366 SKELETON OF THE POWL 



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. 



It 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, 

 ii. The anterior openings of the Eustachian tubes 

 are a pair of smaU 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 

 throtogh 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 promi- 

 nent 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, 

 and a lower, the fenestra rotunda. The 

 former of these lodges during life the inner 

 end of the auditory ossicle, or columella, a 

 slender bar of bone and cartilage, the outer 



