TEGUMENTARY ORGANS. 



477 



embryonic state of all animals, and in the adult 

 condition of many of the lower forms, the in- 

 tegument, constituted as above defined, forms 

 a continuous investment over the surface of 

 the body without any important processes or 

 irregularities. Such is the case in many of 

 the Worms, Polypes, and lower Mollusca. 

 From such simple forms of integument as these 

 the most rudimentary kinds of appendages or 

 tegumentary organs are produced in one of 

 two wavs, either the outer portion of the 

 ecderon is thickened, and as a spine or as a 

 plate projects be\ond the common surface 

 e. g. cells of Hydroid and Polyzoic Polypes ; 

 or the whole integument is developed into a 

 spine-like or plate-shaped process, as in the 

 so-called " bracts " of the Diphydae, and in all 

 the spines, hairs, and scales of the Insecta, 

 Crustacea, and Araclmida. 



The shells and plates of Mollusca and Arti- 

 culata belong principally to the former division, 

 being simple laminated thickenings of theouter 

 portion of the ecderon. In the Vertebrata 

 the integument but rarely possesses appen- 

 dages of so simple a nature. Simple plates 

 of this kind, however, coat the surface of the 

 beaver's tail, in \\hich animal, according to 

 Heusinger, " the epidermis is divided by a 

 great number of clefts into hexagonal por- 

 tions 4 lines long, whose whole edges ad- 

 here to the cutis. They usually consist of a 

 couple of superimposed lamina? identical in 

 structure with the rest of the epidermis " (I. c. 

 p. 168.). The polygonal horny plates of the 

 Chelonia are of the same nature. The scales 

 on the under surface of the tail of the rat and 

 other rodents, and on the tarsi of birds, are 

 similarly constituted ; but here one edge is 

 thrown up, and we have a transition to the scales 

 of the Pangolin, to those of Ophidia and 

 Sauria, and to the nails, claws, hoofs, and 

 hollow horns of Mammals, and the horny 

 sheaths of the beak of Birds, all of which are 

 constructed on essentially the same plan, being 

 diverticula of the whole integument, the outer 

 laver of whose ecderon has undergone horny 

 metamorphosis. 



Among these the nails, horns, and hoofs of 

 mammalia present certain complexities of 

 arrangement which entitle them to particular 

 notice. 



Nails are flattened horny plates developed 

 from the upper surface of the phalangeal in- 

 tegument only ; they are free at their distal 

 extremities, but laterally and at their proximal 

 ends they are enclosed within raised ridges of 

 the whole integuments, the nail ivalls. The 

 enderon beneath them in the space which is 

 called the " bed of the nail " is raised into 

 parallel longitudinal ridges or laminae, which 

 fit into corresponding depressions of the under 

 surface of the ecderon. 



Clftws are nails which embrace a larger 

 portion of the phalanx, being developed, not 

 merely from its upper surface, but also from 

 its extremity, and extending far round on its 

 sides. In the dog and cat (fig. 305. A) the bed 

 of the claw is laminated as in man, but pre- 

 sents no papillae (Gurlt), and a bony plate 



extends from the last phalanx into the pos- 

 terior fold of the nail. 



The transition from the claw to the hoof is 

 readily understood if we suppose the terminal 

 portion of the former to be blunt and cylindri- 

 cal, instead of pointed and conical (fig. 305.). 

 The elephant and rhinoceros do in fact afford 

 an actual passage from the nail to the hoof, 

 inasmuch as their very flat nails are con- 

 tinuous at their edges with the solid horny 

 covering of the sole (Heusinger). 



The solipede hoof has been described in 

 the article SOLIPEDIA; we need therefore only 

 remark here that the wall corresponds with 

 the nail in man, and may, by maceration, be 

 separated from the sole and frog, which are 

 developed from the termination and pos- 

 terior surface of the phalanx. The ridge or 

 " bourrelet " at the upper margin of the wall an- 

 swers to the posterior nail-wall, and, as in the 

 nail, the horny upper layer of the "epidermis" 

 is continued on to the hoof from it. The struc- 

 ture of the bed of the hoof differs in its different 

 parts. That portion which corresponds with 

 the sole and frog merely presents papillae, 

 which fit into depressions of the horny ecde- 

 ron ; that which corresponds with the wall 

 is produced into lamellae like those of the 

 bed of the nail, so that the deep surface of 

 the wall is laminated. In addition, however, 

 long papillae extend from the "bourrelet" 

 through the superficial portion of the wall, so 

 that, on section, it presents a superficial series 

 of canals, around which the horny matter is 

 disposed in concentric layers. 



Fig. 305. 



A. Section of the foot in a kitten. B. In a fetal 

 lamb. 



Each half of the hoof of a ruminant (fg. 305. 

 B), or of the pig, corresponds in general struc- 

 ture with the entire hoof of a solipede, except 



