THE SKIN OF THE DOMESTIC PIG* WILLIAM MONTAGNA, Pn.D. AND JEUNG S. YEN, M.S. Little is known about the anatomy or thehematoxylin and eosin, 0.05% toluidine blue physiology of porcine skin; the only pertinent(buffered at pH 4.5), the PAS technic, Verhoeff's stain, Giemsa stain, PTAH, HIll, Mallory's Azan modern studies are those of Schaffer (1) andtechnique and the DD reaction for —Sil and Kitamura (2), and Kurasumi and Kitamura (3)—S—S groups (l3arrnett and Seligman, 1952 (4)). on the glands of the so-called carpal organs. ThereAlkaline phosphatase, alpha naphthol esterase, is, furthermore, practically no data that sub-naphthol AS esterase (see Gomori (5)), tween esterase (Stowell and Lee, 1950 (6)) and eholines- stantiates the often repeated statement that theterases (Montagna and Ellis, 1957 (7)), were dem- skin of the pig is similar to that of man. Thisonstrated in frozen sections of tissues fixed in brief survey of the histology and histochemistry10% formalin. of the skin of the pig, therefore, should be a useful Succinic dehydrogenase (Farber and Louvierel, reference for those who plan to do further work1956 (5)), monoamino oxidase (Glenner, et at, 1957 (9)), and phosphorylase (Takeuchi and Kuri- on this animal. The pig, a highly specialized mam-aki, 1955 (10)) were studied in frozen sections of mal, with highly specialized habits, has manyunfixed tissues. local, topographic anatomical differentiations in its skin, which are beyond the scope of this study. GENERAL DESCRIPTION We have been more interested here in analyzing the general body skin, to allow us to draw some The skin of the pig has a remarkable number of focal specializations, some of these being asso- generalizations. The skin has a gross resemblance to that ofciated with rich glandular fields. The most signifi- man, particularly after the bristles have beencant specialization has occurred in the rhinarium, removed. Like man, the pig has a sparse cover ofor snout, which has become flattened and ex- ; the has a well-differentiated under- panded, and used to good advantage for routing. sculpture, the has a thick papillary bodyThe snout has the only surface that is compara- and a rich population of elastic fibers. These areble to the friction areas of most non-hooved mam- mals (palms and soles); it is different, however, in specious similarities, however, and it is apparent that in other respects the skin is very differenthaving over its surface widely spaced, short from that of man. vibrissac (Fig. 1). The snout has undergone ad- mirable adaptations: it has a very thick epidermis under which are many tactile endings, end- MATERIALS AND METhODS organs, and sinus hair follicles. Like the friction Specimeus of skin were taken from 6 adult sows surface of other mammals, the surface of the immediately after they had been slaughtered.snout is kept moist by the secretion of special Pieces were removed from the scalp, ear, ,scrous glands deep in the dermis. Midway be- lip, gular region, the entire snout, back, chest and carpal organs and mental organs. Small pieces oftween the bases of mandibles, is a round, raised tissue were fixed in Helly's fluid and T.C.A. andnevus-like structure called mandibular or mental embedded in paraffin. Sections were stained withorgan, which is composed of sebaceous and *Fromthe University of Oregon Medicalapocrine glands, and very coarse vibrissae. Proxi- School, Portland, Oregon, and the Oregon Re-mally on the ventral surface of the carpus, and gional Primate Research Center, Beaverton,extending to just between the two hind toes Oregon. This work was supported in part by grants from(numbers 2 and 3) is a row of large pores into the United States Public Health Service (RG-which open the duets of special serous glands 2125-Cl2), Colgate-Palmolive Co., and Revlon, Inc. clustered around the pores. Together, the glands Publication No. 24, Oregon Regional Primatecomprise the carpal organ. Schaffer (1) (1940), Research Center, Beaverton, Oregon, establishedKitamura (2) (1957) and Kurosumi and Kita- 1960 by grants FR 00163 (formerly R-5129-R-1) and FR 00170 (formerly RD 5690) from the Na-mura (3) (1958) have already described the tional Institutes of Health, U. S. Public Healthglands of the earpal organs, but we have some Service, Department of Health, Education anddetails to add to their observations. Welfare. Received for publication August 1, 1963. The surface of the skin of the pig is creased by 11 12 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY

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Fiu. 1. A whole snout from a freshly killed animal FIG. 2. Epidermis from the back, stained with hematoxylin and eosin THE SKIN OF THE DOMESTIC PIG 13 delicate intersecting sulci, which when shavedtum granulosum and the cells immediately above attain a superficial resemblance to the skin ofthem. The of the snout, the lips man. Though breeds vary in the profusion ofand between the toes abounds in enzymie activ- their , pigs have a moderate to sparse coat ofity. AS esterase activity, present in the malpighian hair, longer and coarser on the back than on thelayer, is absent from the stratum corneum. The belly. The hairs are shorter and more crowdedmost striking feature of the epidermis is the together in those areas where the expansion of thepresence of alkaline phosphatase in the lower skin surface is minimal, such as between theportion of the malpighian layer; the reaction toes, at the base of the ears, on the head, thefades in the upper cells of the spinous layer and is axillary and inguinal regions, etc. Short, coarseabsent from the stratum eorneum (Fig. 3). There vibrissae are widespread over the snout, theis some acetyleholinesterase but no butyryleho- muzzle, and on the upper and lower lips; theylinesterase in the malpighian layer. are much longer on the mental organ. The only outstanding feature of the melano- cytes, when these are present, is their very large The Epidermis size and the conspicuousness of their dendrites. Relatively thick everywhere, the epidermis has a conspicuous undersculpture of more or less The Dermis regularly alternating large and small ridges (Figs. A substantial papillary body consisting of fine 2, 3). It is thickest, and with a correspondinglycollagen fibers rests under the tortuous epidermis. more elaborate understructure, on the glabrousIn the thick reticular layer are woven coarse surface of the lips, on the snout and between thebundles of collagen fibers oriented largely hori- toes. A granular layer, better differentiated in thezontally. The cell population is sparse, and there epidermis between the toes, on the snout and theare no melanotie dermal melanocytes. Elastic glabrous surfaces of the lips, is occasionally dis-fibers are numerous everywhere. In the papillary continuous over the trunk. The stratum eorneumlayer delicate fibers branch toward the epidermis is deep and compact everywhere, and somewhatas they do in the skin of man. In the snout, where resembles that of the friction surfaces of the skinthe dermal ridges are extremely long, many elastic of other mammals. In preparations treated forfibers extend along with capillaries up to the —SR groups the entire lower two-thirds of thepapillary bed. Fine elastic fibers are wound stratum eorneum is reactive. Often even thearound the hair follicles. upper part shows some reactivity. On the snout The dermis has a moderate number of super- and lips the entire layer is intensely colored. Whenficial capillaries which form a plexus inside the treated for —S-—S groups, the entire eornealdermal ridges. The endothelium of these vessels, layer is consistently colored. however, has no alkaline phosphatase activity. The whole malpighian layer, but particularlyIn fact, the only alkaline phosphatase activity in the basal layer, stains well with basic dyes. Ex-capillaries is in those around the sweat cept in occasional isolatedfoei, there is no glyeo-gland, and occasionally some of those around the gen in the malpighian layer. There is always ahair follicles. In contrast, all capillaries have band of glyeogen in the upper cells of the mal-strong ATPase activity. pighian layer in the epidermis of the snout. The Only a few that contain acetyleholin- malpighian layer has pronounced suecinie de-esterase are present in the general body skin. In hydrogenase reactivity; a color reaction in thethe snout, eyelid, lower lip and gular region, stratum corneum may indicate the presence ofthere are numerous free nerve endings and end abundant —SH groups. A thin band of colororgans. Sausage-shaped and bead-shaped end immediately above the re-organs in the snout rest under the epidermal minds one of the sulfhydryl-rieh keratogenous zoneridges. In the snout, fine, free nerve-endings also in the epidermis of man (11). MAO reactivity,run against the long epidermal ridges, and seem weak in the malpighian layer, is strong in theto penetrate them (Fig. 4). stratum granulosum. Phosphorylase activity is In the gular skin and in the , skeletal barely detectable. Nowhere strong, alpha naph-muscle fibers rise to the papillary body from be- thol esterase activity extends throughout the mal-neath; these are encrusted by conspicuous end- pighian layer, with the best reaction in the stra-buttons that are strongly reactive for both buty- 4:

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FIG. 3. Epidermis Gf the snout with abundant alkaline phosphatase activity in the lower part of the malpighian layer. FIG. 4. Pleomorphic end organs just under the epidermis of the snout, and intraepidermal nerve fibers, both showing acetylcholinesterase activity. FIG. 5. Portion of a large cut longitudinally to show the prolongations of the dermal papilla into the bulb, and the dendritic melanocytes in the matrix. FIG. 6. Hair follicle cut longitudinally to show the aggregates of melanocytes in the matrix 14 THE SKIN OF THE DOMESTIC PIG 15 ryleholinesterase and acetyleholinesterase. Nervesalso scattered loosely in the lower part of the that contain eholinesterase are wrapped aroundmatrix (Figs. 5, 6). The population of melano- many of the blood vessels in the reticular layer. cytes within the matrix is neatly separated from that in the bulb. In heavily pigmented follicles, The Pilary System small dendritie melanoeytes are normally found The sparse pelage is composed of thick bristlesthroughout the outer . between which is a relatively small population of Quiescent follicles are about one-half the thin hairs almost as long as the bristles. These,length of the active ones. The relatively short however, do not form a true underhair coat. Evenhair germ may be central or eccentric. A thick fairly hirsute animals have a somewhat exposedepithelial capsule surrounds a very stout club. skin. There are no true glabrous surfaces other Histologically and histoehemically, the fol- than the labial borders. Even the snout, whichlicles are similar to those of most other mammals. appears to be glabrous, is riddled by short vibris-When stained with toluidine blue, the entire sae (Fig. 1). The surface of the snout extends tofollicular connective tissue, not including the the middle dorsal surface as a small, completelyglassy membrane, attains a metachromatie glabrous shield. Sinus hair follicles are found pre-color; the dermal papilla usually stains a meta- dictably only on the surface of the snout and inchromatic color. There is no metachromatie stain- the gular organ. ing inside the cells of the . The hairs may grow in groups of two or three,Glycogen, abundant throughout the outer root but are usually found singly. When in groups ofsheath, from the piliary canal to practically the two, one of the follicles is large, the other small;base of the bulb, is always also present in the when in three, the large hair is flanked by twoupper bulb, just below the keratogenous zone, small ones. The grouping is predictable onlyand in the cells of the of the inner root where the hairs grow close together, as for exam-sheath below the keratogenous zone. Glyeogen ple in the scalp and the skin between the toes.may be found in Huxley's layer, often far up in This suggests that the follicles, which originate inthe follicle. groups, become separated as the animal's surface The distribution of the various enzymes studied expands. On the jowls, as in all of the other ani-is similar to that which we have found in the mals that we have studied, there is no real group-follicles of other animals, except alkaline phos- ing, and most of the follicles grow singly. phatase. The dermal papilla of both quiescent The hairs are composed almost entirely of cor-and active follicles usually abounds in alkaline tex, with rarely an indication of a . Thephosphatase; the basal plate of active follicles is cuticle cells are very small and adhere close to thealso intensely reactive. The few capillaries in the shaft. In pigmented hairs, melanin is found onlydermal papilla have an endothelium rich in in the , the cuticle cells being nonpigmented.alkaline phosphatase, although those around the The hair follicles are stout and not very long.follicle do not. The cornified portions of the inner The bulb of the growing larger follicles restsroot sheath are reactive: thus, Henle's layer is either in the upper portion of the hypodermal fat,reactive down to the middle of the bulb, whereas or entirely within the lower part of the reticularisthe layer of Huxley does not become reactive dermis, seldom extending to its lower portion. until the level of the keratogenous zone. Tween Hair follicles are peculiar in having a thin outeresterase is distributed throughout the outer root root sheath and a thick . In verysheath from the wall of the pilary canal to the large active follicles, the dermal papilla has manymatrix. The arrectores pilorum muscles are also long, narrow prongs that continue upwards insidestrongly reactive for tween esterases. The entire the upper bulb (Fig. 5). Although some musclesouter root sheath, including the pilary canal and are attached to every follicle, the outer rootthe bulb has AS esterase. This is also heavily sheath has practically no bulge for their attach-concentrated at the bottom of the infundibulum, ment. In the follicles that have a large diameter,where the inner root sheath becomes dissipated. the bulb is wide, but relatively short. In pig- Except for some of those in the eyelids, hair mented follicles, large dendritie melanocytes re-follicles have no nerves around them which con- side not only the upper part of the bulb, wheretain eholinesterases. All sinus hair follicles, how- they are normally found in other animals, butever, have a certain number of nerves that contain 16 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY acetyleholinesterase, but the reaction is neversecretory cells contain no glycogen, but the dark very cleareut. cells are full of PAS-positive, non-glycogen granules (Fig. 11); none of the cells has particu- The Sebaceous Glands late or pigmented material. Many of the cells All of the animals that we have studied beinghave scattered evenly in the cytoplasm discrete females, the sebaceous glands were necessarilygranules that stain well only with Heidenhein's or somewhat small. The morphology of the glands iswith Regaud's hematoxylin (Fig. 12). The entire similar to that of the glands of other mammals.gland abounds in both suecinie dehydrogenase They have some glycogen only in the ducts; none(Fig. 10) and MAO. Weak phosphorylase activity is present in the glands. There is much succinicis present only in the secretory portions. The dehydrogenase and MAO reactivity mostly insecretory epithelium has alkaline phosphatase, the undifferentiated cells, and only barely de-alpha naphthol esterase, tween esterases, but no tectable phosphorylase activity. The peripheralAS esterase activity. Dark cells, but not clear cells cells have intense alkaline phosphatase activitymay have great quantities of acetyleholinesterase. but the sebum has none (Fig. 7). The per pheralThe glands are surrounded by acetyleholinester- cells have strong alpha naphthol esterase andase -reactive nerves (Fig. 13). There is no butyryl- AS esterase reactions, but the sebum has a weakeholinesterase. reactivity. Tween esterase is neatly localized in A striking feature of these histoehemieal prop- fat globules (Fig. 8); only the sebum has no reac-erties is that entire nests of glands may abound tion. The differentiating cells have some acetyl-in alkaline phosphatase and acetyleholinesterase, eholinesterase. Instead of meibomian glands, thewhereas others nearby may show only minimal pig has in the eyelids, typical, large, multilobu-amounts. Similarly, in some beds of tubules the lated sebaeeous glands. These have all of thesecretory cells have numerous stainable granules attributes described for the sebaceous glandswhereas in others there may be none. We have the elsewhere. Like the meibom.ian glands in otherimpression that this may reflect some phase of a mammals, these are surrounded by many acetyl-secretory cycle of these glands. The glands which eholinesterase-reactive nerves (Fig. 9). have no granules in them probably correspond to those poor in enzymes; they seem to contain The "Sweat Glands" in the Snout, the Lip and greater quantities of substances in their lumina. Car pal Organ The Apocrine "Sweat Glands" Between the short, widely-spaced vibrissae, the surface of the snout is punctuated by the coarse Generally distributed throughout the hairy orifices of the serous glands that lie deep in theskin, these are often in a one-to-one ratio with subdermal fat. Similar serous glands are alsohair follicles. There are no apoerine glands in the found on the lips and in linearly arranged aggre-glabrous surfaces of the lips and on the snout. gates on the hack of the earpus, comprising theThe ducts of the glands are arranged parallel to carpal organs. All of these glands are attached tothe hair, and open to the surface independently, the underside of the epidermis by stout, funnel-near the follicle orifices (Fig. 14). The long, like dilatations, and pass through the epidermisstraight duet extends from approximately the in a straight path. At the junction of the dermislevel of the bulb to the surface. The secretory with the hypodermal fat, each duct branchescoil rests in the lower portion of the dermis and in several times, and each branch terminates in athe hypodermal fat, below the level of the bulb secretory tubule, lined with a cuboidal or col-of active hair follicles. The size varies everywhere umnar epithelium; outside of the secretory cellson the body surface. Typically, the glands have is a layer of gigantic myoepithelial cells. Thea dilated secretory coil and a much attenuated cytoplasm of some of the secretory cells is weaklyduet. The cuboidal or columnar secretory epi- basophilic; that of other cells takes up basic dyesthelium may protrude in blebs into the lumen, or more avidly, and contains many basophilieit may be flattened. The myoepithelial cells granules of different sizes. Thus, there is a distinctoutside of the secretory cells are often larger than population of clear and dark cells. The dark cellsthe secretory cells themselves. have an oval dense nucleus; the nucleus of the Occasionally, large dendritic, melanotie melan- clear cells is spheroidal and lightly stained. Theocytes are insinuated between the secretory cells THE SKIN OF THE DOMESTIC PIG 17 "S.' e) r -, 44 &a fl.4s 'S • e 1 a' 't4uc 4 -I®

FIG.7. Portion of a , rich in alkaline phosphatase FIG. 8. Sebaceous gland with much tween esterase activity FIG. 9. Large sebaceous gland from the eyelid, surrounded by acetylcholinesterase-reactive nerves FIG. 10. "Eccrine" sweat glands from the snout, rich in succinic dehydrogenase 18 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY

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(Fig. 16). When stained with toluidine blue, the The secretory cells have no glycogen, but the secretory cells contain numerous basophilicducts do, especially in the luminal border. PAS- granules in the basal portion of the cytoplasmreactive, non-glycogen granules are more numer- and around the nuclei (Fig. 15). ous in the flattened cells of dilated tubules; these '•r, 1 W—WJI

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''C''' FIG. 13. Large mass of ecerine glands from the snout, surrounded by acetyleholinesterase-reactive nerves. The tubules in the lower half of the figure have some enzyme reactivity in their cells, but those in the upper part do not. FIG. 14. Duet of an apocrine , opening alongside a hair canal. Thick frozen section show- ing suceinie dehydrogenase reactivity. FIG. 15. Apocrine tubules with basophilie substances in the cytoplasm of the secretory cells; stained with toluidine blue. FIG. 16. Dendritie melanocytes in the secretory tubules of an apocrine gland 19 20 THE JOTJHNAL OF INVESTIGATIVE DEEMATOLOGY glands also contain a PAS-reactive substance inwhich have some resemblance to eecrine sweat their lumina. glands but they are different organs, being The histochemical properties are not unusual.branched tubular glands. These strange glands We have shown all of the enzymes that we haveembody the major features of both eeerine and found in the apoerine glands of other animals.apocrine glands. The epithelial cells, though The secretory, but not the myoepithelial cells,consisting of both clear and dark cells, as con- contain suceinie dehydrogenase and mono-firmed also by others (Kitamura, 1957 (2); Kuro- aminoxidase. There is practically no phosphoryl-sumi and Kitamura, (3) 1958), resemble more ase activity. The cells are reactive for all of theapoerine than eecrine cells. The cells contain various esterases tested. There is some alkalinepractically no glycogen, and are replete with rela- phosphatase in the luminal cells of the duet andtively large granules about one half a micron in some between the secretory cells. Only an occa-size. We have found such granules before only in sional capillary around the glands has an alkalinethe glands in the friction surfaces of the lorises. phosphatase-reactive endothelium. The glandsThe glands are surrounded by nerves that contain contain both butyryl- and aeetyleholinesteraseaeetylcholinesterase, which is correct for ecerine activity but no nerves containing these enzymesglands, but the incidence of such nerves around surround the glands. apoerine glands in man and in other animals are too numerous to list. The secretory segment of DISCUSSION these glands, thus, is one of the rare true inter- Since so much has been made of the similaritiesmediate glands that we have found in any animal. between porcine and skin, it is advisableThe apoerine sweat glands of the pig, like those to review below some of the similarities and dis-of most other mammals, open upon the surface, similarities that we have found. independent of pilary orifices, whereas those of In both animals the pelage is sparse, resultingnian open mostly inside the pilary canals (Mon- in a relatively thick epidermis. The surface oftagna, 1963 (12)). There are considerable differ- both skins is grooved by intersecting lines whichences between the pilosebaceous systems of the form characteristic geometric patterns. The un-two skins, as the details listed under the observa- derside of the epidermis is thrown into manytions show. folds which through resembling that of the epi- This study makes it clear that in spite of a dermis of man is simpler. The epidermis is thickfew similarities, the dissimilarities in morphologic in both animals, but in the pig the corneal layerand histochemical attributes of the skin of the pig remains compact, even in paraffin sections andand that of man are considerable. In the light of has an extensive keratogenous zone. The dermisthis, we should all reflect soberly in the future has a well-differentiated papillary body in bothbefore uttering again the fantasy that the skin of skins, but whereas the dermis of man is riddledthe pig resembles more that of man than that of throughout by an excessive blood supply, that ofany other mammal. To seek a skin similar to the pig has only moderate vaseularity. The endo-that of man, consideration should be given to the thelial tubes of the cutaneous capillaries, typicallyanthropoid primates, and particularly, the apes rich in phosphatase in man, have no such reaction(12). in the pig, except for a few around the apoerine sweat glands. One of the most striking resem- 5UMMARY blances between these two skins is the large con- The skin of the pig, while sharing some ana- tent of elastic tissue in the dermis. Whereas mantomical and histochemical features with that of has mostly ecerine sweat glands over the bodyman, is distinctly different. Here are some of the surface, the pig has only apoerine glands. Thesalient features of similarity and dissimilarity. pilo-sebaceous apparatuses of man are richly 1. The pelage is sparse in both skins, but the vascularized, but those of the pig are not. Thehairs are much coarser in the pig. epidermis and sebaeeous glands of the pig con- 2. The surface of both skins is carved by fine tain alkaline phosphatase, but those of man dointersecting lines that form characteristic pat- not. Nowhere has the pig eccrine sweat glandsterns. like those of man. On the snout and the lips and 3. The epidermis of both skins has an elaborate in the earpal organ, the pig has serous glandsunderstructure of ridges. The outstanding feature THE SKIN OF THE DOMESTIC PIG 21 of the epidermis of the pig is its high content of mcrkungcn Uber die Proktodualdruscn. Urban & Schwarzcnberg, Berlin. 1940. alkaline phosphatase, not found in any other 2. KITAMURA, T.: Histologischc und cytolo- animals studied so far. It also has a thick, very gischeuntersuchungen uber Karpalorgan compact stratum corncum. vomSchwein (in Japanese). Arch. Histol. Jap., 13:299,1957. 4. The dermis, thick in both skins, shares a 3. Kuxosnu, K. AND KITAMURA, T.: Occurrence well-defined papillary body and a large popula- offoldings of plasma membrane ($-mem- braue)in cells of pig's carpal organ as re- tion of elastic fibers. However, that of the pig is vealed by electron microscopy. Nature, poorly vascularized in contrast to that of man, 181:489, 1958. and the endothelium of the surface capillaries of 4. BARRNETT,R. J. AND SELIGMAN, A.M.: Histo- chemicaldemonstration of sulfhydryl and the pig contains no alkaline phosphatase. disulfidc groups ofprotein. J.Nat. Cancer 5. The hair follicles of the pig arc poorly vas- Inst., 14: 769, 1954. cularized; they form a hair which has practically 5.Coaioru, C.:Microscopic Histochcmistry. Chicago,Ill., University of Chicago Press, no medulla. The dermal papilla of the large 1952. follicles has numerous streamers that risc up into 6. STOwELL, R. E. AND Lxx, C. S.: Histochcmical the bulb. In active colored hair follicles, melano- studiesof the mouse liver after single feed- ingof carbon tetrachloride. Arch. Path. cytes are present not only in the bulb, above the (Chicago), 50: 519, 1950. critical level, but a special population of them is 7.MONTAGNA,W. AND ELLIs, R. A.: Histology and cytochemistry of . XII. also found in the matrix. The scbaceous glands, in Cholinesterases in thehairfollicles of the contrast with those of man, which have none, con- scalp. J. Invest. Derm., 29: 151, 1957. tain much alkaline phosphatasc; they have no 8. FARBER, E.ANDL0uvIRRE, D. C.: Histochem- ical localization of specific oxidative en- glycogen in them. zymes, IV. Soluble oxidation-reduction 6. Over the body surface the pig has only apo- dyes as aids in the histochemical localiza- crinc sweat glands which open directly upon the tion of oxidative enzymes with tctrazolium surface. salts. J. Histochcm. Cytochem., 4: 347, 1956. 9. GLENNER, G. G., BURTNER, H. J. AND BRowN, 7. "Eccrine" sweat glands arc found only on G. W., JR.: The histochemical demonstra- the snout, lips and "carpal organ." These arc not tion of monamine oxidase activity by tctra- like the cccrinc glands found in other animals; zolium salts. Ibid., 5: 591, 1957. 10. TARRTJcHI, T. AND KLTRIAKI, H.: Histochem- they are branched tubular glands with a secretory ical demonstration of phosphorylase in cpithclium composed of distinctive clear and dark animal tissues. J.Histochem. Cytochem., cells. 3:153, 1955. 11. MONTAGNA, W.: The Structure and Function REFERENCES ofSkin. New York, Academic Press, 1962. 1. SdHAFFER, J.: Die Hautdrusenorgane der 12.MONTAGNA, W.: The phylogenetic signifi- Saugetiere, mit besondere Bcriicksichtigung cance of the skin of man. Arch. Dcrm. ihres histologischem Aufbaues und Be- (Chicago), 88: 1, 1963.