I20 THE SKULL [chap. 



vertical plates, thick and rounded below at first, but 

 gradually becoming more compressed. These are continued 

 still further backwards by the compressed pterygoid bones 

 {Pt), ending in the backward-projecting hamular processes, 

 and supported externally by the descending (pterygoid) plate 

 of the alisphenoid. The groove between these descending 

 lamellse of bone continues the narial passage backwards. It 

 has for its roof the vomer ( V6) in front, then the presphenoid 

 {PS), and posteriorly a portion of the basisphenoid {BS) ; 

 but the palatines and pterygoids arch over so much towards 

 the middle line that they only leave a small strip of these 

 bones exposed. Inferiorly, this groove is not closed by 

 bone, but in the living animal the soft palate is stretched 

 across it. 



The base of the skull, behind this " mesopterygoid " 

 fossa, presents in the middle a nearly flat elongated surface, 

 consisting of the basisphenoid [BS) and ba.sioccipital {BO) ; 

 the latter, roughened for the attachment of muscles, and 

 terminating posteriorly at the inferior border of the foramen 

 magnum {fni), flanked on each side by the occipital condyles 

 {oc). The nearly straight lateral edges of the anterior half 

 of the basioccipital rise up to abut against the prominent 

 smooth rounded auditory bullae {Ty), which form so con- 

 spicuous a feature in this region of the skull, and which are 

 produced outwards into the lower wall of the external 

 auditory meatus {earn). In the antero-internal angle of the 

 bulla is seen the irregular orifice of the Eustachian canal. 

 Close to the inner side of this is an oval aperture, which is 

 at the same time the anterior extremity of the carotid canal 

 and the entrance to the foramen lacerum medium {f I m) 

 through which the internal carotid artery enters the cranial 

 cavity. In front, and rather to the outer side of this, is the 

 foramen ovale {fo) piercing the alisphenoid, and imniedi- 



