44 THE BONES. 



Development. The tuberous portion of the temporal bone is developed 

 from two principal centres of ossification which are consolidated at birth, and 

 which are often described as two distinct portions : the one as the petrous or 

 stony portion, the other as the mastoid portion. 



The faces, borders, summit and inner side of the base of the bone are 

 formed by the petrous part, which contains the cavities of the internal ear 

 and furnishes the inner wall of the middle ear. 



The mastoid portion constitutes almost entirely the base of the temporal 

 pyramid ; to it belongs the external auditory canal, the mastoid process, the 

 sheath of the hyoid prolongation, and the styloid process; it forms the 

 external wall and circumference of the case of the tympanum. 



For the tuberous portion of the temporal bone there are also two small 

 complementary nuclei : one for the vaginal process, whose base is united 

 to the petrous portion, and another forming the ring of the tympanum. 



Structure. The petrous portion is the hardest mass of bone in the 

 skeleton, and scarcely contains any spongy tissue, except at the centre of the 

 mastoid process ; in the mastoid portion it may be said not to exist. 



In the other domesticated animals, the tuberous portion of the temporal 

 bone is always consolidated with the squamous, and the summit of the 

 zygomatic process only articulates with the malar bone. 



BONES OF THE FACE. 



The face is much more extensive than the cranium in the majority of the 

 domesticated animals, and is composed of two jaws, a bony apparatus that serves 

 as a support to the passive organs of mastication the teeth. The superior 

 or anterior jaw, traversed in its entire length by the nasal cavities, is formed by 

 nineteen wide bones, only one of which, the vomer, is a single bone ; the 

 pairs are : the superior and intermaxillaries (or premaxillaries), the palate, 

 pterygoid, zygomatic, lachrymal, nasal, and superior and inferior turbinated bones. 

 Of these only four, the maxillaries, are intended for the implant ition of 

 the teeth ; the others form the union between the cranium and the superior 

 maxilla, or concur in the formation of- the nasal cavities. The lower jaw has 

 for its base a single bone, the inferior maxilla or maxillary bone. 



1. Great Supermaxilla, or Superior Maxillary Bone. 



This bone, the most extensive in the upper jaw, is situated on the 

 side of the fa.ce, and is bordered above by the frontal, palate, zygomatic, and 

 lachrymal bones ; below, by the premaxillary bones ; in front, by the nasal 

 bone ; behind and within, by that of the opposite side. It is elongated 

 vertically, is irregularly triangular, and exhibits two faces, two borders, and 

 two extremities. 



Faces. The external face, which is more convex in the young than the 

 old animal, presents : 1, On the level of the fourth and fifth molar teeth, a 

 vertically elongated ridge which is continued above with the inferior border 

 of the zygomatic bone ; this is the supermaxillary spine ; 2, The inferior 

 orifice of the supermaxillo-dental canal, or infra-orbital foramen. 



The internal face concurs in forming the external parietes of the 

 nasal cavities. We observe, above and in front, a deep, wide, and diverticu- 

 lated excavation, forming part of the maxillary sinus ; above and behind, a 

 surface roughened by fine lamellae and dentations to correspond with the 

 palate bone, and traversed from above to below by a fissure which forms, 

 in uniting with a similar fissure in the latter bone, the palatine canal. For 

 the remainder of its extent it is unequally smooth, covered by the membrane 



