue begins to of the Mm. submentalis, genioglossus basalis, fold; it then drops below 50%as the lingual tip and genioglossus medialis change. As the lin- approaches the symphysis. As the mouth opens gual tip passes the symphysis, the activity of further, the activity of the M. depressor man- TONGUE PROTRUSION IN MARINE TOADS 217

4

DM 0 z 0 - 4 l- -a 4 !! ! ! CT >a IMP 0 LL 0 4 I- 4 zw - HG 0 0 I LL LL w 4 0 4 0

STH 0

4L !! ! ! 5 10 15 20 25 30 35 40 45 50 TIME (7 rnsec/unit)

Fig. 10. Bufo marinus. Coefficients of variation for spike the Mm. petrohyoideus and omohyoideus have been omitted. number (above line) and amplitude (below line) ofthe EMGs Note that variability for the M. hyoglossus appears to be for each of the intervals of a lingual flip. The values are for least during retraction. the flip sequences and muscles as shown in Figure 6, though

dibulae first increases to 70% but then drops 40% as the retrusion phase ends and the lin- to below 40%. Activity of the M. intermandi- gual pad reaches the back of the oral cavity. bularis posterior increases to 75 to loo%, as The M. genioglossus medialis shows occasional the pad approaches the symphysis, but then bursts of low-level activity below 10%. The M. drops to below 60% as the pad enters the buccal depressor mandibulae becomes silent before cavity. Activity of the M. sternohyoideus first the tongue reaches the back of the oral cavity. remains below 30%,but then increases to 50% Whereas the Mm. geniohyoideus lateralis, hy- to 90% when the pad crosses the symphysis. oglossus, and sternohyoideus are active into As the soft tissues of the tongue are retracted the closing phase, the M. petrohyoideus ante- over the symphysis, activity of the Mm. sub- rior is silent. Activity of the M. geniohyoideus mentalis, genioglossus basalis, intermandi- lateralis varies markedly as the lingual mass bularis posterior, and geniohyoideus medialis folds posteriorly past the symphysis to reach decreases to levels below 40%; it remains below the back of the oral cavity, showing bursts of 218 C. GANS AND G.C. GORNIAK activity ranging from 10 to 50%. Activity of ative magnitude may pose problems in anal- the M. hyoglossus is 60 to 80% as the tip passes ysis. Presumably, the number of fibers re- the symphysis but then decreases to below 40% cruited is a function of the imposed loading. as the retraction phase ends. Activity of the Consequently, a muscle might be expected to M. sternohyoideus is at 60 to 70% as the lin- fire at different levels if it were active during gual tip passes the symphysis and remains both extension and retraction. Flipping of the active at this level to the end of the retraction tongue might well require less effort than, for phase. instance, its retraction while lifting objects of During the retraction phase, changes in the varied mass. mean number of spikes again generally show Three lines of evidence may explain the con- a positive correlation with mean spike ampli- trast of constancy and variation. (1) The an- tude of the Mm. submentalis, genioglossus bas- ticipatory firing during the preparatory phase alis, and hyoglossus. The mean number of seems to correlate with the toad's perception spikes of the Mm. geniohyoideus medialis and of prey. Toads sometimes move forward and lateralis, sternohyoideus, and intermandibu- then wait until more than one moving meal- laris posterior remain relatively constant; worm is adjacent; the tongue is then flicked to whereas their mean spike amplitudes changes. capture all at once. Apparently the variation In general, the mean amplitudes of these latter of firing pattern relates to differences in the muscles are more informative than their mean initial position of the tongue. (2) Quite regu- spike numbers; however, the product of the larly the Mm. genioglossus basalis and medi- means is most informative. The standard er- alis show a low-level pulse during the prepa- rors of the M. hyoglossus are lowest during ratory phase, prior to the much higher pulse retraction (Fig. 10). observed during the tongue flip. We interpret this preliminary pulse as a positioning of the The closing phase muscle fibers into the appropriate longitudinal During closing of the mouth, the Mm. de- relationships for the flip. Such preliminary pressor mandibulae, geniohyoideus medialis, standardization of the system presumably ac- and intermandibularis posterior are silent. counts for the low variability of EMGs ob- The Mm. submentalis, genioglossus basalis, served among successive flips. (3) Variability genioglossus medialis, geniohyoideus later- during retraction and during the closing and alis, and hyoglossus show bursts of activity, post-closing periods apparently reflects the the levels of which vary considerably but are need for repositioning the flabby tongue and below 40%. However, activity of the M. ster- the prey. nohyoideus increases to 75 to 100% during the The EMG could always be correlated with first half of closing and then becomes highly mechanical activity (in toads with body tem- variable. The M. petrohyoideus anterior is si- peratures near 25°C) following 5 msec or less lent throughout most of the closing phase, but upon the signal. This is in good correlation the M. omohyoideus may show an occasional with reports that mechanical activity in iso- burst of low-level activity. However, these metric contraction is temperature sensitive muscles are active at the very end of closing and occurs 5 msec after stimulation at 20°C and during the subsequent buccal oscillations. (Abbott and Brady, 1964). In addition, the Mm. geniohyoideus medialis and lateralis, intermandibularis posterior and submentalis show low levels of activity coin- Mechanical synthesis of lingual protrusion cident with the buccal oscillations; however, The first series of events occurs during the the Mm. genioglossus basalis and medialis are preparatory phase and is clearly the antici- then silent. This differential activity also pro- patory effect that bunches the lingual tissues vides an indicator of whether electrodes lie near the mandibular symphysis and presum- within the genioglossal muscles or in muscles ably orients the extensible soft tissue mass in ventral to them. anticipation of the flip. This bunching is also reported in cinefluorograms (Emerson, 1977). DISCUSSION Low-level activity of the M. genioglossus me- Meaning of the EMG dialis just before the flip seems to stiffen these Almost all of the buccal muscles fire at dif- long fibers into a rod. ferent levels during each of the activity pe- The Mm. geniohyoideus medialis and later- riods. Consequently, the relative magnitude of alis would presumably act along a straight line activity rather than its onset and cut-off proves between their origins on the symphysis and to be the critical item. However, even the rel- insertions on the hyoid. Even though the M. TONGUE PROTRUSION IN MARINE TOADS 219

geniohyoideus medialis lies on the dorsal sur- physis. This supports the concept that the face of the M. submentalis, neither it nor the forces required for tha anterior bending must M. geniohyoideus lateralis (which inserts near be applied near the symphysial region. the lateral margin of the M. submentalis) Coincident with this ventral bending, one would restrict deformation of M. submentalis sees the stiffened lingual rod lift from the floor at the start of protrusion; the M. geniohyoideus of the mouth and rotate around and over the medialis shows high-level activity only during depressed mandibular symphysis (Fig. 3). The late protrusion. While both Mm. genio- lingual rod is very clearly formed by the long- hyoidei assist in opening the mouth, the M. fibered M. genioglossus medialis; the two rod- geniohyoideus lateralis also moves the hyoid shaped parts markedly increase in activity plate anteriorly. This anterior movement re- during the flip and are the only elements in a duces tension in the soft tissues that will be position to form this rod. What then rotates flipped (and thus allows their full and most the lingual rod about the symphysis? rapid extension) and reduces the length to The basal rod is tied to the symphysial re- which the long, central fibers of the M. hy- gion and its anterior end and would be ex- oglossus will be stretched (and with this im- pected to be depressed as the symphysis is de- proves the length-tension characteristics for formed ventrally (Fig. 3C). The activity of the its retraction). The low level of activity of the very short fibered M. genioglossus basalis, M. sternohyoideus fixes the hyoid, facilitating which lies immediately deep to the lingual rod, depression of the jaw by providing a stable or- would stiffen and thicken it, forming a trans- igin for action of the M. geniohyoideus later- verse, rod-shaped mass between the mandi- alis. bles, though this transverse rod would not in- The major action of the M. geniohyoideus duce transverse tension. Simultaneously, the lateralis (but not the medialis) deserves atten- more posteriorly positioned, short fibered, M. tion, for not only does it move the hyoid an- submentalis forms a second rigid and trans- teriorly, but also adds a vector depressing the verse bar. Action of the Mm. submentalis and symphysis. In contrast, the lower level activity geniohyoideus lateralis induces symphysial of this muscle during ventilatory oscillations depression; however, the M. submentalis does appears to lack the depressing factor, and the not itself move ventrally, but its mass shifts contraction of the M. submentalis then lifts the anteriorly and upward. When the M. submen- mandibular symphysis (and the mentomeck- talis is relaxed, its mass extends much further elian bones, where unfused), and with this clo- posteriorly than anteriorly between its two lat- ses the nares (de Jongh and Gans, 1969). eral attachments. Activation then brings the Opening of the mouth apparently involves ends together, but also shifts the mass ante- minor activity of the M. depressor mandibulae. riorly; it changes the cross-section of the mus- In those flips in which the symphysial region cle from a horizontal to a vertical oval. The starts bending ventrad as the mouth opens, dorsal surface of the M. submentalis rises far one notes initial high levels of activity in the above the level of the mandible and lifts the Mm. submentalis and geniohyoideus lateralis; M. genioglossus basalis farther dorsally. While the M. genioglossus basalis fires at or within the anterior end of the rod formed by the M. milliseconds before this interval. Apparently submentalis is depressed, the slightly more the M. submentalis acts to pull the mandibular posterior one is raised. tips together, thus deforming the mentomeck- Combined EMG and cinematography indi- elian region and deflecting it ventrad so that cate that the M. genioglossus medialis reaches there is an obvious bend of the tip of the jaws. high levels of activity and forms a rigid rod The contraction of the M. geniohyoideus later- just after the activation of high-level activity alis, acting as it does on the more lateral por- of the Mm. submentalis and genioglossus bas- tion, provides further ventrad deformation of alis. Consequently, the rise of these two mus- the symphysial zone. This anterior symphysial cles exerts an upward force on the more pos- flexibility of Bufo marinus is apparently equiv- terior portion of the rod, and the depression of alent to the symphysial motility, which is in the mandibular symphysis exerts a downward other species (Ranapipiens) induced by shifts force on its anterior portion. The resulting of the mentomeckelian elements. The bending force-couple induces a strong moment that of the anterior zone is reduced as the mouth would then rotate the rod-shaped M. geniog- opens farther; i.e., a ventral shift of the pos- lossus medialis through 180" around the man- terior portion of the mandibles, due to action dibular symphysis. of the M. depressor mandibulae, follows, rather TWOfactors induce the rapid rotation of the than precedes, downward deflection of the sym- rod-shaped M. genioglossus medialis. First, the 220 C. GANS AND G.C. GORNIAK stiffness of the Mm. submentalis and geniog- mandible. Indeed, in a number of flips in which lossus basalis reduces delay in the force trans- toads were misled by prey seen in mirrors and mission for producing movement. This stiff- the tongue never contacted a surface, the cen- ness greatly reduces any time lag due to initial tral rod always rotated 180", and the soft tis- compression of soft tissues. Secondly, these two sues rotated slightly more. The tongue never rigid elements act near the point of rotation continued to move and hit the ventral surface of the tongue. In a lever system, displacement of the throat. Clearly, the rod remains rigid as closer to the fulcrum is amplified at the tip of it passes the symphysis and mainly imparts the rotating arm. Consequently, elevation of propalinal momentum; its more ventral com- the rigid M. submentalis, acting against a sec- ponent is retained as the soft tissues slip over ond rigid element (M. genioglossus basalis), its surface. produces simultaneous and marked rotation of As the tongue straightens, so must its in- the distal tip of the rigid lingual rod and rap- ternal sinus and the initially folded M. hy- idly protrudes the tongue. oglossus; the fibers of this muscle come to lie The remaining soft tissues of the tongue, in a more nearly straight line once the tongue that were bunched anteriorly by the prepara- has been fully extended. Activity of the Mm. tory activity of the several muscles, are carried geniohyoideus lateralis and medialis during along by the rotational movement (Fig. 30. the flip would presumably shift the hyoid an- They are accelerated as the rod lifts out of the teriorly and subsequent activation of the M. buccal floor and tend to reach maximum ac- sternohyoideus would stablize the hyoid, thus celeration by the time they pass the symphy- reducing the need for extension of the M. hy- sial region. As the rod rotates farther (past 90") oglossus. As the tongue extends, the nerves its propalinal vector obviously becomes less, and blood vessels flanking the central sinus and the downward one increases. also straighten from their complexly curved Preparatory activity in the tongue has state. bunched it onto the tip of the rod, and the fluids in the central sinus have been shifted into the lingual base by action of the Mm. geniohyoidei; this soft mass which was carried along by the Mechanical synthesis of prey pick-up and lifting rod and is now propelled straight for- lingual retraction ward in a whiplash action due to the momen- If the tongue of the toad hits the prey ap- tum entrained in its soft tissues (Fig. 3D). propriately, the lingual pad impacts so that the As in a whip, the momentum is conserved medial groove deforms around it (Fig. 3E and and passes longitudinally through the soft tis- 4). The inertia of the lingual mass would tend sues. The soft tissues of the anterodorsal sur- to expel any air; good contact is then induced face of the tongue are apparently the first to and the mucous contents of some of the intrin- reach their tensile limit and stop in an ex- sic glands squeezed out. Adhesion of tongue tended position. The orientation of the lingual and prey during retraction involves several surface shifts as the symphysis is crossed and factors. The fluid between tongue and prey not the remaining portions of the inverted tongue only is sticky, so that it will wet the surfaces continue their forward travel. It is interesting contacted, but also has high viscosity and sur- that they travel in an almost completely face tension; thus, the bonds between tongue straight line that represents an anterior ex- and prey will be maintained during retraction, tension of the mandible and only slight depres- and no air will enter the cavity between tongue sion below the mandibular level. The direction and prey. The combination of stickiness and of the line of propulsion is indicated by the viscosity is clearly the critical item. Toads long anteriormost extent of the genioglossal rod. If have been known to have much more difficulty the tongue extends farther than one head- in ingesting moist objects, such as earthworms, length, one can see this rod well defined in than dry ones, such as beetles. The fluid-fluid high-speed films. The more anterior portions interface provides a zone of discontinuity along of the tongue will then be narrowed beyond the which separation occurs. The serous secretion zone of the M. genioglossus medialis up to the of the intermaxillary gland apparently does level at which the thickened lingual pad de- not participate in adhesion. fines the end of the extended tongue. While its The major mass of the M. hyoglossus consists momentum causes this pad to deform around of long fibers that insert as a series of fascicles the surface upon which it impacts, there is lit- on the center of the lingual pad. The M. hyo- tle ventrad deformation below the level of the glossus clearly initiates retraction, and it is TONGUE PROTRUSION IN MARINE TOADS 221 likely that there is a differentiation of its ac- instance, the muscular arrangement in the tivation time with the central fibers firing first. tongue of ranids and bunfonids is quite dis- Films of flips that did not achieve full contact tinct. of prey onto substrate show a marked dimpling These present observations also confirm of the extended surface. This dimpling further stimulation experiments indicating that the suggests that there may be some central suc- activity of the muscles attaching to the sym- tion-effect, rather than a reliance on the prop- physial region is sufficient to flip the tongue. erties of the fluid film. The detailed motor pat- It is then unnecessary to invoke various levels tern within the M. hyoglossus clearly deserves of hydrostatic mechanisms (Hartog, 1901a, b). more attention, as there is also the possibility Fluids contained in the basal lymph sac might of local feedback between sensors in the sur- be displaced into the tongue during the con- face that contacts the prey and the underlying traction of the muscles of the buccal floor. Once fascicles of this muscle. there, they might be entrained during rotation Lingual movements are asymmetrical, and and would then add to the mass of the tongue retraction is not an exact reversal of protrac- in the same way as does the sliding lead weight tion. During retraction, contraction of the M. in special billies. However, such a role would hyoglossus pulls the soft tissues of the ex- be essentially passive; it would not produce the tended tip posteriorly, while the basal rod re- flip, nor do any of the photographs indicate mains rigid (note the low-amplitude and du- that liquid is being returned. ration of activity of the M. genioglossus basalis Similarly, there is no evidence here for elas- and medialis during the beginning of retrac- tic recoil mechanisms involving the hyoid, in tion). Consequently, the portion of the tongue the version of either Gnanamuthu (1933) or holding the prey has been repositioned deep in Emerson (1977). It has been shown that the the buccal cavity, although much of the basal hyoid moves anteriorly during the flip and re- rod has not yet returned to the buccal cavity. turns to its resting position during retraction. Once the lingual pad passes the symphysis, This return movement may indeed have an activity in the M. genioglossus and in both the elastic component, although the retraction in- Mm. geniohyoideus lateralis and medialis volves action of the Mm. sternohyoideus and drops markedly. The basal mass starts to ro- omohyoideus. However, the model of Emerson tate, but then “melts” rather than rotating as (1977) is clearly invalid, as the M. sterno- a rigid element over the symphysial region. hyoideus shows only insignificant activity The difference in their insertions explains prior to, as well as during, the tongue flip; the difference in actions of the Mm. genioglos- whereas the M. geniohyoideus lateralis (its sus and hyoglossus. Contraction of the M. gen- antagonist) then shows high levels of activity. ioglossus distalis should mainly pull the ton- Thus the M. sternohyoideus does not retract gue to the symphysis, shortening and bunching and then release the hyoid at this time. it. The contraction of the M. hyoglossus would Furthermore, there is a fundamental diffi- act primarily and directly on the region of the culty with all propulsive mechanisms proposed extended tip of the tongue and only secondarily for the hyoid of Bufo and Rana (but not for upon its more basal portions. This action sug- Rhinophrynus, Trueb and Gans, 1981). The gests that it is the basal portion of the tongue hyoid is connected directly to the tongue only that would be lifted up and first rotated out of by the M. hyoglossus. Contraction of this mus- the mouth during protrusion, while the re- cle would draw the tongue posteriorly (and not versing action of the M. hyoglossus would in- bunch it as is seen prior to the flip). However, itially affect the extended tip during retrac- its relaxation could not induce forward mo- tion. mentum within the tongue unless (1) a second longitudinal muscle would then pull the ton- Previous explanations gue towards the symphysis, or (2) there were The preceding results confirm the view ex- a mass of loose tissue that could be accelerated pressed by Gaupp, Barclay, Magimel-Pellon- in order to transmit the momentum from hyoid ier, Tatarinov, Severtzov, and Gans (among to tongue. Neither condition is met. Also a re- others) that the tongue is propelled and re- coil mechanism would not explain the stiff- tracted directly by the action of intrinsic mus- ening and rotation of the basal portion of the cles, although the detailed mechanism devel- tongue, nor its lifting from the buccal floor. oped represents a variant of those previously Furthermore, the hyoid shift is much slower described. This may indeed reflect the diversity than the movement of the tongue. Finally, of species examined by these authors, as, for there is the observation that the tongue does 222 C. GANS AND G.C. GORNIAK not flip out and around the symphysis as a soft fur Physiologen, Ante und Studierende. F. Vieweg und mass, nor does it tend to continue ventrally to Sohn, Braunschweig. Emerson, S. 1977 Movement of the hyoid in frogs during contact the throat when prey is missed; rather feeding. Am. J. Anat., 149:115-120. one sees the firm skeleton of the rod rigidly Ewert, J. P., I. Speckhart and W. Amelang 1970 Visuelle extended toward the location of the prey. Inhibition und Exzitation im Beutefangverhalten der The anterior and then posterior movements Erdkrote Bufo bufo (1).Z. vergl. Physiol., 68:84-110. Ferdinand, L. i894.Z~Anatomie der Zunge. Eine verglei- of the hyoid, the observation of which appears chend-anatomische Studie. Munich. to have contributed substantially to the for- Fischl, E., and R. H. Kahn 1928 Untersuchungen an einem mation of the Emerson hypothesis, appear to Nerv-Muskel-Praparate zur Beobachtung einzelner quer- reflect different factors and perhaps involve a gestreiRer Muskelfasern. Pfliiger’s Arch. ges. Physiol., 219:33-46. different advantage. Hyoid movement just be- Fixsen, C. 1857 De Linguae Raninae Textura Disquisitiones fore and during the flip appears to be coinci- Microscopicae. Diss. inaug. Dorpati, Livonorum. dent with the contraction of the geniohyoid Francis, E. T. B. 1961 The sources and nature of salivary muscles that have (as proposed by Tatarinov, secretions in Amphibia. Proc. Zool. SOC. Lond., 136:453-478. 1957) a major, but not a unique, component in Fulton, G. P., and B. R. Lutz 1940 The neuro-motor mech- symphysial depression. However, its anterior anism in the small blood vessels of the frog. Science, movement both reduces tension on the soft tis- 92:223-224. sues during the flip (and thus allows full and Gans, C. 1961 The bullfrog and its prey. Nat. Hist., 70:26-37. most rapid extension) and reduces the length Gans, C. 1962 The tongue protrusion mechanism in Rana to which the hyoglossus will be stretched, and catesbeiana. Am. Zool., 2:524 (Abstract). with this improves its length-tension charac- Gaupp, E. 1896 A. Ecker’s und R. Wiedersheim’s Anatomie teristics. des Frosches. Erste Abteilung. Vieweg, Braunschweig. Gaupp, E. 1901 Ueber den Muskelmechanismus bei den ACKNOWLEDGMENTS Bewegungen der Froschzunge. Anat. Anz., 19:385-396. Gelfan, S. 1930 Studies on single muscle fibers. I. The all- We are grateful to Mr. Mervin F. Roberts for or-none principle. Am. J. Physiol., 93:1-8. use of the early photographs of tongue protru- Gnanamuthu, C. P. 1933 The anatomy ofthe tongue ofRana heradactyla. Rec. Ind. Mus., 35:125-144. sion (originally commissioned and reproduced Hartog, M. 1901a Sur le mecanisme de la propulsion de la in part by Natural History magazine); to Dr. langue chez les amphibiens anures. C. r. hebd. Acad. Sci. Max K. Hecht for loan of a Russian translation; Paris, 132:588-589. to Dr. C. Schubert for permission to examine Hartog, M. 1901b The mechanism of the protrusion of the tongue of the Anura. Preliminary note. Am. Mag. Nat. strobe photos of the flip in B. blombergi (Shub- Hist. 7:501-503. ert, 1974);to R. Thomas and J. See for technical Holmes, S. J. 1906 The Biology of the Frog. Macmillan & assistance; and to R. Marlow, R. A. Nussbaum, Co., New York. 0. M. Sokol and L. Trueb for comments on the Ingle, D. 1976 Behavior correlates of central visual function in Anurans. In: Frog Neurobiology. R. Llinas and W. manuscript. Supported by NSF grant DEB-80- Precht, eds. SpringerVerlag, Berlin, pp. 435-451. 03678 and DHEW grant PHS-G-IROIDEO5112- Klein, E. 1850 Beitrage zur Anatomie der ungeschwanzten 03. Batrachier. Jahrb. Verein. vaterl. Naturk. Wiirttemberg 6:l-84. LITERATURE CITED Magimel-Pellonier, 0. 1924 La Langue des Amphibiens (Anatomie et Ontogenie Comparees de la Forme et des Abbott, B. C., and A. J. Brady Amphibian muscle. In: 1964 Muscles). Thesis, Paris. Physiology of the Amphibia. J. A. Moore, ed. Academic Noble, G. K. The Biology of the Amphibia. McGraw- Press, New York, Vol. pp 1931 1, 329-369. Hill, New York. Alexander, T. R. Observation on the feeding behavior 1964 Pratt, F. H., and M. A. Reid A method for working on of Bufo marinus (Linne). Herpetologica, 20:255-259. 1930 Barclay, 0. R. 1942 Feeding of the common toad. Nature, the terminal nerve-muscle unit. Science, 72:431-433. Regal, P., and C. Gans 1977 Functional aspects of the 1513376. evolution of frog tongues. Evolution, Biesladecki, A. v., and A. Herzig 1859 Die verschiedenen 30:718-734. Formen der quergestreiften Muskelfasern. Sber. k. Akad. Schubert, C. 1974 Behabiger Riese mit schneller Zunge. Das Wiss. Wien., math. natunv. Kl., 33:146-149. Tier, 14:30-31, 68. Boker, H. 1937 Einfuhrung in die vergleichende Anatomie Severtzov, A. S. 1961 On the mechanism of tongue protru- der Wirbeltiere. Vol. 2 (Die Biologie der Ernahrung.). G. sion in anuran amphibians. Doklad. Akad. Nauk SSSR. Fischer, Jena. 140:(translation)256-259. Clark, R. D. 1974 Food habits oftoad, genus Bufo (Amphibia: Tatarinov, L. P. 1957 Tongue movement mechanism in tail- Bufonidae). Am. Midland Naturalist, 91:140-147. less Amphibia. Doklad. Akad. Nauk SSSR, 116r707-709. de Jongh, H. J., and C. Gans 1969 On the mechanisms of (Translation 916-918). respiration in the bullfrog Rana catesbeiana: A reassess- Trewavas, E. 1933 The hyoid and larynx of the Anura. Phil. ment. J. Morph., 127r259-297. Trans. Roy. SOC.London B, 222:401-527. Dug&, A. 1827 Rechhrches anatomiques et physiologiques Trueb, L., and C. Gans 1981 Tongue propulsion in Rhino- sur la deglutition des reptiles. Am. Sci Nat., 12t337-345. phrynus: A mechanism unique in frogs. Am. Zool 21:970. Dug&, A. 1834 Recherches sur l’osteologie et la myologie Waller, A. 1849 Minute structure of the papillae and nerves des batraciens A leurs differents bges. Mem. Acad. Roy. of the tongue of the frog and toad. Phil. Transact. London, Sci. Inst. France, Sci. Math. Phys., 6:l-216. 139. Dumeril, A. M. C., and G. Bibron 1841 Erpetologie generale Zug, G. R., and P. B. Zug 1979 The marine toad Bufo mar- et Histoire Naturelle des Reptiles. Roret, Paris, vol. 8. inusr Natural history resume of native populations. Ecker, A. 1882 Die Anatomie des Frosches. Ein Handbuch Smithsonian Contrib. Zool., No. 284, 58 pp.