THE SKULL. 407 



b. The ventral surface of the cranium. 



i. The base of the skull is wide behind. The basi- 

 sphenoid is covered by the broad transverse 

 basi-temporal, a membrane-bone which forms 

 part of the boundaries of the tympanic cavities, 

 and probably corresponds to the posterior part 

 of the para- sphenoid of the frog. 



In front of the basi-temporal the base of 

 the skull is narrow, and formed by the i-os- 

 trum, or thickened ventral border of the inter- 

 orbital septum, which prolDably represents the 

 anterior part of the para-sphenoid. 



ii. The anterior openings of the Eustachian canals 

 are a pair of small holes, close to the median 

 plane, and opening into a deep notch between 

 the anterior end of the basi-temporal bone and 

 the rostrum. 



iii. The foramina of exit for the carotid canals 



are a pair of small holes, near the middle line, 

 and immediately to the outer sides of the 

 Eustachian apertures. They are connected 

 with each other by a canal, traversing the 

 base of the skull. > 



Pass a bristle along tJie carotid canal of one side, and 

 through the transverse canal from side to side. 



c. The side of the cranium. 



i. The tympanic cavity is a liemispherical depres- 

 sion, at the side of the hinder end of the skull, 

 bounded above by the squamosal and below by 

 the basi-temporal. The tympanic membrane 

 is attached just within its prominent Jip. 



Within the tympanic cavity can be seen 

 the following, 

 u. The fenestral recess is an oval depression in 

 the middle of the cup, divided by a parti- 

 tion into an upper hole, the fenestra oralis, 



