THE PHYSIOLOGY OF HEARING 



621 



the more effective propagation of the sonorous vibrations and their intensi- 

 fication by resonance; and, in fact, the whole of the acoustic apparatus may 

 be shown to have reference to these principles. 



The external ear and the auditory passages influence the propagation of 

 sound to the tympanum by collecting from the atmosphere the sonorous undu- 

 lations that strike against the external ear and by transmitting them by the 

 air in the passage to the membrana tympani. 



In animals living in the atmosphere, the sonorous vibrations are con- 

 veyed to the auditory epithelium through three different media in series; 



membrana tectoria, 



outer hair-cells 



inner rod vas basilar outer ceils of Deiten 

 membrane rod 



FIG. 438. Semidiagrammatic Representation of the Organ of Corti and Adjacent Structures. 

 (Merkel-Henle.) a. Cells of Hensen; b, cells of Claudius; c, internal spiral sulcus; x, Nuel's space. 

 The nerve fibers (dendrites of cells of the spinal ganglion) are seen passing to Corti's organ through 

 openings (foramina nervosa) in the bony spiral lamina. The black dots represent longitudinally 

 running branches, one bundle lying to the inner side of the inner pillar, a second just to the outer 

 side of the inner pillar within Corti's tunnel, the third beneath the outer hair cells. 



namely, the air of the external ear and meatus, which sets in vibration the 

 tympanic membrane, the solid chain of auditory ossicles, and the fluid of the 

 labyrinth. Sonorous vibrations are imparted too imperfectly from air to 

 the solid structures of the body as a whole for the propagation of sound to 

 the internal ear to be adequately effected by that means alone. In passing 

 from air directly into water, sonorous vibrations suffer also a considerable 

 diminution of their strength; but if a tense membrane exists between the 

 air and the water, the sonorous vibrations are communicated from the former 

 to the latter medium with very great intensity, This fact, of which Miiller 

 gives experimental proof, furnishes at once an explanation of the use of the 

 fenestra ovalis and of the membrane closing it. It is the means of com- 

 municating, in full intensity, the vibrations of the ear bones, or, in their absence, 

 of the air in the tympanum, to the fluid of the labyrinth. The vibration of 

 the fluid, the perilymph and endolymph, of the internal ear, sets the basilar 

 membrane in vibration and in consequence stimulates the sensory apparatus 



