176 HORSE [ANATOMY. 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 im mense numbers for at least several thousands of years. AVe 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 differ ences quite as striking as those exhibited by their external form, and more than perhaps might be expected from the similarity of their cerebral organization. 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 the extinct Hipparion or Anckitkerium, 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 which, whatever its origin, certainly cannot in many instances, as the cases of entire limbs super- added, 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 article will only admit of the most salient points being indicated, particularly those in which the horse differs from the other Ungulata. Unless other wise 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 com pared 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 communi cating 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 superior maxilla, giving support to the long row of molar teeth, and having a prominent ridge running forward from below the orbit for the attachment of the masseter muscle. The lacrymal occupies a considerable space on the flat surface of the cheek in front of the orbit, and below it the malar does the same. The latter sends a horizontal or slightly ascending process backwards below the orbit to join the FTJi FIG. 4. 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, malar bone ; L, lacrymal bone ; Fr, frontal bone ; >q, squamosal bone ; Pa, parietal bone ; oc, occipital condyle ; pp. paroecipital process : i 1 , z 2 . and t 3 , the three incisor teeth; r, the canine tooth ; pm^, the situation of the rudimentary first prrmolar, which has been lost in the lower, but is present in the upper jaw ; )im 2 . pin 3 , and pm*, the three fully-developed premolar teeth ; in 1 , in 2 , and m 3 , the three true molai teeth. 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 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 by 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 premaxillai. 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
