THE HEAD. 43 



apparatus. In the meantime, only the exterior 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 whose base is 

 turned downwards and a little backwards. It will be studied successively in 

 its 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 of the lateral wall of the cerebellar cavity. 

 It presents the canal or internal auditory hiatus (meatus 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 

 aquceductus 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 continuous 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 mastoid process. This border is 

 traversed by a slit, the mastoid fissure, 1 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 lacerated 

 foramen ; above, and under the mastoid process, the stylo-mastoid or pre-mastoid 

 foramen, the external orifice of the aqueduct of Fallopius ; below, the 

 subuliform (or styloid) process for the attachment of the stylo-staphyleus 

 (tensor palati) muscle and the Eustachian tube : this is a long, thin, and pointed 

 process presenting, at its base and within, a canal which enters the cavity of the 

 tympanum, and which is incompletely partitioned by a small bony plate into 

 two parallel portions ; in the centre, the hyoid prolongation or vaginal process, 2 

 a little cylindrical eminence surrounded by a bony sheath, and the mastoid 

 protuberance or process, a slightly salient, 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 

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

 arterial branches they lodge are described. 



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



2 This process is prolonged by a cartilage that unites it to the styloid bone. 



