234 



ON THE STRUCTURE OF THE SKULL. 



Eustachian tubes have separate openings into the pharyngeal 

 cavity, and curve upwards and backwards from the latter round 

 the inferior and posterior edges of the quadrate bones to open 

 into the tympana. 



In Birds the tympanic cavity is roofed over by the squa- 

 mosal, while a more or less complete floor is furnished to it by 

 the basi-sphenoicl, and a back wall by the produced ex-occipital 

 (and opisthotic ?). It may be completed in front by fibro- 

 cartilage or even by bone, and the memhrana tympani is fastened 

 to the outer margin of these boundaries of the tympanum and 

 not to the quadrate bone. 



The Eustachian passages ordinarily traverse the basi-sphenoid, 

 and when they reach the base of the skull unite into a single, 

 cartilaginous, common Eustachian tube, which opens in the 

 middle line, on the roof of the mouth. 



Fig. 96. 



Fig. ( Jfi. — Vertical and transverse sections of the left tympanic cavity of Crocodilus bipor- 

 catus. A, posterior, B, anterior segment; a, bristle passed into the small lateral 

 Eustachian passage leading from b, the posterior tympanic passage, which opens into 

 c, the common Eustachian passage; d, a bristle thrust into the air-passage which 

 traverses the supra-occipital ;/, bristle passed into the anterior tympanic passage; 

 <<i,Ca', carotid canal; Ct, fossa for the extremity of the cochlea; Tut, inner division 

 nt' the tympanic cavity. 



