The Nervous System 149 



is stretched taut and while it does not, as in the 

 higher vertebrates, lie in a bony groove, it possesses 

 around its periphery a strong thickening of circular 

 fibers, the annul us tympanicns, by means of which 

 it is closely united with the lining membrane of 

 the outer ear passage. The drum is attached chiefly 

 to the quadrate but in part to the squamosal bone. 



The middle ear is divided into an outer part, 

 the tympanic cavity proper, and a part next to the 

 labyrinth, the recessus cavi tympani. Within the 

 tympanic cavity, besides blood-vessels and nerves, 

 is found the columella with its appendage (found 

 in all Reptilia), the recessus scales, tympani. The 

 tympanic cavity is formed mainly by the quadrate, 

 though the exoccipital and squamosal bones take 

 some part. In outline it might be compared to a 

 truncated, four-sided pyramid, with its base below, 

 its truncated apex above, and with an anterior, a 

 posterior, a mesial, and a lateral side. 



From the floor of each tympanic cavity a 

 Eiistachian tube leads towards the throat. These 

 tubes unite and connect with the throat by a single 

 small opening just behind the posterior nares, as 

 shown in the figures of the skull. 



The semicircular canals with their ampulla lie in 

 the usual positions as seen in other vertebrates : the 

 anterior vertical, posterior vertical, and horizontal. 

 The details in structure of the inner ear cannot be 

 given here. The nervous epithelium is said to have 

 the same characteristics as in other vertebrates. 



