Behind the glenoid cavity is found the external auditory canal, and, further back still, the mastoid process. This latter, but slightly developed in the carnivora, a little more so in the ruminants, and still more in the horse, has its external surface traversed by a crest, the mastoid crest, which, after becoming blended with the antero-posterior root of the zygomatic process, proceeds with this latter to join the superior occipital curved line.
Below the auditory canal is situated a round prominence, highly developed in carnivora; this is the tympanic bulla, also called the mastoid protuberance; it is an appendage of the tympanum.
The Face
The bone of this region, around which all the others come to be grouped, is, as in man, the superior maxillary. The relations of this maxillary with the neighbouring bones is not exactly the same in all animals; for example, in the ox, sheep, and horse, in which the bones of the nose are wide in their upper part, and in which the lachrymal bone, which is very highly developed, encroaches on the face, the superior maxillary does not meet the frontal bone; it is separated from it by the above-named bones. It unites with it, on the other hand, in the dog and the cat. In the bear, it is separated from the bones of the nose by a small tongue of bone which springs from the anterior border of the frontal—a process which we have noticed in connection with this latter.
Fig. 62.—The Skull of the Ox: Left Lateral Aspect.
1, Occipital condyle; 2, jugular process; 3, parietal bone; 4, frontal bone; 5, osseous process, which serves to support the horn (horn-core); 6, orbital cavity; 7, external auditory canal, in front of which is found the zygomatic process; 8, temporal fossa; 9, superior maxillary bone; 10, intermaxillary or incisor bone; 11, nasal bone; 12, anterior orifice of the cavity of the nasal fossæ; 13, malar bone; 14, lachrymal bone; 15, inferior maxillary bone; 16, condyle of the inferior maxillary bone; 17, incisor teeth; 18, molar teeth.
In the pig, ox, sheep, and horse, the external surface is traversed, to a greater or less extent, by a crest which is situated on the prolongation of the inferior border of the malar bone. This crest, which is straight in the horse, but curved with its convexity upwards in the ox and the sheep, is known as the maxillary spine or the malar tuberosity: it gives attachment to the masseter muscle, and, in the horse, is distinctly visible under the skin. It does not exist in the carnivora. On the same surface is situated the sub-orbital foramen.
The inferior border is hollowed out into alveoli, in which are implanted the superior molar and canine teeth. This border is prolonged forwards from the alveolus, which corresponds to the first molar tooth, to terminate, after a course more or less prolonged, at the alveolus of the canine. This space, more or less considerably expanded, which thus separates these teeth is called the interdental space; but this denomination is not applicable to ruminants, because these latter possess neither canine nor incisor teeth in the upper jaw (see [p. 125], dentition of the ox and sheep). The superior maxillary bone of one side and that of the opposite side do not meet in the median line in the region which corresponds to the incisor teeth; they are separated by a bone which, in the human species, is present only at the commencement of life, and afterwards coalesces with the maxilla; this is the intermaxillary or incisor bone. This bone, which is paired, is formed of a central part, which bears the superior incisor teeth; it is prolonged upwards and backwards by two processes: one, external, which insinuates itself between the superior maxillary and the nasal bone, except in the sheep, in which it remains widely separated from the latter; the other, internal, which is united to that which belongs to the bone of the opposite side to form part of the floor of the cavity of the nasal fossæ; the external border of this process, which is separated from the body of the bone by a notch, forms the internal boundary of the corresponding incisor opening or the incisor slit. Owing to the absence of superior incisors in ruminants, the intermaxillary bone presents no alveoli.