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. The malar bone, and the os unguis or lachrymal, are more or less developed according to the species considered. With regard to the malar bone, it is most important to notice the part which it takes in the formation of the zygomatic arch, and that its inferior border contributes to form the crest to which is attached the masseter muscle. As for the nasal bones, they present differential characters which, as they affect the form of the region which they occupy, are worthy of notice. [Illustration: FIG. 63.--SKULL OF THE HORSE: LEFT LATERAL ASPECT. 1, Posterior surface of the occipital bone; 2, external occipital protuberance; 3, occipital condyle; 4, jugular process; 5, parietal bone; 6, frontal bone; 7, orbital cavity; 8, zygomatic process of the temporal bone; 9, external auditory canal; 10, mastoid process; 11, superior maxillary bone or _maxilla_; 12, intermaxillary or incisor bone; 13, nasal bone; 14, anterior orifice of the cavity of the nasal fossæ; 14´, malar bone; 15, lachrymal bone; 16, inferior maxillary bone or _mandible_; 17, inferior maxillary fissure; 18, condyle of the inferior maxillary bone; 19, coronoid process of the inferior maxillary bone; 20, incisor teeth; 21, canine teeth; 22, molar teeth.]