60 THE BONES. 



against %yhich rests the maxillary condyle when this bone is drawn backwards ; it 

 is immediately above this eminence that the inferior orifice of the parieto-temporal 

 canal (mastoid foramen) opens. The external face of the zygomatic process is 

 smooth and convex ; the internal, concave, is also smooth, and bordered outwards 

 by the temporal fossa. Its anterior border is sharp and convex ; the posterior, very 

 short, is thick and roughened. Its summit is ilattened from before to behind, and 

 marked by notches on its two faces ; it somewhat resembles a wedge, fixed as it is 

 between the orbital process of the frontal bone and the zygoma ; it comes in contact 

 with the maxillary bone, and by a small portion of its anterior face, which is 

 deprived of notches, it concurs in circumscribing the orbital cavity. In the 

 domesticated animals, as in Man, the zygomatic process appears to arise from the 

 surface of the bone by two roots — one, the inferior or transverse, is represented by 

 the condyle ; the other, the siqjerior, forms a sharp crest which is continuous with 

 the anterior border of the process, and above, joins the lateral crest of the occipital 

 protuberance. 



The internal or cerebral face of the squamous portion is divided into two parts 

 by an almost vertical channel, which terminates above the supra-condyloid 

 process, and which, meeting a similar furrow on the parietal bone, forms the 

 parieto-temporcd canal. The superior portion is of small extent, and of a 

 triangular form ; it articulates by a simple harmonia suture with the external 

 face of the petrous portion. The inferior part, the widest, present in its middle 

 some cerebral impressions. For the remainder of its extent or circumference, it 

 is cut into a wide, dentated, and lamellar bevel, which brings it in contact with 

 the surrounding bones. 



Circumference. — This may be divided into tivo borders .- one, anterior, is 

 convex and united with the parietal and frontal bones ; the other, posterior, articu- 

 lates with the sphenoid in its inferior moiety, and is provided, above the level of 

 the supra-condyloid process, with a deep notch which receives the external 

 auditory canal. Superiorly, the two borders unite at the summit in a thin point, 

 which rests on the occipital bone. 



Structure. — The squamous portion of the temporal bone is formed of two 

 very thin compact plates, which have but little spongy tissue between them ; the 

 latter, however, is very abundant in the body of the zygomatic process. 



Development. — It is developed from a single nucleus of ossification. 



Petrous Portion. — This is one of the most interesting parts of the 

 skeleton for study, in consequence of its containing two systems of cavities which 

 enclose the essential organs of hearing. One of these systems is named the cavity 

 of the tympanun or middle ear ; the other forms the interned ear. These cavities 

 will be studied when we come to speak of the auditory apparatus. In the mean 

 time, only the external surface, and the structure and development of this portion 

 of the temporal bone, will be noticed. 



It is wedged between the antero-lateral border of the occipital bone, the 

 lateral border of the parietal, and the superior part of the internal face of the 

 temporal shell. It represents a quadrangular pyramid, the base of which is turned 

 downwards and a little backwards. It will be studied successively in four faces, 

 a summit, and base. 



Faces. — The anterior face is united by harmonia suture to the parietal bone. 

 The posterior face articulates in the same manner with the occipital bone. The 

 external face lies against the squamous portion of the bone. The internal face, 

 slightly concave and marked by very superficial digital impressions, forms a part 



