HEARING. 



561 



to its work, some regulative apparatus, in the form of a nerv- 

 ous and muscular mechanism, would have been evolved in the 



Pig. 404.— Diagrammatic representation illustrating auditory processes (after Beaunis). 

 A, external ear; B, middle ear; C, internal ear; 1, auricle; 2, esitemal auditory 

 meatus; 3, tympanum; 4, membrana tympanl: 5, Eustachian tube; 6, mastoid 

 cells; 10, foramen rotondum; 11, foramen ovale; 12, vestibule; 13, cochlea; 14, 

 scala tympani; 16, scala vestibuli; 10, semicircular canals. 



N. B. — The ear is so complicated an organ that it is almost impossible to give a dia- 

 grammatic representation of it at once simple and complete in a single figure. 

 A comparison of the whole series of cuts is therefore desirable. It is essential to 

 understand how the end-organ within the scala media is stimulated. 



higher groups of animals. Such is found in the tensor tym- 

 pani, laxator tympani, and stapedius muscles, as well as the 

 Eustachian tube. 



Muscles of the Middle Ear. — The tensor tympani regulates 

 the degree of tension of the drum-head, and hence its ampli- 

 tude of vibration, having a damping efPect, and thus preventing 

 the ill results of very loud sounds. 



Ordinarily, this is, doubtless, a reflex act, iu which the fifth 

 is usually the afferent nerve concerned. It is well-known that, 

 when we are aware that an explosion is about to take place, we 

 are not as much affected by it, which would seem to argue a 

 voluntary power of accommodation ; but of this we must speak 

 with caution. 



According to some authorities the laxator tympani is not a 

 36 



