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 annulus tympanicus, 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 scale 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 
Eustachian 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 ampulle 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 
givenhere. The nervous epithelium is said to have 
the same characteristics as in other vertebrates. 
