ANATOMY

the anatomical structure of the horse has been described in detail in several works mentioned in the bibliography at the end of this section, though these have generally been written from the point of view of the veterinarian rather than of the comparative anatomist. the limits of the present article will only admit of the most salient points being indicated, particularly those in which the horse differs from other ungulata. unless otherwise specified, it must be understood that all that is stated here, although mostly derived from observation upon the horse, applies equally well to the other existing members of the group. _skeleton._--the skull as a whole is greatly elongated, chiefly in consequence of the immense size of the face as compared with the hinder or true cranial portion. the basal line of the cranium from the lower border of the foramen magnum to the incisor border of the palate is nearly straight. the orbit, of nearly circular form, though small in proportion to the size of the whole skull, is distinctly marked, being completely surrounded by a strong ring of bone with prominent edges. behind it, and freely communicating with it beneath the osseous bridge (the post-orbital process of the frontal) forming the boundary between them, is the small temporal fossa occupying the whole of the side of the cranium proper, and in front is the great flattened expanse of the "cheek," formed chiefly by the maxilla, giving support to the long row of cheek-teeth, and having a prominent ridge running forward from below the orbit for the attachment of the masseter muscle. the lachrymal occupies a considerable space on the flat surface of the cheek in front of the orbit, and below it the jugal does the same. the latter sends a horizontal or slightly ascending process backwards below the orbit to join the under surface of the zygomatic process of the squamosal, which is remarkably large, and instead of ending as usual behind the orbit, runs forwards to join the greatly developed post-orbital process of the frontal, and even forms part of the posterior and inferior boundary of the orbit, an arrangement not met with in other mammals. the closure of the orbit behind distinguishes the skull of the horse from that of its allies the rhinoceros and tapir, and also from all of the perissodactyles of the eocene period. in front of the brain cavity, the great tubular nasal cavities are provided with well-developed turbinal bones, and are roofed over by large nasals, broad behind, and ending in front in a narrow decurved point. the opening of the anterior nostrils is prolonged backwards on each side of the face between the nasals and the elongated slender premaxillae. the latter expand in front, and are curved downwards to form the semicircular alveolar border which supports the large incisor teeth. the palate is narrow in the interval between the incisor and molar teeth, in which are situated the large anterior palatine foramina. between the molar teeth it is broader, and it ends posteriorly in a rounded excavated border opposite the hinder border of the penultimate molar tooth. it is mainly formed by the maxillae, as the palatines are very narrow. the pterygoids are delicate slender slips of bone attached to the hinder border of the palatines, and supported externally by, and generally welded with, the rough pterygoid plates of the alisphenoid, with no pterygoid fossa between. they slope obliquely forwards, and end in curved, compressed, hamular processes. there is a distinct alisphenoid canal for the passage of the internal maxillary artery. the base of the cranium is long and narrow; the alisphenoid is very obliquely perforated by the foramen rotundum, but the foramen ovale is confluent with the large foramen lacerum medium behind. the glenoid surface for the articulation of the mandible is greatly extended transversely, concave from side to side, convex from before backwards in front, and hollow behind, and is bounded posteriorly at its inner part by a prominent post-glenoid process. the squamosal enters considerably into the formation of the temporal fossa, and, besides sending the zygomatic process forwards, it sends down behind the meatus auditorius a post-tympanic process which aids to hold in place the otherwise loose tympano-periotic bone. behind this the exoccipital gives off a long paroccipital process. the periotic and tympanic are welded together, but not with the squamosal. the former has a wide but shallow floccular fossa on its inner side, and sends backwards a considerable "pars mastoidea," which appears on the outer surface of the skull between the post-tympanic process of the squamosal and the exoccipital. the tympanic forms a tubular meatus auditorius externus directed outwards and slightly backwards. it is not dilated into a distinct bulla, but ends in front in a pointed rod-like process. it completely embraces the truncated cylindrical tympanohyal, which is of great size, corresponding with the large development of the whole anterior arch of the hyoid. this consists mainly of a long and compressed stylohyal, expanded at the upper end, where it sends off a triangular posterior process. the basi-hyal is remarkable for the long, median, pointed, compressed "glossohyal" process, which it sends forward from its anterior border into the base of the tongue. a similar but less developed process is found in the rhinoceros and tapir. the lower jaw is large, especially the region of the angle, which is expanded and flattened, giving great surface for the attachment of the masseter muscle. the condyle is greatly elevated above the alveolar border; its articular surface is very wide transversely, and narrow and convex from before backwards. the coronoid process is slender, straight, and inclined backwards. the horizontal ramus, long, straight, and compressed, gradually narrows towards the symphysis, where it expands laterally to form with the ankylosed opposite ramus the wide, semicircular, shallow alveolar border for the incisor teeth. [illustration: fig. 1.--side view of skull of horse, with the bone removed so as to expose the whole of the teeth. pmx, premaxilla. mx, maxilla. na, nasal bone. ma, jugal or malar bone. l, lacrymal bone. fr, frontal bone. sq, squamosal bone. pa, parietal bone. oc, occipital condyle. pp, paroccipital process. i^1, i^2, and i^3, the three incisor teeth. c, the canine tooth. pm^1, the situation of the rudimentary first premolar, which has been lost in the lower, but is present in the upper jaw. pm^2, pm^3, and pm^4, the three fully developed premolar teeth. m^1, m^2, and m^3, the three true molar teeth.] the vertebral column consists of seven cervical, eighteen dorsal, six lumbar, five sacral, and fifteen to eighteen caudal vertebrae 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. [illustration: 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 papillae or villi, and take the greatest share in the formation of the hoof; the intermediate part constituting the front and side of the foot (14), corresponding with the wall of the hoof, is covered with parallel, fine longitudinal laminae, which fit into corresponding depressions 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, mainly grouped in a concentric manner around the vascular papillae of the subcorneous integument, 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 farther removed from the seat of growth, the more indistinct the structure becomes. small round or oval plates of horny epithelium called "chestnuts," callosities 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 equidae, and in the horse (_e. caballus_) similar structures occur near the upper extremity of the inner face of the metatarsus. they are evidently rudimentary structures which it is suggested may represent glands (lydekker, _proc. zool. soc. london_, 1903, vol. i.). [illustration: fig. 3.--longitudinal and transverse section of upper incisor of horse. p, pulp cavity. d, dentine or ivory. e, enamel. c, outer layer of cementum or crusta petrosa. c', inner layer of cementum, lining a, the pit or cavity of the crown of the tooth.] _dentition._--the dentition of the horse, when all the teeth are in place, is expressed by the formula _i._ 3/3, _c._ 1/1, _p._ 4/4 _m._ 3/3 = 44. the incisors of each jaw are placed in close contact, forming a semicircle. the crowns are broad, somewhat awl-shaped, and of nearly equal size. they have all the great peculiarity, not found in the teeth of any other mammal, and only in the equidae of comparatively recent geological periods (see also palaeontology), of an involution of the external surface of the tooth (see fig. 3), by which what should properly be the apex is carried deeply into the interior of the crown, forming a pit, the bottom of which becomes partially filled with cement. as the tooth wears, the surface, besides the external enamel layer as in an ordinary simple tooth, shows in addition a second inner ring of the same hard substance surrounding the pit, which adds greatly to the efficiency of the tooth as an organ for biting tough, fibrous substances. this pit, generally filled in the living animal with particles of food, is conspicuous from its dark colour, and constitutes the "mark" by which the age of the horse is judged, as in consequence of its only extending to a certain depth in the crown it becomes obliterated as the latter wears away, and then the tooth assumes the character of that of an ordinary incisor, consisting only of a core of dentine, surrounded by the external enamel layer. it is not quite so deep in the lower as in the upper teeth. the canines are either rudimentary or absent in the female. in the male they are compressed, pointed, and smaller than the incisors, from which they are separated by a slight interval. the teeth of the cheek series are all in contact with each other, but separated from the canines by a considerable toothless space. the anterior premolars are quite rudimentary, sometimes not developed at all, and generally fall by the time the animal attains maturity, so that there are but six functional cheek teeth,--three that have predecessors in the milk-dentition, and hence are considered as premolars, and three molars, but otherwise, except the first and last of the series, not distinguishable in form or structure. these teeth in both upper and lower jaws are extremely long-crowned or hypsodont, successive portions being pushed out as the surface wears away, a process which continues until the animal becomes advanced in age. the enamelled surface is infolded in a complex manner (a modification of that found in other perissodactyles), the folds extending quite to the base of the crown, and the interstices being filled and the surface covered with a considerable mass of cement, which binds together and strengthens the whole tooth. as the teeth wear, the folded enamel, being harder than the other constituents, the dentine and cement, forms projecting ridges on the surface arranged in a definite pattern, which give it great efficiency as a grinding instrument (see fig. 2, in article equidae). the free surfaces of the upper teeth are quadrate, except the first and last, which are nearly triangular. the lower teeth are much narrower than the upper. the milk-dentition consists of i. 3/3, c. 0/0, m. 3/3 = 24,--the canines and first or rudimentary premolars having apparently no predecessors. in form and structure the milk-teeth much resemble the permanent ones, having the same characteristic enamel-foldings. their eruption commences a few days after birth, and is complete before the end of the first year, the upper teeth usually appearing somewhat earlier than the lower. the first teeth which appear are the first and second milk-molars (about five days), then the central incisor (from seven to ten days); this is followed by the second incisor (at one month), then the third molar, and finally the third incisor. of the permanent teeth the first molar appears a little after the end of the first year, followed by the second molar before the end of the second year. at about two and a half years the first premolar replaces its predecessor. between two and a half and three years the first incisor appears. at three years the second and third premolars, and the third molar have appeared, at from three and a half to four years the second incisor, at four to four and a half years the canine, and, finally, at five years, the third incisor, completing the permanent dentition. up to this period the age of the horse is clearly shown by the condition of dentition, and for some time longer indications can be obtained from the wear of the incisors, though this depends to a certain extent upon the hardness of the food or other circumstances. as a general rule, the depression caused by the infolding of the surface of the incisor (the "mark") is obliterated in the first or central incisor at six years, in the second at seven years, and in the third at eight years. in the upper teeth, as the depressions are deeper, this obliteration does not take place until about two years later. after this period no certain indications can be obtained of the age of the horse from the teeth. _digestive organs._--the lips are flexible and prehensile; and the membrane that lines them and the cheeks smooth. the palate is long and narrow; its mucous surface has seventeen pairs of not very sharply defined oblique ridges, extending as far back as the last molar tooth, beyond which the _velum palati_ extends for about 3 in., having a soft corrugated surface, and ending posteriorly in an arched border without a uvula. this embraces the base of the epiglottis, and, except while swallowing food, shuts off all communication between the cavity of the mouth and the pharynx, respiration being, under ordinary circumstances, exclusively through the nostrils. between the mucous membrane and the bone of the hard palate is a dense vascular and nervous plexus. the membrane lining the jaws is soft and corrugated. an elongated raised glandular mass, 3 in. long and 1 in. from above downwards, extending backwards from the root of the tongue along the side of the jaws, with openings on the surface leading into crypts with glandular walls, represents the tonsil. the tongue, corresponding to the form of the mouth, is long and narrow. it consists of a compressed intermolar portion with a flat upper surface, broad behind and becoming narrower in front, and of a depressed anterior part rather shorter than the former, which is narrow behind and widens towards the evenly rounded apex. the dorsal surface generally is soft and smooth. there are two large circumvallate papillae near the base, rather irregular in form, about a quarter of an inch in diameter and half an inch apart. the conical papillae are small and close set, though longer and more filamentous on the intermolar portion. there are no fungiform papillae on the dorsum, but a few inconspicuous ones scattered along the sides of the organ. of the salivary glands the parotid is by far the largest, elongated in the vertical direction, and narrower in the middle than at either end. its upper extremity embraces the lower surface of the cartilaginous ear-conch; its lower end reaches the level of the inferior margin of the mandible, along the posterior margin of which it is placed. its duct leaves the inferior anterior angle, at first descends a little, and runs forward under cover of the rounded inferior border of the lower jaw, then curves up along the anterior margin of the masseter muscle, becoming superficial, pierces the buccinator, and enters the mouth by a simple aperture opposite the middle of the crown of the third premolar tooth. it is not quite so thick as a goose-quill when distended, and nearly a foot in length. the submaxillary gland is of very similar texture to the last, but much smaller; it is placed deeper, and lies with its main axis horizontal. it is elongated and slender, and flattened from within outwards. its posterior end rests against the anterior surface of the transverse process of the atlas, from which it extends forwards and downwards, slightly curved, to beneath the ramus of the jaw. the duct which runs along its upper and internal border passes forwards in the usual course, lying in the inner side of the sublingual gland, to open on the outer surface of a distinct papilla, situated on the floor of the mouth, half an inch from the middle line, and midway between the lower incisor teeth and the attachment of the fraenum linguae. the sublingual is represented by a mass of glands lying just beneath the mucous membrane of the floor of the mouth on the side of the tongue, causing a distinct ridge, extending from the fraenum backwards, the numerous ducts opening separately along the summit of the ridge. the buccal glands are arranged in two rows parallel with the molar teeth. the upper ones are the largest, and are continuous anteriorly with the labial glands, the ducts of which open on the mucous membrane of the upper lip. the stomach of the horse is simple in its external form, with a largely developed right _cul de sac_, and is a good deal curved on itself, so that the cardiac and pyloric orifices are brought near together. the _antrum pyloricum_ is small and not very distinctly marked. the interior is divided by the character of the lining membrane into two distinct portions, right and left. over the latter the dense white smooth epithelial lining of the oesophagus is continued, terminating abruptly by a raised crenulated border. over the right part the mucous membrane has a greyish-red colour and a velvety appearance, and contains numerous peptic glands, which are wanting in the cardiac portion. the oesophageal orifice is small, and guarded by a strong crescentic or horseshoe-like band of muscular fibres, supposed to be the cause of the difficulty of vomiting in the horse. the small intestine is of great length (80 to 90 ft.), its mucous membrane being covered with numerous fine villi. the caecum is of conical form, about 2 ft. long and nearly a foot in diameter; its walls are sacculated, especially near the base, having four longitudinal muscular bands; and its capacity is about twice that of the stomach. it lies with its base near the lower part of the abdomen, and its apex directed towards the thorax. the colon is about one-third the length of the small intestine, and very capacious in the greater part of its course. as usual it may be divided into an ascending, transverse, and descending portion; but the middle or transverse portion is folded into a great loop, which descends as low as the pubis; so that the colon forms altogether four folds, generally parallel to the long axis of the body. the descending colon is much narrower than the rest, and not sacculated, and, being considerably longer than the distance it has to traverse, is thrown into numerous folds. the liver is tolerably symmetrical in general arrangement, being divided nearly equally into segments by a well-marked umbilical fissure. each segment is again divided by lateral fissures, which do not extend quite to the posterior border of the organ; of the central lobes thus cut off, the right is rather the larger, and has two fissures in its free border dividing it into lobules. the extent of these varies, however, in different individuals. the two lateral lobes are subtriangular in form. the spigelian lobe is represented by a flat surface between the postal fissure and the posterior border, not distinctly marked off from the left lateral by a fissure of the ductus venosus, as this vessel is buried deep in the hepatic substance, but the caudate lobe is distinct and tongue-shaped, its free apex reaching nearly to the border of the right lateral lobe. there is no gall-bladder, and the biliary duct enters the duodenum about 6 in. from the pylorus. the pancreas has two lobes or branches, a long one passing to the left and reaching the spleen, and a shorter right lobe. the principal duct enters the duedenum with the bile-duct, and there is often a second small duct opening separately. _circulatory and respiratory organs._--the heart has the form of a rather elongated and pointed cone. there is one anterior vena cava, formed by the union of the two jugular and two axillary veins. the aorta gives off a large branch (the anterior aorta) very near its origin, from which arise--first, the left axillary, and afterwards the right axillary and the two carotid arteries. under ordinary circumstances the horse breathes entirely by the nasal passages, the communication between the larynx and the mouth being closed by the velum palati. the nostrils are placed laterally, near the termination of the muzzle, and are large and dilatable, being bordered by cartilages upon which several muscles act. immediately within the opening of the nostril, the respiratory canal sends off on its upper and outer side a blind pouch ("false nostril") of conical form, and curved, 2 to 3 in. in depth, lying in the notch formed between the nasal and premaxillary bones. it is lined by mucous membrane continuous with that of the nasal passage; its use is not apparent. it is longer in the ass than in the horse. here may be mentioned the guttural pouches, large air-sacs from the eustachian tubes, and lying behind the upper part of the pharynx, the function of which is also not understood. the larynx has the lateral sacculi well developed, though entirely concealed within the alae of the thyroid cartilage. the trachea divides into two bronchi. _nervous system._--the brain differs little, except in details of arrangement of convolutions, from that of other ungulates. the hemispheres are rather elongated and subcylindrical, the olfactory lobes are large and project freely in front of the hemispheres, and the greater part of the cerebellum is uncovered. the eye is provided with a nictitating membrane or third eyelid, at the base of which open the ducts of the harderian gland. _reproductive system._--the testes are situated in a distinct sessile or slightly pedunculated scrotum, into which they descend from the sixth to the tenth month after birth. the accessory generative glands are the two vesiculae seminales, with the median third vesicle, or _uterus masculinus_, lying between them, the single bilobed prostate, and a pair of globular cowper's glands. the penis is very large, cylindrical, with a truncated, expanded, flattened termination. when in a state of repose it is retracted, by a muscle arising from the sacrum, within the prepuce, a cutaneous fold attached below the symphysis pubis. the uterus is bicornuate. the vagina is often partially divided by a membraneous septum or hymen. the teats are two, inguinally placed. the surface of the chorion is covered evenly with minute villi, constituting a diffuse non-deciduate placenta. the period of gestation is eleven months. authorities.--r. i. pocock, "the species and subspecies of zebras," _ann. mag. nat. hist._ ser. 6, vol. xx., 1897, and "a new arrangement of the existing species of equidae," op. cit. ser. 7, vol. x., 1902; r. lydekker, "notes on the specimens of wild asses in english collections," _novitates zoologicae_, vol. xi., 1904; b. salensky, "on equus przewalskii," _mem. acad. st petersburg_, 1902; m. s. arloing, "organisation du pied chez le cheval," _ann. sci. nat._, 1867, viii. 55-81; h. burmeister, _los caballos fosiles de la pampa argentina_ (buenos aires, 1875); chauveau and arloing, _traite d'anatomie comparee des animaux domestiques_ (paris, 1871), and english edition by g. fleming (1873); a. ecker, "das europaische wildpferd und dessen beziehungen zum domesticirten pferd," _globus_, bd. xxxiv. (brunswick, 1878); major forsyth, "beitrage zur geschichte der fossilen pferde besonders italiens," _abh. schw. pal. ges._ iv. 1-16, pt. iv.; george, "etudes zool. sur les hemiones et quelques autres especes chevalines," _ann. sci. nat._, 1869, xii. 5; e. f. gurlt, _anatomische abbildungen der haussaugethiere_ (1824), and _hand. der vergleich. anat. der haussaugethiere_ (2 vols., 1822); huet, "croisement des diverses especes du genre cheval," _nouv. archives du museum_, 2nd ser., tom. ii. p. 46, 1879; leisering, _atlas der anatomie des pferdes_ (leipzig, 1861); o. c. marsh, "notice of new equine mammals from the tertiary formation," _am. journ. of science and arts_, vol. vii., march 1874; _id._, "fossil horses in america," _amer. naturalist_, vol. viii., may 1874; _id._, "polydactyle horses," _am. journ. sci. and arts_, vol. xvii., june 1879; franz muller, _lehrbuch der anatomie des pferdes_ (vienna, 1853); r. owen, "equine remains in cavern of bruniquel," _phil. trans._ vol. clix., 1870, p. 535; w. percivall, _the anatomy of the horse_ (1832); g. stubbs, _anatomy of the horse_ (1766); w. h. flower, _the horse_ (london, 1891); ridgeway, _origin of the thoroughbred horse_ (1905). (w. h. f; r. l.*)