770 



THE SENSE OF HEARING 



pharynx involving this tube, as may be gathered from the diminution 

 in the acuity of hearing then commonly experienced. In many in- 

 stances, these simple catarrhal affections pave the way for suppurative 

 processes which spread from the lining of the tympanum to the ossicles, 

 destroying them in part or causing them to become ankylosed. The 

 exudations formed in the course of this process most commonly burrow 

 their way through the eardrum, but without permanently destroying 

 the oscillatory qualities of this membrane. The greatest danger of 

 an infection of this kind lies in the fact that it may spread to the 

 adjoining mastoid cells and, unless the latter are freely drained, 

 give rise to a septic infection of the neighboring meninges. 



The Inherent Muscles of the Ear. — Besides the different muscles 

 attached to the pinna, the ear also contains two muscles which are 

 intimately concerned with the transmission of the sound waves through 

 the tympanum. These muscles are the tensor tympani and the 

 stapedius. The former is placed in a long furrow above the Eustachian 

 tube and is inserted by means of a long tendon 

 into the neck of the malleus directly below the 

 axis of rotation of this bone. It is innervated 

 by fibers derived from the trigeminus and re- 

 legated to the otic ganglion. When this muscle 

 contracts, it pulls the eardrum inward, thereby 

 placing it under a greater tension. This ful- 

 fills two purposes, namely to accommodate the 

 drum to sounds of higher pitch, and to lessen 

 its vibratory power whenever sounds of great 

 intensity are received. It need scarcely be 

 mentioned that a sound of high pitch can only 

 be transferred in its true form if the tension of 

 the drum is increased sufficiently to correspond 

 to its wave-length. In the second place, it 

 must be evident that a tense membrane is more 

 resistant than a flaccid one and cannot, there- 

 fore, be made to vibrate so easily. For this reason, the tensor tympani 

 muscle may also be regarded as a protective means against the activa- 

 tion of the organ of Corti by sounds of unusual intensity. Conse- 

 quently, its function is very similar to that of the iris which, by the 

 contraction of its radial fibers, lessens the size of the pupil, thereby 

 preventing the entrance of a bundle of fight of injurious intensity to 

 the retina. 



The stapedius muscle arises from the inner wall of the tympanum 

 near the fenestra ovahs. Its tendon passes forward and is inserted 

 upon the posterior aspect of the neck of the stapes. On contraction 

 it pulls this bone over in a lateral direction so that the hinder part 

 of its base is pressed more firmly into the membrane closing this 

 foramen. In accordance with the degree of its deviation, it increases 

 the tenseness of this membrane until its vibration is finally prevented 



Fig. 387. — Diagram Il- 

 lustrating THE Action of 

 THE Stapedius Muscle. 



A, state of relaxation; 

 B, state of contraction; s, 

 stapes; m-s, muse, sta- 

 pedius. 



