570 HANDBOOK OF PHYSIOLOGY. 



movable external ears. The ossicula seem to be vestigial relics utilized 

 for the auditory function. In land animals they vary in shape accord- 

 ing to the type of the animal rather than in relation to its acuteness of 

 hearing. I have never found a muscular laxator tympani in any ani- 

 mal, but the tensor exists as a ligament in whales where the malleus is 

 fixed." (Alban Doran.) 



The bones of the ear are covered with mucous membrane reflected over 

 them from the wall of the tympanum; and are movable both altogether 

 and one upon the other. The malleus moves and vibrates with every 

 movement and vibration of the membrana tympani, and its movements 

 are communicated through the incus to the stapes, and through it to the 

 membrane closing the fenestra ovalis. The malleus, also, is movable 

 in its articulation with the incus; and the membrana tympani moving 

 with it is altered in its degree of tension by the laxator and tensor tym- 

 pani muscles. The stapes is movable on the process of the incus, when 

 the stapedius muscle acting, draws it backwards. The axis round which 

 the malleus and incus rotate is the line joining the processus gracilis of 

 the malleus and the posterior (short) process of the incus. 



Fio. 383. Interior view of the tympanum, with membrana tympani and bones in natural posi- 

 tion. 1, Membrana tympani; 2, Eustachian tube; 3, tensor tympani muscle; 4, lig. mallei superior; 

 5, lig. mallei super.; 6, chorda-tympanic nerve; a, 6, and c, sinuses about ossicula. (Schwalbe.) 



(3.) The Internal Ear. The proper organ of hearing is formed 

 by the distribution of the auditory nerve within the internal ear, or 

 labyrinth, a set of cavities within the petrous portion of the temporal 

 bone. The bone which forms the walls of these cavities is denser than 

 that around it, and forms the osseous labyrinth ; the membrane within 

 the cavities forms the membranous labyrinth. The membranous laby- 

 rinth contains a fluid called endolymph ; while outside it, between it and 

 the osseous labyrinth, is a fluid called perilymph. 



The osseous labyrinth consists of three principal parts, namely, the 

 vestibule, the cochlea, and the semicircular canals. 



The Anatomy of the Internal Ear. The vestibule is the middle 

 cavity of the labyrinth, and the central organ of the whole auditory ap- 



