336 THE BIOLOGY OF THE FROG CHAP 



masses of similar crystals which are commonly called the 

 otoliths. 



The different parts of the ear are all supplied by the 

 branches of the auditory nerve. In the regions of the nerve 

 endings the epithelium contains sensory cells with hairlike 

 processes at their outer ends. These are especially abun- 

 dant and well developed in the walls of the ampullae. The 

 whole labyrinth is filled with a fluid called the endolymph, 

 and it is surrounded by another fluid, the perilymph* which 

 fills the space between the labyrinth and the wall of the 

 auditory capsule. On the outer side the wall of the audi- 

 tory capsule is perforated by an aperture, \hefenestra ovalis, 

 which is closed with a small plug of cartilage. 



The accessory auditory apparatus consists of the structures 

 which in higher forms constitute the middle ear, there being 

 nothing in the frog which corresponds to the external ear of 

 mammals. The cavity of the middle ear communicates 

 through the Eustachian tube with the mouth cavity near the 

 angle of the jaw. It is closed externally by the tympanic 

 membrane, which is readily seen at the side of the head 

 behind the eye. This membrane is nearly circular in form 

 and is attached by its outer margin to a ring of cartilage, the 

 annulus tympanicus. Near its middle it gives attachment 

 to the outer head of a rod, the columeUa t which extends 

 across the cavity of the middle ear, and joins the small car- 

 tilage lying over the fenestra ovalis. It is through this rod 

 that the vibrations which are produced by the sound waves 

 impinging upon the tympanic membrane are carried to the 

 inner ear. Here they set the contents of the labyrinth into 

 vibration and thus stimulate the sensory end organs of the 

 auditory nerve. The nervous impulses set up in this way 

 are conveyed to the brain, where they give rise to the sensa- 

 tion of sound. 



