THE APPENDAGES OF THE SKIN. 921 



soft flexible horn at this point obviates tearing of the sole from the wall, and 

 fracture of the former. The cells of the horny laminae are more or less fusiform. 



The beautiful lateral leaflets on the sides of the horny laminte — corresponding 

 to those on the vascular leaves — were observed by me in 1858 ; in 1862 they 

 were described by Rawitsch and Ercolani, and, at a later period, by Colin of 

 Alfort and Leisering of Dresden. They are very conspicuous in a well-prepared 

 section — Fig. 496, c", c", d". 



It should be observed that the growth of the wall of the hoof is indefinite, 

 but that the sole and frog, after attaining a certain thickness, exfoliate. For 

 complete details as to the physiology of the Horse's foot, the student is referred 

 to the Veterinarian for 1871. 



It may here be noted that hairs and horn-tissue contain a substance named 

 keratin, which is remarkable for the large proportion of sulphui' existing in it in 

 a loose or free state.) 



2. The Claws of Rumikants and Pachyderms. 



In the Ox, Sheep, and Pig, the plantar cushion covers the bulb of the heel of each digit, 

 wliere it furms a convex mass ; it extends to the insertion of the deep flexor tendons of the 

 phalanges, in becoming triangular in shape, and thinner. 



In the Camel, the foot rests on the ground by the lower face of the two last phalanges. 

 These are protected by a double elastic cushion, but covered behind by a single horny sandal. 

 Tlie third phalanx is continued in a short and very convex claw, resembling somewhat a Dog's 

 claw. 



The keratogenous membrane has the same regions as in the Horse. The periople is separated 

 from the coronary cushion by a well-marked groove, and has very large papillae. The parietal 

 coronary cushion is wide and flat, of a brownish tint in its upper moiety, white iuferiorly ; it 

 blends behind with the velvety tissue of the plantar cushion, and ends in a point inwardly. 

 The papillae covering it are smaller than those of the periople and velvety tissue. The laminae 

 of the podophyllous tissue are nearer each other, and proportionately wider than in Solipeds; 

 they are furnished with somewhat atrophied laminellse. 



The liorny envelope enclosing the extremity of the digits of the Ox, Sheep, and Pig, 

 is a kind of copula, having almost the form of the third phalanx ; it is usually named the claw. 



The claw of the Ox has an outer face resembling the wall of the Horse's hoof, and an inner 

 face which is slightly concave, and marked by undulating grooves ; owing to this concavity 

 the two claws of each foot only touch at the extremities of their adjacent faces. The plantar 

 region of the claw is slightly depressed, and but little developed ; it is chiefly made up of the 

 plantar cushion, which is covered by a thin layer of supple tubular horn. On the interior of 

 the claw is seen a wide shallow cutigeral cavity, perforated by very fine openings, and laminae 

 thinner and more numerous than in the Horse's hoof. The tubes of the claw-horn are very 

 small, being surpassed in diameter by those of the periople and sole. 



Above and behind each claw are two little rudimentary horny capsules, which are named 

 ergots. They are absent in the Camel. (Each ergot contains a small bone, which is not 

 attached to the skeleton in Ruminants. In the Pig, these rudimentary claws are larger, and 

 are connected with the bones of the leg. In this animal the horn of the claws is altogether 

 thinner, softer, and less resisting than in Solipeds. The ergot is the representative of those 

 digits which are apparently absent in the solid and cloven foot.) 



3. The Claws of Caenivora. 



In these animals, the third phalanx of the digits is enveloped in a conical horny sheath 

 that curves downwards like the bone itself. This covering is designated the claw, or nail, and 

 off'ers somewhat the same organization as the horns of Ruminants ; it is developed, and grows 

 in the same manner, as the hoof of Solipeds, its matrix being a prolongation of the corium 

 which extends over the third phalanx, after dipping into the circular furrow at the base of that 

 bone. 



Placed at the extremity of the digital region, the claw in these animals is not utilized in 

 locomotion, as the foot does not rest on the ground by the extremities of the digits, but by the 

 whole plantar surface. Therefore it is, that we find on this face a kind of epidermic sandal 

 covering five fibro-adipose tubercles— four small ones placed along the four principal digits 



