The plantar tubercle is larger than the preceding. It is of a more or less rounded form; sometimes it is triangular, and then comparable in outline to the ace of hearts, the point of which is, in this case, turned towards the claws; its margin being sometimes strongly indented, it may also have a trilobate form. It is on it that rest the metacarpo-phalangeal or metatarso-phalangeal articulations, according to the limb studied. The tubercle of the carpus, situated at the level of the posterior surface of this latter, is less important than the preceding, the region which it occupies not reaching the ground during walking. But it is not to be neglected from the point of view of external form, because of the prominence which it produces. [Illustration: FIG. 93.--CLAW OF THE DOG: INFERIOR SURFACE. 1, Horny lamina of the claw; 2, plantar nail; 3, tubercle of the corresponding digit.] [Illustration: FIG. 94.--LEFT HAND OF THE DOG: INFERIOR SURFACE, PLANTAR TUBERCLES. 1, 1, 1, 1, 1, Tubercles of the fingers; 2, plantar tubercle; 3, tubercle of the carpus.] In the ungulates the terminal extremity of the limb is, as we have above pointed out, enclosed in a horny envelope which is no other than the hoof. We will first study the hoof of the horse--a hoof which is single for each of the limbs, inasmuch as in this animal each of these has but a single digit. =Hoofs of the Solipeds.=--We will first study the hoof as regarded in a general way--that is, without taking into account the limb to which it belongs. We will afterwards point out the differences presented when the hoofs of the fore and hind limbs are compared. In connection with the external forms of the horse, the study which we are now commencing is of great importance. But, before entering upon it, it appears to us necessary to rapidly examine what the hoof contains (Fig. 95). [Illustration: FIG. 95.--VERTICAL ANTERO-POSTERIOR SECTION OF THE FOOT OF A HORSE. 1, Third phalanx; 2, fibro-cartilage; 3, podophyllous tissue; 4, inferior part of the wall; 5, section of the wall of the hoof; 6, cutigerous cavity; 7, tendon of the anterior extensor of the phalanges; 8, reinforcing band coming from the suspensory ligament of the fetlock; 9, tendon of the superficial flexor of the phalanges; 10, tendon of the deep flexor of the phalanges.] In the interior of this horny box we find the third phalanx, a small sesamoid bone placed opposite to the posterior border of the latter, a portion of the inferior extremity of the second phalanx, and the tendons, which terminate at this region.