202 GENERAL ZOOLOGY 



through the length of the scala vestibuli to the apex of the 

 cochlea and then down through the scala tympani. 



Scala Media. — The sound receptors are located in the scala 

 media or cochlear duct, which in man is triangular in cross sec- 

 tion (Fig. 1305). On its upper side the scala media is separated 

 from the scala vestibuli by the thin vestibular membrane, also 

 known as Reissner's membrane. The outer side of the scala 

 media is firmly attached to the wall of the bony labyrinth by the 

 spiral ligament. The floor of the scala media is formed by the 

 basilar membrane which, with a narrow bony shelf extending 

 from the axis of the bony cochlea, separates the scala media from 

 the scala tympani. The organ of Corti, resting on the basilar 

 membrane, is a complicated structure provided with sensory hair 

 cells, which are the sound receptors through which nerve fibers 

 of the cochlear branch of the auditory nerve are stimulated. 



Middle Ear. — The middle ear is a chamber derived from the 

 first gill cleft of fishes. It is connected with the pharynx by 

 the Eustachian tube, by means of which air pressure within the 

 chamber is kept constant. Its inner wall is in contact with the 

 bony labyrinth at the fenestra ovalis and the fenestra rotunda. 

 Its outer wall is provided with a circular tympanic membrane, 

 which in the frog is visible externally at the side of the head. 

 From the tympanic membrane a chain of small bones extends 

 across the cavity to the fenestra ovalis. In the frog these con- 

 sist of a rod-shaped columella, extending from the center of the 

 tympanic membrane to a small cartilage lying in the fenestra 

 ovalis. In man three bones are present : (1) the malleus, attached 

 by an arm to the tympanic membrane; (2) the incus, extending 

 from the malleus to (3) the stapes, a stirrup-shaped bone fitting 

 into the fenestra ovalis. A middle-ear cavity is absent in snakes. 



External Ear. — Mammals alone have external ears. The 

 external ear consists of (1) the pinna, supported by cartilages 

 and provided with muscles by means of which it may be moved, 

 and (2) the auditory meatus, a canal leading to the tympanic 

 membrane. The external ear increases the effectiveness of the 

 ear by collecting and directing sound waves against the tympanic 

 membrane. 



Auditory Function. — Sound vibrations are transmitted from 

 the tympanic membrane across the middle ear by the ear bones 

 to the fenestra ovalis, where they are taken up by the perilymph 



