406 THE HORSE OF AN AU IN ITS VARIOUS RELATIONS.
Frank states chiefly that the plication of the enamel-margin in the upper molars of the ass is less complex than in the Oriental horse, and that the so-called spur is here wholly wanting. He considers the best characteristic to be the fact that the distance from the anterior margin of the foramen occipitale to the median point of the vomer incision is shorter in the ass than the distance from the same point on the vomer to the end of the palatine suture. In the horse this dimension. is much greater.
Nehring also agrees with Frank as to the great value of this characteristic.
Dugés calls attention to the greater convexity of the forehead of the ass. According to him the face is shorter in comparison with the horse, and the orbits triangular. A perpendicular to the plane of support falls far behind the condyli of the occiput. The free part of the nasalia reaches to the posterior edge of the corpus maxillare and is therefore very long in the ass; then the spur is wanting on the teeth. In the horse, on the other hand, according to him, the forehead is flat, the orbits round and the occipital line touches the condyli. The free part of the nasalia does not reach to the middle of the incisive edge. The " spur " is characteristic of the teeth.
Monfalet gives nothing on the characteristics of the face, but only on those of the brain-skull.
The most accurate work, especially as regards dentition, is that of X. Lesbre. He is the first to distinguish between the teeth of the adult animal and of the young. He finds (p. 6o) that in the horse, at the age of ten months, the " spur, " which he calls " pli cabalin," is already clearly developed. In the young ass this is always wanting.
Lesbre confirms a shortening of the teeth of the adult ass through a kind of atrophy of the posterior pillar, but he considers the disposition of the enamel plications more important. (I) In the upper molars the internal lobule is less developed in the ass than in the horse, all proportions remaining the same; it is, in the first place, shorter towards the rear, so that its base stands median or almost median, and not, as in the horse, on the forward part of the tooth. In the first molar the plication is round in both animals, only in the ass it is less obliquely inclined toward the back than in the horse. (2) The exterior sides of the tooth seen from the grinding surface are narrow and simple in the ass, broad and compressed in the middle in the horse, especially the premolars. (3) The " spur " or pli cabalin is wanting in the ass at all ages, or is very inconspicuous, while in the horse it is often double, and disappears only at an advanced age, and earlier in the molars than in the premolars. (4) The crescentic islands are simpler in the ass than in the horse, less plicated and complex, but they often vary. (5) The outer channels are not so deep in the ass as in the horse. On the molars of the lower jaw Lesbre finds that the 8 formed by plications I and 2, has both of its loops generally round in the ass, and usually somewhat flattened and angular in the horse. In the ass both the loops of the 8 are equally large; in the horse the forward one is longer than the posterior, and they are separated by a sharp angle instead of by a curve. An exception is the first premolar, in which the posterior
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