170 ANIMALS 



which almost touches the processes of the sensory cells. This 

 membrane is free at one edge, but attached at the other to a 

 non-sensory portion of the cochlea. The nerve fibres supplying 

 this organ end in free nerve terminations around the sensory 

 cells. The basilar membrane becomes wider toward the apex 

 of the cochlea, and the fibres of the basilar membrane become 

 correspondingly longer. 



399. The entire labyrinth lies in a cavity of approximately 

 the same shape, in the petrosal portion of the temporal bone. 

 This is the bony labyrinth. It is considerably larger than the 

 membranous labyrinth, and the space between is filled with 

 perilymph. A small opening in the wall of the bony labyrinth 

 is covered by a membrane, and a small, movable bone, the 

 stapes. By the vibrations of these parts the perilymph is 

 disturbed and through it the fibres of the basilar membrane and 

 thus the cells in the organ of Corti are stimulated. It is 

 supposed that the difference in the lengths of the basilar mem- 

 brane fibres corresponds to differences in the lengths of sound 

 waves, and that, therefore, sounds of a given pitch stimulate 

 only that part of the organ of Corti in which fibres of a cor- 

 responding length occur. 



400. Certain accessory organs, by which the sound waves 

 in the air are transmitted to the fluids of the labyrinth are 

 found in all animals having a well-developed sense of hearing, 

 and the condition of these organs is a very good index as to the 

 degree of perfection of the sense itself. 



401. The auricle, or shell, of tfie outer ear is found only in 

 Mammals. Its function is manifestly to gather the sound 

 waves and direct them to the auditory meatus, the tube which 

 leads to the ear drum. The external auditory meatus is also 

 found in Birds and some Reptiles. But in the frogs the ear 

 drum is on a level with the surface of the head. Some Reptiles 

 (snakes), some Amphibia (salamanders) and Fishes have 

 neither outer nor middle ear. 



