186 HAND-BOOK OF PHYSIOLOGY. 



interior of the ampulla. The branches for the cochlea enter it through 

 orifices at the base of the modiolus, which they ascend, and thence suc- 

 cessively pass into canals in the osseous part of the lamina spiralis. In 

 the canals of this osseous part or zone, the nerves are arranged in a plexus, 

 containing ganglion cells. Their ultimate termination is not known with 

 certainty; but some of them, without doubt, end in the organ of Corti, 

 probably in cells. 



PHYSIOLOGY OF HEAEING. 



All the acoustic contrivances of the organ of hearing are means for 

 conducting the sound, just as the optical apparatus of the eye are media 

 for conducting the light. Since all matter is capable of propagating sono- 

 rous vibrations, the simplest conditions must be sufficient for mere hear- 

 ing; for all substances surrounding the auditory nerve would communi- 

 cate sound to it. The whole development of the organ of hearing, there- 

 fore, can have for its object merely the rendering more perfect the propa- 

 gation of the sonorous vibrations, and their multiplication by resonance; 

 and, in fact, all the acoustic apparatus of the organ may be shown to have 

 reference to these two principles. 



Functions of the External Ear. The external auditory passage 

 influences the propagation of sound to the tympanum in three ways: 1, 

 by causing the sonorous undulations, entering directly from the atmos- 

 phere, to be transmitted by the air in the passage immediately to the 

 membrana tympani, and thus preventing them from being dispersed; 2, 

 by the walls of the passage conducting the sonorous undulations imparted 

 to the external ear itself, by the shortest path to the attachment of the 

 membrana tympani, and so to this membrane; 3, by the resonance of the 

 column of air contained within the passage; 4, the external ear, especially 

 when the tragus is provided with hairs, is also, doubtless, of service in 

 protecting the meatus and membrana tympani against dust, insects, and 

 the like. 



1. As a conductor of undulations of air, the external auditory passage 

 receives the direct undulations of the atmosphere, of which those that 

 enter in the direction of its axis produce the strongest impressions. The 

 undulations which enter the passage obliquely are reflected by its parietes, 

 and thus by reflexion reach the membrana tympani. 



2. The walls of the meatus are also solid conductors of sound; for 

 those vibrations which are communicated to the cartilage of the external 

 ear, and not reflected from it, are propagated by the shortest path through 

 the parietes of the passage to the membrana tympani. Hence, both ears 

 being close stopped, the sound of a pipe is heard more distinctly when its 

 loAver extremity, covered with a membrane, is applied to the cartilage of 

 the external ear itself, than when it is placed in contact with the surface 

 of the head. 



