402 SPECIAL PHYSIOLOGY. 



Certain sounds produced by the concussion of solids against solids, 

 reach the fluid of the labyrinth directly through the cranial bones ; 

 e.g., the note of a tuning-fork held against the teeth or sides of the 

 head, gives rise to sonorous vibrations, which are much more powerful 

 than when transmitted to the ear through the air, and which can even 

 be heard after its first sound has ceased to be distinguishable through 

 the air. It is in this manner that sonorous vibrations are trans- 

 mitted to the ear, when this is placed on the ground, and it is also 

 of importance in hearing one's own voice. When the ears are closed, 

 the sound of our voice transmitted through the cranial bones is power- 

 ful, but its tone is altered. The ticking of a watch heard when placed 

 between the teeth, and the noise produced by striking the teeth toge- 

 ther, are further examples of sounds conveyed through the cranial 

 bones. The bones of the head also conduct sounds when these are 

 transmitted to it through water, as in the ease of divers. When the 

 head is submerged and the ears are closed, the noise produced by 

 the knocking together of two stones in the water, is very distinctly 

 heard. 



Such sounds as are produced by atmospheric undulations of suffi- 

 cient power to affect the cranial walls, as, e.g., the report of artillery 

 and the sound of thunder, are transmitted to the internal ear, partly 

 in a direct manner by the cranial bones, partly by the tympanum ; 

 for, though the ear be tightly closed such sounds are still audible. 

 The external ear is, however, the proper inlet for almost all air-borne 

 sonorous vibrations, and the tympanum is their proper path to the 

 labyrinth. The particular use fulfilled by each part of the complex 

 auditory apparatus in the conduction of sound-waves to the nerve of 

 hearing, may now be considered. 



The pinna or auricle, from its varied form, must receive and partly 

 reflect the atmospheric sound-waves from and in many directions. 

 Most of these undulations must be reflected externally ; but the size 

 of the auricle, its position, and external projection from the head, 

 the direction of its general concavity, which is turned somewhat for- 

 wards, its dense, firm, structure, the tightness and smoothness of the 

 skin investing it, and lastly, the form of the concha, and the relation 

 of that recess to the external auditory canal, clearly indicate its 

 office of collecting sound. The sonorous waves collected by the con- 

 cha are reflected from it, so as to impinge upon the inner surface of 

 the tragus, and are again reflected from the latter into the meatus. 

 The auricle, moreover, conducts a certain number of sonorous undu- 

 lations through its cartilaginous walls ; this, perhaps, affords some ex- 

 planation of the various and singular disposition of its surfaces, of 

 its eminences and depressions, which are probably intended to meet 

 the slighter sound-waves in such opposite and conflicting directions as 

 to enable them to neutralize each other, so that those which are prop- 

 erly conveyed to the membrana tympani through the air of the 

 meatus, may not be confused. This view is favored by the fact that 

 the cartilage of the auricle has only one narrow point of connection 

 with that of the meatus, and that it is cut up by many fissures, which 

 partially separate its different portions. Some, however, suppose that 



