412 SKELETON OF THE FOWL. 



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 ros- 

 trum, or thickened ventral border of the inter- 

 orbital septum, which probably 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 the 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 hemispherical 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 lip. 



Within the tympanic cavity can be seen 

 the following : 



a. The fenestral recess is an oval depression in 

 the middle of the cup, divided by a parti- 

 tion into an upper hole, the fenestra ovalis, 



