HORSE 825 of the lips, which man has availed himself of to introduce the bit, hy which this animal is subjugated to his uses; in the young animal there are also deciduous molars. The different species of equus, as the zebras and the asses, so resemble each other in outward form and internal economy that the description of the typical species, the horse, will answer for all, with the exception of a few structural pecu- liarities; they are so nearly related to each other that they will breed together, producing more or less fertile hybrids, as in the cases of the horse and ass, and both with the zebra, &c. The skull of the horse is remarkable for the great width between the orbits, its flatness, the length of the face compared with the crani- um, and the vertical depth of the lower jaw ; the intermaxillaries project considerably be- yond the nasal bones, the latter overhanging the cavity of the nostrils ; the temporal arch is short, straight, and situated in the posterior third of the skull. The cervical vertebrae are of large size, and the posterior are oblong with short processes, so as to secure great freedom of motion in the neck ; the dorsals are 18, with short transverse processes, and very long spi- nous anteriorly to afford origins for the liga- ment which supports the head ; the lumbar are six (but five in the ass), broad and firmly joined together, with remarkably well developed pro- cesses, especially the transverse ; the sacrum is a single bone, made up of five consolidated ver- tebras, in a continuous line with the rest of the spine, and united to the last lumbar by the very large articulating oblique processes of the latter, securing a springiness in this region in leaping and galloping ; the caudals vary from 17 to 21, having the form of vertebrae only in the upper ones. The chest is capacious, com- pressed laterally in front, and prolonged in ad- vance of the first rib so as somewhat to resem- ble the thorax of a bird ; in the middle and posterior portions it is rounded, and extends far back toward the pelvis; the ribs are 18 pairs, the anterior broad and massive (8 being true), and the posterior more slender. The clavicles are absent, and the coracoid process very rudimentary ; the shoulder blades are tri- angular, with a prominent spine, closely ap- proximated to the chest, transmitting the weight of this half of the body perpendicu- larly to the ground ; the arm bone is short and strong ; the forearm consists almost entirely of the greatly developed radius, the ulna be- ing a mere appendage consolidated in the adult animal to its posterior surface, though its ole- cranon process is of large size, affording a pow- erful purchase to the extensor muscles ; there are no movements of pronation and supination, but only of hinge-like flexion and extension. The carpus or wrist has seven bones in two rows, four in the upper and three in the low- er; the metacarpus consists of a single long bone, the shank or cannon bone, and of two smaller supplementary pieces ; this long bone represents the middle-finger metacarpal of the human hand, and the others the ring and fore- finger metacarpals, those of the thumb and little finger being absent. The fore foot is made up of three bones representing the tbree phalanges of a middle finger, called respective- ly the great and little pastern and coffin bones, the latter large and crescentic, supporting the hoof; there are also three sesamoid bones im- planted in the flexor tendon of the foot. The pelvis is remarkable for the elongation of tbe ilium and the outward extension of the crest and spine ; the thigh bone is massive, and so short that it is entirely concealed under the integuments of the trunk, what is commonly called the thigh being in reality the leg; the leg is formed almost entirely by the tibia, which is very strong at its upper portion, the fibula being a long slender bone among the muscles lost about the lower third of the tibia; the tarsus consists of six bones, the astragalus or cockal bone, the os calcis or heel bone, the cuboid, the navicular, and the middle and lesser cuneiform bones, the internal or great cunei- form being absent with the great toe which it supports ; the metatarsus and the hind foot are constituted as in the anterior limb, and the bones have received the same names. The muscular system of the horse is very different from that of man, and has been described mi- nutely in treatises on veterinary medicine. The panniculus carnosus, of which the platysma myoides of man is a rudiment, is greatly devel- oped and very movable, affording support and protection to various organs. The spinal mus- cles are of great extent and strength, especially in the neck and tail, which admit of much pre- cision and grace of motion ; the extensors of the forearm, the glutens mediw (the kicking muscle), and the muscles of the loins, extremi- ties, and neck are generally very powerful ; the muscles of the face, particularly those of the lips and nostrils, are largely developed, giving the well known variety of facial expression in this animal. The molar teeth of the horse may be known from those of other herbivora by the arrangement of the patches of enamel above re- ferred to, and by their great length before they divide into fangs. The incisors are close together in a circle at the end of the jaws, slightly curved, with long simple fangs ; the crowns are broad, thick, and short, of an elliptical form before they are much worn ; a fold of enamel pene- trates the crown like the inverted finger of a glove, which presents an island of enamel en- closing a cavity partly filled with cement and partly by the food; this is called the "mark," and is useful in determining the age of the ani- mal, disappearing in very old horses, whose teeth get worn below the penetrating fold ; accord- ing to Owen, it is usually obliterated in the middle incisors of the second set at the sixth year, and in the next and outer pairs in the seventh and eighth years respectively in the lower jaw, remaining longer in the upper, and in both its place is indicated for years by the darker color of the cement, even to the age of
