420 



ANATOMY FOR NURSES 



[Chap. XX 



The fibres of the vestibular nerve haw their orighi in the gray 

 matter of the pons \ arolii. Some of these fibres extend to the 

 cerebelkim and to motor centres of the si)inal nerves. Other fibres 

 extend to tlie vestibnle and are (Hstril)uted around the liair-eells 

 of the saccule, utricle, and the ampulla of the semicircuhir canals. 

 Physiology of hearing. — All bodies which produce sound are 

 in a state of vibration, and communicate their vibrations to the 

 air with which they are in contact. 



When these air-waves, set in motion by sonorous bodies, enter 

 the external auditory canal, they set the drum-membrane vibrat- 

 ing, stretched membranes taking up vibrations from the air with 

 great readiness. These vibrations are communicated to the chain 

 of tiny bones stretched across the middle ear, and their oscilla- 

 tions cause the membrane leading into the internal ear to be 

 alternatively pushed in and drawn out, and vibrations are in this 

 way transmitted to the perilymph. The movements of the peri- 

 lymph are transmitted to the basilar membrane, and set some of 

 the strings in motion. In some unknown way tliese movements 

 are transmitted to the hair-cells and through them to the nerve- 

 fibres at their base. By means of the nerve-fibres the stimulus 

 is conveyed to the brain and'interpreted there, so that it is with 

 the brain that we hear. 



The sense of equilibrium. — Among the various means (such as 



sight, touch, and muscular sense) 

 whereby we are enabled to maintain 

 our equilibrium, coordinate our move- 

 ments, and become aware of our po- 

 sition in space, one of the most im- 

 portant is the action of the vestibule 

 and semicircular canals. Though 

 these structures are found in the inner 

 ear and communicate with the coch- 

 lea, it is now thought that they are 

 not connected with the sense of hear- 

 ing. Just how they perform their 

 function is not known, but it is 

 thought that movements of the head 

 set up movement in the endol^'mph of the canal, and this acts as 

 a stimulus to the nerve-endings around the hair-cells. 



Fig. 196. — Diagram show- 

 ing Relative Position of the 

 Planes in which the Semi- 

 circular Canals Lie. Rt., 

 right ear; Lt., left ear; A.V., 

 anterior vertical canal; P.V., 

 posterior vertical canal ; H., 

 horizontal canal ; a, ampulla of 

 Rt. anterior vertical canal ; a', 

 ampulla of Lt. posterior vertical 

 canal. 



