THE HEAD. 61 



of the lateral wall of the cerebellar cavity. It presents the canal or internal audi- 

 tory hiatus {rneatus auditorius internus), a small fossa, the bottom of which is 

 pierced by several foramina for the transmission of nerves ; the largest of these 

 is the internal orifice of the aqimdudus Fallopii — a flexuous canal which passes 

 through the bone and opens at the external surface of its base ; the other 

 foramina penetrate the cavities of the internal ear. 



These faces are separated from each other by so many borders or plane angles, 

 two of which more particularly merit attention ; one of these isolates the external 

 from the posterior face, and the other separates the anterior from the internal 

 face. The first is thick and rugged, and constitutes the mastoid crest ; it is con- 

 tinuous above with the lateral ridge of the occipital bone, after being united to 

 the superior root of the zygomatic process, and terminates, near the base of the 

 bone, by a tuberosity for muscular insertion, to which has been given the name 

 of (poster io7-) mastoid process. This border is traversed by a slit, the mastoid fissure^ 

 which passes under the squamous portion and enters the parieto-temporal canal. 

 The second is thin, and, with the superior part of the lateral border of the parietal 

 bone, forms the crest which establishes the line of demarcation between the 

 cerebral and cerebellar cavities of the cranium ; it gives attachment to the 

 tentorium cerebelli. 



Summit. — This is slightly denticulated, and articulates with the occipital bone. 



Base. — This is very irregular, and offers : outwardly, the external auditory 

 canal which penetrates the middle ear, and the external orifice of which has been 

 named in veterinary anatomy the external auditory hiatus ; inwardly, a sharp 

 crest which circumscribes the external contour of the foramen lacerum basis cranii ; 

 above, and under the mastoid process, the stylo-mastoid or pre-mastoid forammi, the 

 external orifice of the aqueduct of Fallopius ; below, the subuliform (or styloid) 

 process for the attachment of the tensor palati muscle and the Eustachian tube : 

 this is a long, thin, and pointed process presenting, at its base and within, a canal 

 (styloid foramen) which enters the cavity of the tympanum, and which is incom- 

 pletely partitioned by a small bony plate into two parallel portions ; in the centre, 

 the hyoid prolongation or vaginal process ^ — a little cylindrical eminence surrounded 

 by a bony sheath, and the mastoid protuberance or auditory bulla — a slightly sahent, 

 smooth, and round eminence hollowed internally by numerous cells, which form 

 part of the middle ear. 



The several small and very remarkable canals which pass through the petrous 

 portion of the temporal bone, will be noticed when the nervous and arterial 

 branches they lodge are described. 



Development. — The petrous 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 belong 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. 

 ' This is the analogue of the mastoid canal in Man. 



* This process is prolonged by a cartilage that unites it to the styloid hone. 

 7 



