156 REPTILIA. 



tympanic cavity is absent. The malleal cartilage is tripartite in 

 the Crocodile, and round in the Chelonia. No external ear is pres- 

 ent in Reptiles, and it is only the highest form of them, the Croc- 

 odile, that possesses a rudiment of this structure, under the condition 

 of a double tegumentary fold or flap, the superior portion of which 

 contains in its interior a bony plate, and can be shut down by a muscle 

 like a valve. 



All Reptiles have a bony labyrinth lined by a membrane, and 

 completely separated from the cranial cavity, with which it is merely 

 associated by the openings for the passage of nerves ; it is situated 

 within the temporal bone, and partly within the latero-iriferior 

 pieces of the occipital bone. The vestibule is of varied form and size, 

 and receives the semicircular canals by four or five openings ; the 

 external canal is placed horizontally, and the anterior and posterior 

 stand perpendicularly, and have one of the crura common to them 

 both. Two grooves are usually found in the vestibule, and the sac 

 lying within it encloses a friable crystalline cretaceous mass, and 

 in rare instances harder lithic bodies ; the membranous canals ex- 

 pand into ampullae. The cochlea appears to be entirely absent in 

 the naked Amphibia, but on the contrary to be found in all the 

 Squamigera. It is found in its simplest condition, as a rounded 

 cavity with a sac in the interior containing a watery fluid, in 

 the Chelonia. Yet even in them, a round or cochlear fenestra, 

 separated from the fenestra ovalis by a thin septum, is found 

 placed in the direction backward, and closed by a second tympanic 

 membrane. In the Sauria and Ophidia the cochlea is a hollow 

 cone, blunt and somewhat dilated at the apex ; it includes a pair of 

 cartilages, which turned toward each other, are clothed by a plicated 

 membrane, upon which, as upon the spiral plate of the higher animals, 

 the auditory nerve expands into delicate filaments ; at the extremity 

 of the bony sphere is situated a peculiar retort-shaped sac (lagena) 

 which contains the fluid of the labyrinth, and receives, like the 

 vestibular sac, a twig from the acoustic nerve. The branches of the 

 portia dura pass only though the tympanic cavity, and there ap- 

 pears to be a true chorda tympani present. Hollow cells are fre- 

 quently found in the tympanic and mastoid bones, and stand in com- 

 munication with the internal ear. 



