Nails are flattened horny plates developed from the upper surface of the phalangeal in tegument only ; they are fi.ee at their distal extremities, but laterally and at their proximal ends they are enclosed within raised ridges of the whole integuments, the nail walls. The enderon beneath them in the space which is called the " bed of the nail" is raised into parallel longitudinal ritlges or laminw, vi hich fit into corresponding depressions of the under surface of the ecderon.
aims 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 papillx (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 Cornier 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 sw ers 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 papillm, which fit into depressions of the horny ecde ron ; that which corresponds with the wall is produced into laniellw like those of the bed of the nail, so that the deep surface of the wall is laminated. In addition, however, long papillm 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.
Each half of the hoof of a ruminant (fig. 305. B), or of the pig, corresponds in general struc ture with the entire hoof of a solipede, except that the frog is rudimentary. The horny ecderon presents both tubuli and larnium.
The excrescences on the inner surface of the leg of the horse are identical with the sole of the foot in structure — consisting of a horny mass penetrated by long papillm.
The hollow horns of the Ruminantia are, to all intents and purposes, Claws. The super ficial cellular ecderon (epidermis) is continued upon them, and, when this is removed, we come to a laminated fibrous horny mass, which is formed and increased by apposition from the subjacent process of the enderon, supported by its bony axis — a process of the frontal bone. The enderon has neither villi nor latnellm, presenting only small irregular ridges (Gurlt).
The horn of the rhinoceros is commonly said to be constituted by a mass of hairs which have coalesced. However, it consists of an aggregation of tubes, round which the horny 'natter is arranged in concentric laminm, as in the horny excrescence of the horse's leg; and as there is no evidence of its having ever been enclosed within a sae, it is more probable that it belongs to the series of the claws and nails.
Glands, hairs, and feathers. — The Hairs and Spines of mammals, the Feathers of birds, and the Integumentary Glands agree in one essential point, that their development is preceded by that of an involution of the ecderon, within which they are formed, and by which the former are, at first, entirely en closed.
At an early period, the rudiments of the hairs, and those of the cutaneous glands of a fmtal mammal, are indistinguishable. They alike consist of solid processes of the ecderon, consisting of a homogeneous matrix, in which lie closely-set endoplasts, bounded internally by a clear, narrow, transparent " basement membrane," which at once sepa rates them from, and connects them with, the enderon.* Externally these processes are con tinuous with the rete mucosum of the ecderon. In the fcetal lamb, in which I have carefully traced the development of these processes, they increase in size without change of struc ture, until, in the ordinary hairs, they have attained a length of T*77 inch ; for the vi brissm, that of 7,1,T, inch. Having reached this length, it is seen that an accumulation of the indifferent tissue of the enderon has taken place around their ccecal ends, which gradually become pushed in, so that, from being rounded, they appear truncated in section, and present a bulb with a hemisphe rical involution, the rudiment of the papilla,. In the ordinary hair no special accumulation of indifferent tissue takes place around the body of the involution; but in the vibrissae, which are ultimately to possess a thick outer capsule, its foundation appears in this form, and a capillary loop may be seen penetrating the rudimentary papilla.