After the preceding remarks, our study will be found to fall into a natural division, and it is in the order which we have just followed for the purpose of indicating its existence that we now proceed to study the nature and form of the different elements which complete or protect the digital extremities of the thoracic and abdominal limbs.
Claws.—These horny coverings of the third phalanges, which we have to consider only in the dog and cat, may be compared with the nails of man, with which, however, they present, as is well understood, characteristic differences.
The claws are compressed laterally, curved on themselves, and are terminated in front by a sharp point in the felide, but more blunted in the dog. Their superior border is convex and thick. We may say, therefore, that a claw is a sort of hollow tube, in the form of a cone flattened in the transverse direction, in which the third phalanx is set, and which is itself set in a groove formed by a kind of osseous hood which occupies the base of this third phalanx (see [Fig. 37], p. 57).
Fig. 93.—Claw of the Dog: Inferior Surface.
1, Horny lamina of the claw; 2, plantar nail; 3, tubercle of the corresponding digit.
This definition is exact, as regards the general appearance; but, when more closely scrutinized, it is not sufficient. The tube in question is not formed of a single piece; each of the claws is formed by a lamina laterally folded, but of which the borders are not exactly joined together inferiorly; they leave between them a small interval, and this is filled by a layer of more friable horny substance, to which has been given the name of plantar nail. This arrangement, which is clearly defined in the dog ([Fig. 93]), is comparable to that which we shall afterwards meet with in connection with the sole of the hoof of the horse (see [Fig. 100], p. 257). In the dog and the cat, the weight of the limb resting on the inferior surface of the phalanges, it was necessary that the region of the plantar surface of the foot corresponding to these latter should be protected; this is the function of certain fibro-adipose pads, which are situated there, and which are designated by the name of plantar tubercles.
