tough membrane stretched between the processes. The 

 infundibulum rests upon the upper surface of the rostrum 

 of the basisphenoid, the rest of the sella turcica is formed 

 in soft structures, and in front of this the brain box is car- 

 tilaginous except for its roof, and the alisphenoid plates of 



The acustico-jugular cavity (cavutn acustico jugular*) 



[see especially figure 50] is divided by two tough mem- 

 branes into three portions, a vago-accessory canal behind, 

 a jugular sinus in front, and the medial portion of the 

 tympanic cavity between the two. The two membranes 

 may be referred to as the anterior and posterior septa. The 

 posterior septum (septum postcriits) is attached below to 

 theexoccipital; its attachment being commonly marked bya 

 ridge; above, to the same bone parallel to and j list behind the 

 opisthotico-exoccipital suture, its medial attachment to the 

 opisthotic in front of the vago-accessory foramen is rarely 

 indicated by a ridge. The anterior septum (septum 

 anterius) is attached below to a ridge on the pterygoid, 

 which, commencing at the hinder and outer corner of the 

 prootic surface passes back and laterally and is continued 

 across the inner aspect of the quadrate in front of the inner 

 end of the columellar sulcus. From the top of the inner 

 end of that groove the septum springs across to the opis- 

 thotic, leaving a gap between itself and the quadrate 

 through which the jugular vein passes. The line of its 

 attachment to the opisthotic is usually indicated by a 

 groove on that bone just behind the inner portion of the 

 quadrato-opisthotic suture. The medial attachment, to the 

 prootic in front of the fenestra ova lis is also indicated by 

 a ridge in most specimens. 



The inner wall of the medial portion of the tympanic 

 cavity may be exposed by removal of quadrate (see Fig. 50). 

 In such a preparation besides exposing that wall the jugular 



