There may be nineteen rib-bearing vertebrae, in which case five
only will be reckoned as belonging to the lumbar series. The
odontoid process of the axis is wide, flat, and hollowed above, as in the
ruminants. The bodies of the cervical vertebrae are elongated,
strongly keeled, and markedly opisthocoelous, or concave behind
and convex in front. The neural laminae are broad, the spines
almost obsolete, except in the seventh, and the transverse processes
not largely developed. In the trunk vertebrae the opisthocoelous
character of the centrum gradually diminishes. The spinous processes
of the anterior thoracic region are high and compressed. To
these is attached the powerful elastic ligament (ligamentum nuchae,
or “paxwax”) which, passing forwards in the middle line of the
neck above the neural arches of the cervical vertebrae—to which it
is also connected—is attached to the occiput and supports the weight
of the head. The transverse processes of the lumbar vertebrae are
long, flattened, and project horizontally outwards or slightly forward
from the arch. The metapophyses are moderately developed, and
there are no anapophyses. The caudal vertebrae, except those
quite at the base, are slender and cylindrical, without processes and
without chevron bones beneath. The ribs are eighteen or nineteen
in number on each side, flattened, and united to the sternum by
short, stout, tolerably well ossified sternal ribs. The sternum consists
of six pieces; the anterior or presternum is compressed and
projects forwards like the prow of a boat. The segments which
follow gradually widen, and the hinder part of the sternum is broad
and flat.
As in all other ungulates, there are no clavicles. The scapula is long and slender, the supra-scapular border being rounded, and slowly and imperfectly ossified. The spine is very slightly developed; rather above the middle its edge is thickened and somewhat turned backwards, but it gradually subsides at the lower extremity without forming any acromial process. The coracoid is a prominent rounded nodule. The humerus is stout and rather short. The ulna is rudimentary, being represented by little more than the olecranon. The shaft gradually tapers below and is firmly welded to the radius. The latter bone is of nearly equal width throughout. The three bones of the first row of the carpus (scaphoid, lunar and cuneiform) are subequal in size. The second row consists of a broad and flat magnum, supporting the great third metacarpal, having to its radial side the trapezoid, and to its ulnar side the unciform, which are both small, and articulate inferiorally with the rudimentary second and fourth metacarpals. The pisiform is large and prominent, flattened and curved; it articulates partly with the cuneiform and partly with the lower end of the radius. The large metacarpal is called in veterinary anatomy “cannon bone”; the small lateral metacarpals, which gradually taper towards their lower extremities, and lie in close contact with the large one, are called “splint bones.” The single digit consists of a moderate-sized proximal (os suffraginis, or large pastern), a short middle (os coronae, or small pastern), and a wide, semi-lunar, ungual phalanx (os pedis, or coffin bone). There is a pair of large nodular sesamoids behind the metacarpo-phalangeal articulation, and a single large transversely-extended sesamoid behind the joint between the second and third phalanx, called the “navicular bone.”
The carpal joint, corresponding to the wrist of man, is commonly called the “knee” of the horse, the joint between the metacarpal and the first phalanx the “fetlock,” that between the first and second phalanges the “pastern,” and that between the second and third phalanges the “coffin joint.”
In the hinder limb the femur is marked, as in other perissodactyles, by the presence of a “third trochanter,” a flattened process, curving forwards and arising from the outer side of the bone, about one-third of the distance from the upper end. The fibula is reduced to a mere rod-like rudiment of the upper end. The lower part is absent or completely fused with the tibia. The calcaneum has a long and compressed calcaneal process. The astragalus has a large flat articular surface in front for the navicular, and a small one for the cuboid. The navicular and the external cuneiform bones are broad and flat. The cuboid is small, and the internal and middle cuneiform bones are small and united together. The metapodals and phalanges resemble very closely those of the fore limb, but the principal metatarsal is more laterally compressed at its upper end than is the corresponding metacarpal. The joint between the femur and tibia, corresponding to the knee of man, is called the “stifle-joint”; that between the tibia and tarsus, corresponding to the ankle of man, the “hock.” The bones and joints of the foot have the same 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 pronation and supination, or opposition of digits, being possible or needed. The muscles therefore which perform these functions 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. 2), that of the anterior extensor of the phalanges (corresponding to the extensor communis digitorum of the arm and extensor longus digitorum of the foot of man) passes down over the metacarpal bone and phalanges, to be inserted mainly 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 digiti of man) receives a slip from that of the principal extensor, and is inserted into the first phalanx. In the hind-leg (where it is the homologue apparently of the peroneus brevis of man) the tendon becomes blended with that of the large extensor.
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| Fig. 2.—Section of Foot of Horse. |
| 1, | Metacarpal bone. |
| 2, | First phalanx (os suffraginis). |
| 3, | Second phalanx (os coronae). |
| 4, | Third or ungual phalanx (os pedis, or coffin bone). |
| 5, | One of the upper sesamoid bones. |
| 6, | Lower sesamoid or navicular bone. |
| 7, | Tendon of anterior extensor of the phalanges. |
| 8, | Tendon of superficial flexor (fl. perforatus). |
| 9, | 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. |
| 16, | Plantar cushion. |
| 17, | Hoof. |
| 18, | Fatty cushion of fetlock. |
A strong ligamentous band behind the metapodium, arising from near the upper extremity of its posterior surface, 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 the extensor tendon of the phalanges. This is called in veterinary anatomy the “suspensory ligament of the sesamoids,” or of the “fetlock” (10 in fig. 2); but its attachments 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, modified 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 (8) 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 phalange. 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 calcaneum. The perforating tendon is derived from the muscle corresponding with the long flexor of man, and the smaller tendon of the oblique flexor (tibialis porticus 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 complete and 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 away by friction. The terminal phalange 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 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 fatty plantar cushion in the median part. These structures are all enclosed in the middle subcorneous integument, a continuation of the ordinary skin of the limb, but extremely vascular, and having its superficial extent greatly increased by being developed into papillae or laminae. 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
