E. zebra (Fig. 161) is the smaller of the two (about 4 feet high at the shoulders), and has longer ears, a tail more scantily clothed with hair, and a shorter mane. The general ground colour is white, and the stripes are black; the lower part of the face is bright brown. With the exception of the abdomen and the inside of the thighs, the whole of the surface is covered with stripes, the legs having narrow transverse bars reaching quite to the hoofs, and the base of the tail being also barred. The outsides of the ears have a white tip and a broad black mark occupying the greater part of the surface, but are white at the base. Perhaps the most constant and obvious distinction between this species and the next is the arrangement of the stripes on the hinder part of the back, where there are a number of short transverse bands passing from the median longitudinal dorsal stripe towards, and sometimes joining with, the uppermost of the broad stripes which run obliquely across the haunch from the flanks towards the root of the tail. There is often a median longitudinal stripe under the chest.

E. burchelli (Fig. 162) is a rather larger and more robust animal, with smaller ears, a longer mane, and fuller tail. The general ground colour of the body is pale yellowish-brown, the limbs nearly white, the stripes dark brown or black. In the typical form they do not extend on to the limbs or the tail; but there is a great variation in this respect, even in animals of the same herd, some being striped quite down to the hoofs (this form has been named E. chapmani). There is a strongly marked median longitudinal ventral black stripe, to which the lower ends of the transverse side stripes are usually united, but the dorsal stripe (also strongly marked) is completely isolated in its posterior half, and the uppermost of the broad haunch stripes runs nearly parallel to it. A much larger proportion of the ears is white than in the other species. In the middle of the wide intervals between the broad black stripes of the flanks and haunches fainter stripes are generally seen.

E. grevyi.—Under this name a Zebra has been described which was sent in 1882 to Paris from the Galla country, lying to the south of Abyssinia, the most northern locality in which Zebras have previously been met with. In many of its characters it resembles E. zebra, but the stripes are much finer and more numerous than in the typical examples of that species, and it has a strong, black, and isolated dorsal stripe. Even allowing for the great variations that are met with in the markings of animals of this group, the aberrant characters of this individual are quite sufficient to separate it specifically from the true Zebra of South Africa. Other similar specimens have been recently brought from the Somali country.

The flesh of the Zebras is relished by the natives as food, and their hides are very valuable for leather. Although the many attempts that have been made to break in and train these animals for riding or driving have sometimes been rewarded with partial success, they have never been domesticated in the true sense of the word.

There are thus at least seven modifications of the Horse type at present existing, sufficiently distinct to be reckoned as species by all zoologists, and easily recognised by their external characters. They are, however, all so closely allied that each will, at least in a state of domestication or captivity, breed with perfect freedom with any of the others. Cases of cross breeds are recorded between the Horse and the Quagga, the Horse and Burchell’s Zebra, the Horse and the Hemionus or Asiatic wild Ass, the common Ass and the Zebra, the common Ass and Burchell’s Zebra, the common Ass and the Hemionus, the Hemionus and the Zebra, and the Hemionus and Burchell’s Zebra. The two species which are perhaps the farthest removed in general structure, the Horse and the Ass, produce, as is well known, hybrids or Mules, which in some qualities useful to man excel both their progenitors, and in some countries, and for certain kinds of work, are in greater requisition than either. Although occasional instances have been recorded of female Mules breeding with the males of one or other of the pure species, it is doubtful if any case has occurred of their breeding inter se, although the opportunities of doing so must have been great, as Mules have been reared in immense numbers for at least several thousands of years. We may therefore consider it settled that the different species of the group are now in that degree of physiological differentiation which enables them to produce offspring with each other, but does not permit of the progeny continuing the race, at all events unless reinforced by the aid of one of the pure forms.

The several members of the group show mental differences quite as striking as those exhibited by their external form, and more than perhaps might be expected from the similarity of their cerebral organisation. The patience of the Ass, the high spirit of the Horse, the obstinacy of the Mule, have long been proverbial. It is very remarkable that, out of so many species, two only should have shown any aptitude for domestication, and that these two should have been from time immemorial the universal and most useful companions and servants of man, while all the others remain in their native freedom to this day. It is, however, still a question whether this really arises from a different mental constitution causing a natural capacity for entering into relations with man, or whether it may not be owing to their having been brought gradually into this condition by long-continued and persevering efforts when the need of their services was keenly felt. It is quite possible that one reason why most of the attempts to add new species to the list of our domestic animals in modern times have ended in failure is that it does not answer to do so in cases in which existing species supply all the principal purposes to which the new ones might be put. It can hardly be expected that Zebras and Quaggas fresh from their native mountains and plains can be brought into competition as beasts of burden and draught with Horses and Asses, whose naturally useful qualities have been augmented by the training of thousands of generations of progenitors.

Not unfrequently instances occur of domestic Horses being produced with a small additional toe with complete hoof, usually on the inside of the principal toe, and, though far more rarely, three or more toes may be present. These malformations are often cited as instances of reversion to the condition of some of the earlier forms of equine animals previously mentioned. Such explanations, however plausible they appear at first sight, are nevertheless very doubtful. All the feet of polydactyle horses which we have examined bear little resemblance to those of Hipparion or Anchitherium, but look rather as if due to that tendency to reduplication of parts which occurs so frequently as a teratological condition, especially among domestic animals, and, whatever its origin, certainly cannot in many instances, as the cases of entire limbs superadded, or of six digits in man, be attributed to reversion.

Anatomy.—The anatomical structure of the Horse has been described in great detail in several works devoted to the subject, which will be mentioned in the bibliography, 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 work will only admit of the most salient points being indicated, particularly those in which the Horse differs from the 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 (Fig. 163) 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 very 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 postorbital 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 or malar 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 postorbital 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 the Rhinoceros and Tapir, and also from all of the Perissodactyles of the Eocene period. In front of the cerebral cavity, the great tubular nasal cavities are provided with well-developed turbinal bones, and are roofed over by very large nasals, broad behind, and ending in front in a narrow decurved point. The opening of the anterior nares is prolonged backwards on each side of the face between the nasals and the elongated slender premaxillæ. The latter expand in front, and are curved downwards to form the semicircular alveolar border supporting the large incisor teeth. The palate is narrow in the interval between the incisor and cheek-teeth, in which are situated the large anterior palatine foramina. Between the cheek-teeth it is broader, and it ends posteriorly in a rounded excavated border opposite the hinder edge of the penultimate molar. It is mainly formed by the maxillæ, 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 ankylosed to, the rough pterygoid plates of the alisphenoid, with no pterygoid fossa between. They slope very obliquely forwards, and end in curved, compressed, hamular processes. There is a distinct alisphenoid canal for the passage of the internal maxillary or main branch of the external carotid 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 very long paroccipital process. The periotic and tympanic are ankylosed 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 styliform process; and completely embraces the truncated cylindrical tympanohyal, which is of great size, in correspondence 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 basihyal 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. The mandible is largely developed, 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.

The vertebral column consists of seven cervical, eighteen dorsal, six lumbar, five sacral, and fifteen to eighteen caudal vertebræ. There may be nineteen rib-bearing vertebræ, in which case five only will be reckoned as belonging to the lumbar series. The odontoid process of the atlas is wide, flat, and hollowed above, as in the Ruminants. The bodies of the cervical vertebræ are elongated, strongly keeled, and markedly opisthocœlous, or concave behind and convex in front. Their neural laminæ are very broad, the spines almost obsolete, except in the seventh, and the transverse processes not largely developed. In the trunk vertebræ the opisthocœlous 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 nuchæ, or “paxwax,” which passing forwards in the middle line of the neck above the neural arches of the cervical vertebræ, to which it is also connected, is attached to the occiput and supports the weight of the head. The transverse processes of the lumbar vertebræ are long, flattened, and project horizontally outwards or slightly forwards from the arch. The metapophyses are moderately developed, and there are no anapophyses. The caudal vertebræ, 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 being extremely compressed, and projecting 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 suprascapular border is 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 process is a prominent rounded nodule. The humerus is stout and rather short, and has a double bicipital groove. The ulna is quite rudimentary, being only represented by little more than the olecranon. The shaft gradually tapers below, and is firmly ankylosed to the radius. The latter bone is of nearly equal width throughout. The three bones of the first row of the carpus (the scaphoid, lunar, and cuneiform) are subequal in size. The second row consists of a very 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 distally with the rudimentary second and fourth metacarpals. The pisiform is large and prominent, flattened, and curved; articulating 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 very short middle (os coronæ, or small pastern), and a wide, semilunar, 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.”[265]

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 hind limb the femur is marked, as in other Perissodactyles, by the presence of a “third trochanter,” a flattened process, curving forwards, 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 styliform rudiment of the upper end; its lower part being 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 very small one for the cuboid. The navicular and the external cuneiform bones are very broad and flat. The cuboid is small, and the internal and middle cuneiform bones are small and united together. The metapodials 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”; while that between the tibia and tarsus, corresponding to the ankle of man, is termed 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[266] 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 mammals 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. 164), 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.

A very 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. 164); 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 short flexor muscle 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 long 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 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 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 posticus 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 very solid case to the expanded termination of the toe, giving a firm basis of support formed of a nonsensitive substance, which is continually renewed by the addition of material from within as its surface wears away 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 papillæ or laminæ. 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 papillæ 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 laminæ, fitting 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 papillæ of the keratogenous membrane, so that a section near the base of the hoof, cut transversely to the long axis of these papillæ, 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 epidermis called “chestnuts,” growing like the hoof from enlarged papillæ of the skin, are found on the inner face of the fore limb, above the carpal joint, in all species of Equidæ, and in the Horse (E. caballus) alone similar formations occur near the upper extremity of the inner face of the metatarsus. Their use is unknown.

Behind the joint between the metapodium and the first phalanx is a prominence formed by the fatty cushion of the fetlock (18 in Fig. 164). On the middle of this is a small bare patch covered with thickened epidermis, the ergot or spur, generally concealed beneath the long hair which grows around it. This is the functionless vestige of the large callous pad found in this situation in the Tapir, and in fact in all mammals in which this part reaches the ground in walking.

Dentition.—The dentition of the Horse, when all the teeth are in place, is, as stated before, expressed by the formula i ³⁄₃, c ¹⁄₁, p ⁴⁄₃, m ³⁄₃ = 42. 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 living mammal, of an involution of the external surface of the tooth (see Fig. 165) forming a deep fossa or pit, the bottom of which becomes partially filled up 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, thus of course adding 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 extending only to a certain depth, it becomes obliterated as the crown wears away, when the tooth assumes the character 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 quite rudimentary or entirely 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, often, especially in the lower jaw, not developed at all, and generally fall by the time the animal attains maturity, so that there are but six functional grinding teeth—three that have predecessors in the milk-dentition, and hence are considered as premolars, and three true 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 (Fig. 158), 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, see Figs. 155, 167), 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. 157, b and c). 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 they much resemble the permanent teeth, 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 those of the lower jaw. The first teeth to 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 by the third molar, and finally by the third incisor. Of the permanent teeth the first true 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 true 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 state of the dentition, and for some time longer indications can be obtained from the wear of the incisor teeth, though this depends to a certain extent upon the hardness of the food or other accidental 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. The membrane that lines them and the cheeks is quite 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 inches, having a soft corrugated surface, and ending posteriorly in an arched border without uvula. This embraces the base of the epiglottis, and shuts off all communication between the cavity of the mouth and the nasal passages, respiration being, under ordinary circumstances, carried on 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 fauces is soft and corrugated. An elongated raised glandular mass, 3 inches long and 1 inch from above downwards, extending backwards from the root of the tongue along the side of the fauces, with openings on the surface leading into crypts with glandular walls, represents the tonsil. The tongue, corresponding to the general 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 but widens towards the evenly rounded apex. The dorsal surface generally is very soft and smooth. There are two large circumvallate papillæ near the base, rather irregular in form, about a quarter of an inch in diameter and half an inch apart. The conical papillæ are very small and close set, though longer and more filamentous on the intermolar portion. There are no fungiform papillæ on the dorsum, but a few not very conspicuous 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 upper or lower extremity. 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 mandibular ramus, 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 frænum linguæ. 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 frænum backwards, and 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 off. The interior is divided by the character of the lining membrane into two very distinct portions, right and left. Over the latter the dense white smooth epithelial lining of the œsophagus is continued, terminating abruptly by a raised crenellated border. Over the right part (rather the larger portion) the mucous membrane has a grayish-red colour and a velvety appearance, and contains very numerous peptic glands, which are wanting in the cardiac portion. The œsophageal orifice is very small, and is guarded by a strong crescentic or rather horse-shoe-like band of muscular fibres, which is supposed to be the cause of the difficulty of vomiting in the Horse. The small intestine is of great length (80 to 90 feet), its mucous membrane being covered with numerous fine villi. The cæcum is of conical form, about 2 feet long and nearly a foot in diameter; its walls are sacculated, especially near the base, having four longitudinal fibrous 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 (Fig. 166) is tolerably symmetrical in its 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 subdividing it into lobules. The extent of these varies, however, in different individuals, being not usually so marked as in the figure, which is from a fœtal specimen. The two lateral lobes are subtriangular in form. The Spigelian lobe is represented by a flat surface between the portal 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. In most works on the anatomy of the Horse this has been confounded with the Spigelian lobe of man. There is no gall-bladder (as in all other Perissodactyles), and the biliary duct enters the duodenum about 6 inches 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 duodenum with the bile-duct, and there is often a second small duct which opens separately near to this.

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 very 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 diverticulum or blind pouch (called “false nostril”) of a conical form, and curved, 2 to 3 inches 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, but its use is not apparent. It is longer in the Ass than in the Horse. A similar structure is found in the Rhinoceros, and in a much more developed condition in the Tapir. Here may be mentioned the guttural pouches, large air sacs, diverticula from the Eustachian tubes, and lying behind the upper part of the pharynx. These are likewise found in other Perissodactyles, but their use is also still not clearly understood. The larynx has the lateral sacculi well developed, though entirely concealed within the alæ of the thyroid cartilage. The trachea divides into two bronchi, one for each lung.

Nervous System.—The brain differs little, except in details of arrangement of convolutions, from that of other Ungulates. The cerebral 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 the ducts of the Harderian gland open.

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 vesiculæ 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 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 mammæ (as in other members of the suborder), 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.

Family Rhinocerotidæ.

Although the existing members of this family are readily distinguished from the other living representatives of the suborder by the simple crescentoid form assumed by the ridges of the lower cheek-teeth, yet it is exceedingly difficult to give a definition by which they can be distinguished from the Lophiodontidæ, from some members of which they are, indeed, probably derived. The outer columns of the upper molars (Fig. 167) are, however, so excessively flattened as to produce a continuous thick and nearly straight outer wall, which is often produced in advance of the anterior transverse ridge; both transverse ridges being but little curved, and intimately connected with the outer wall. The upper premolars are in most cases nearly or quite as complex as the molars, and the ridges of the lower cheek-teeth are crescentoid. The last lower molar has no third lobe. The height of the crowns of the cheek-teeth is variable. The skull is large, with the orbit confluent with the temporal fossa. There are either three or four digits in the manus, and three in the pes. One or more dermal horns are attached to the fronto-nasal region of the skull of existing forms, but these were wanting in some of the fossil species.