178 HOUSE ANATOMY. The joint between the femur and tibia, corresponding to the knee of man, is called the " stifle joint"; tint between the tibia and tarsus, corresponding to the ankle of man, is called the "hock." The bones and joints of the foot have the s ime names as in the fore limb. The horse is eminently " digitigrade," standing on the extremity of the single digit of each foot, which is kept habitually in a position approaching to vertical. The muscles of the limbs are modified from those of the ordinary mammalian type in accordance with the reduced condition of the bones and the simple requirements of flexion and extension of the joints, no such actions as pro- riitiou and supination, or opposition of digits, being possible or needed. The muscles therefore which perform these f unctions in other quadrupeds are absent or rudimentary. Below the carpal and tarsal joints, the fore and hind limbs correspond almost exactly in structure as well as function. On the anterior or extensor surface of the limb a powerful tendon (7 in fig. 5), that of the anterior extensor FIG. 5. Section of foot of liors-j. 1, metacarpal bone ; 2, first ph i mix (os suffra- ginis) ; 3, second phalanx (os corona) ; 4, third or ungual phalanx (os pedis,or coffin b >ne) ; 5, one of the upper sesa noid bones; fi, lower scsamoid or navi- cular bone; 7, tendon of anterior extensor of the phalanges; 8, tendon of snperfici.tl flexor (/. perforatus); 0, tendon of deep flexor (fl. perforans); 10, suspensory ligament of fetlock; 11, inferior or short sesamoid ligament; 12, derma or skin of the foot, covered with hair, and continued into 13, the coronary cushion, 14, the podophyllous or laminar membrane, and 15, the keratogenous membrane of the sole; 10, plantar cushion ; 17, hoof; 18, fatty cushion of fetlock. of tlie phalanges (corresponding to the extensor communis digitorum of the arm and extensor long us digitorum of the foot of man) passes down over the metacarpal bone and phalanges, to be inserted nuinly into the upper edge of the anterior surface of the last phalanx or pedal bone. There is also a much smaller second extensor on the outer side of this in each limb, the lateral extensor of the phalanges. In the fore leg the tendon of this muscle (which corresponds with the extensor minimi digitiol man) receives a slip from that of the principal extensor, and is inserted into the first phalanx. In the hini le^ (where it is the homologue apparently of the jxroneus brevis of man) the tendon becomes blende 1 with that of the large extensor. A very strong ligamentous band behind the metapodium, arising from near the upper extremity of its posterior svirface, divides into two at its lower end, and each division, being first connected with one of the paired upper sesamoid bones, passes by the side of the first phalanx to join tha extensor tendon of the phalanges. This is called in veterinary anatomy the " suspensory ligament of the sssamoids," or of the "fetlock " (10 in fig. 5); but its attach ments and relations, as well as the occasional presence of muscular fibres in its substance, show that it is the homologue of the interosseous muscles of other mammals, curiously modified both in structure and function, to suit the requirements of the horse s foot. Behind or superficial to this are placed the two strong tendons of the flexor muscles, the most superficial, or flexor perforatus (S), dividing to allow the other to pass through, and then inserted into the middle phalanx. The flexor perforans (9) is as usual inserted into the terminal phalanx. In the fore leg these muscles correspond with those similarly named in man. In the hind leg, the perforated tendon is a continuation of that of the plantaris, passing pulley-wise over the tuberosity of the os calcis. The perforating tendon is derived from the muscle corresponding v/ith the long flexor of man, and the small LT tendon of the oblique flexor (tibialis portions of man) is united with it. The hoof of the horse corresponds to the nail or claw of other mammals, but is so constructed as to form a com plete and very solid case to the expanded termination of the toe, giving a firm basis of support formed of a non- sensitive substance, which is continually renewed by the addition of material from within, as its surface wears a>;ay by friction against the ground. The terminal phalanx of the toe is greatly enlarged and modified in form to support this hoof, and the size of the internal framework of the foot is further increased by a pair of lateral fibro-cartilaginous masses attached on each side to the hinder edges of the bone, and by a fibro-cellular and adipose plantar cushion in the median part. These structures are all enclosed in the keratogenous membrane or " subcorneous integument," a continuation of the ordinary derma of the limb, but extremely vascular, and having its superficial extent greatly increased by being developed into papillae or laminje. From this the horny material which constitutes the hoof is exuded. A thickened ring encircling the upper part, called coronary cushion (13), and the sole (15), are covered with numerous thickly-set papillae or villi, and take the greatest share in the formation of the hoof ; the intermediate part con stituting the front and side of the foot (14), corresponding with the wall of the hoof, is covered with parallel, fine longitudinal lamina?, which fit into corresponding depres sions in the inner side of the horny hoof. The horny hoof is divided into a wall or crust consisting of the front and sides, the flattened or concave sole, and the frog, a triangular median prominence, notched posteriorly, with the apex turned forwards, situated in the hinder part of the sole. It is formed of pavement epithelial cells, which are mainly grouped in a concentric manner around the vascular papillae of the keratogenous membrane, so that a section near the base of the hoof, cut transversely to the long axis of these papillae, shows a number of small circular or oval orifices, with cells arranged concentrically round them. The nearer the surface of the hoof, or further re moved from the seat of growth, the more indistinct the structure becomes. Small round or oval plates of horny epithelium called " chestnuts," growing like the hoof from enlarged papillae of the skin, are found on the inner face of the fore arm, above the carpal joint in all species of EqiiiJce, and in the horse (E. caballus) similar formations occur near the upper extremity of the inner face of the metatarsus. Their use is unknown. Dentition. The dentition of the horse, when all the teeth are in place, is, as stated before, expressed by the formula i. -|, c. T , p. -, m. | = 44. The incisors of each jaw are placed in close contact, forming a semicircle. The crowns are broad, somewhat awl-shaped, arid of nearly equal size. They have all the great peculiarity, not found in the teeth of any other mammal, and only in the Eyiiiihe of com paratively recent geological periods, of an involution of the external surface of the tooth (see fig. (5), by which what should properly be the apex is carried deeply into the
interior of the crown, forming a fossa or pit, the bottom
