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Observations on Jacobson's Organ and Its Innervation in Vipera Berus by A

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Observations on Jacobson's Organ and Its Innervation in Vipera Berus by A 167 OBSERVATIONS ON JACOBSON'S ORGAN AND ITS INNERVATION IN VIPERA BERUS BY A. BELLAIRS INTEREST has been aroused in the Jacobson's organ of snakes by the recent experimental work which suggests that this structure is one of the most important sense organs which these animals possess. It was originally sug- gested by Broman (1920) that the bifurcated tongue of snakes could be flickered into the organ of Jacobson or its duct, situated in the roof of the mouth, carrying odorous particles whose nature could thus by appreciated. There is now considerable evidence that this organ plays an important part in the feeding reactions of snakes and of some lizards (Kahmann, 1932; Noble & Kumpf, 1936; Wilde, 1938), in trailing the female in courtship (Noble, 1937), and a less important part in identifying other members of the same species in aggregation behaviour (Noble & Clausen, 1936). At present it appears that no microscopic investigation of the innervation of Jacobson's organ, and the role (if any) of the nervus terminals, has yet been made in snakes. The following observations are based upon the examination of a series of transverse sections through the head of an adult Vipera berus, cut at 15,u, and stained according to the de Castro technique. THE ORGAN OF JACOBSON This organ is first seen in Amphibia, being present in Anura and Gymno- phiones; it is-interesting that the latter possess -a peculiar retractile facial tentacle, which, according to Noble (1931), can be brought into contact with the epithelium in the neighbourhood of the organ of Jacobson, paralleling the function of the tongue tips in snakes. There is some doubt as to the identity of the food-taster of Urodeles, some regarding it as true vomeronasal epithelium which has migrated from the medial to the lateral side of the nasal cavity, while others, admitting its functional similarity, hold that it is derived from the existing epithelium of the lateral nasal sinus. According to Francis (1934), the latter view is now more generally held. The organ is rudimentary in birds, crocodiles and some lizards, but is fairly well developed in other living reptiles, and relatively enormous in snakes. It is also present in most groups of mammals, and is most prominent in the mono- treme Ornithorhynchus; in fact among the vertebrates it probably reaches its greatest development in this animal and in the snakes. In our specimen of Vipera berus, Jacobson's organ, exclusive of its capsule, was about 1 mm. in antero-posterior length, and it can be seen as a black spot 168 A. -Bellairs beneath the epithelium of the roof of the mouth in preserved specimens. As in all snakes, the organ is completely closed off from the nasal cavity by its capsule, although, of course, it is continuous with it in the early stages of its development. The superior and lateral parts of the capsule are formed by the septomaxilla, while, medially and in front, the floor is formed by the paired ?I.e. t9,za. o.n. 0 ,~g Fig. 1. Transverse section ofthe left side of the head through the anterior part of Jacobson's organ. d.o.g. duct of great orbital gland. d.o.g.' duct of great orbital gland (upper part). d.o.J. duct of Jacobson's organ. ec.c. ectochoanal cartilage. gr. paramedian groove into which duct of Jacobson's organ opens. hy.c. hypochoanal cartilage. J.O. Jacobson's organ. (In Fig. 1 the epithelium is cut transversely, and much pigment is seen among the epithelitl cells). l.g. labial glands. mx. maxilla. na. nasal bone. na.g. nasal gland. n.x. nasal capsule cartilage. n.8. nasal septum. o.e. olfactory epithelium. p.f. prefrontal. smx. septomaXilla. v. vomer. vo.n. vomero- nasal nerve. V.br. branch of V nerve. vomer. Further back the bony floor is incomplete and the organ is supported by two cartilages. The more lateral of these is the ectochoanal cartilage (de Beer, 1937), while the medial, which projects into the lumen of the organ with its epithelial covering forming a concha-like prominence, represents the hypo- choanal cartilage as described by Peyer (1912) for Vipera aspis, and Brock Jacobson's organ in Vipera berus 169 (1929), for Leptodeira hotamboia. According to Broman (1920), pressure on the floor of the organ in feeding causes compression of the lumen which opens up again when the pressure is relaxed, sucking in fluid containing food particles from the mouth. The experiments of Wyilde (1938) upon Thamnophis have shown, however, that the mere presence of stimulating particles upon the palate is sufficient to excite the organ, without the application of any pressure to its floor. I Fig. 2. The same as Fig. 1, a few sections farther back, showing the duct of the organ, and the duct of the great orbital gland opening into it. Lettering as in Fig. 1. The paraseptal cartilages which participate in the capsule of the organ in lizards and Sphenodon, and form a complete cylindrical investment in Orni- thorhynchus, are absent in snakes. The organ of Jacobson opens to the mouth by a duct which passes almost directly downwards some distance in front of the posterior nasal opening. It opens immediately into a long groove which can be seen in preserved specimens on either side of the torus medialis; the groove itself extends forward for a millimetre or so in front of the opening of the duct, and is readily accessible to the tips of the tongue (Fig. 2). The duct itself is exceedingly narrow, with one or two blind diverticula opening into it as well as the duct of the orbital gland; 170 A. Bellairs the walls were actually contiguous throughout most of its length, and the opening into the lumen of Jacobson's organ on the lateral side of the vomer was only patent in one section, so it would seem doubtful if the tips of the tongue could be inserted into the lumen of the organ itself. According to Broman (1920), the duct in mammals never opens directly into the mouth, but communicates with it through the palatine duct. The great orbital gland ('Harderian' gland) No intrinsic glands associated with Jacobson's organ such as are present in mammals were found, although, according to Broman (1920), they appear in early embryos of Watrix, and later regress. The posterior part of the great orbital gland, which characteristically lies behind and outside the orbit, consists, in Vipera, of a small nodule lying under cover of the adductor mandibulae externus anterior muscle. It then passes forwards medial to the post-orbital bone and extends forwards beneath the optic nerve to reach the anterior boundary of the orbit, where the duct arises. This is at first greatly dilated, but later becomes narrower as it passes across the anterior margin of the prefrontal bone, and swings- medially beneath the capsule of Jacobson's organ. It finally opens into the medial side of the lower part of the duct of the organ, and this terminal part is also continuous posteriorly with the nasal cavity. / This gland varies greatly in size and shape among different snakes, but its relations are essentially similar. In all species it reaches the orbit by passing medially to the post-orbital bone, when this is present, and then divides in most species into two finger-shaped lobes which embrace the optic nerve. The lower of these is the longer, and passes forward to the front of the orbit. In the genus Vipera, the gland is relatively small and the superior process hardly developed, but in some species, notably the egg-eaters Dasypeltis and Elachis- todon, it is very large. Its size bears no relationship to the size of the eye, being fairly large in Typhlops (where the organ of Jacobson is said to be well developed) and other burrowing forms where the eyes are vestigial (Smith & Bellairs, unpublished observations). The great orbital gland is frequently referred to as the Harderian gland, but it is doubtful whether it is homologous with the Harderian gland of other vertebrates where it is developed from the medial wall of the conjunctiva and discharges its -secretion into the conjunctival sac, for the latter is absent in snakes. In lizards, a gland variously referred to as the lachrymal (Broman, 1920), or the Harderian (Schwartz-Karsten, 1937), is present, lying mainly in front of the orbit and communicating, according to Broman (192aY), with the duct of Jacobson's organ, as in snakes, and according to Schwartz-Karsten (1937) with the conjunctival space. The latter suggests that the orbital gland of snakes has been derived from the lacertilian condition by posterior migra- tion. Since this gland did not appear to discharge into the orbit in our Vipera it seems unlikely that it constitutes a true Harderian gland. Jacobs'on's organ in Vipera berus 171 True lachrymal glands lying lateral to the lid-angle and discharging into the conjunctival sac have been described in the lizards, Lacerta and Ophisops by Schwartz-Karsten (1937), but no vestige of these could be found in Vipera. The vomeronasal nerve This nerve has been described by McCotter (1917) in Rana catesbyana and Chrysemys marginate, and its course and relations appear to be essentially similar in both forms, though the entire vomeronasal apparatus is relatively better developed in reptiles. Fig. 3. Graphic reconstruction of right side of the orbito-nasal region. d.o.g. duct of orbital gland. d.o.J. duct of Jacobson's organ. o. boundary of orbit. o.b. olfactory bulb. p.n.o. posterior nasal opening. II. optic nerve.
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