906 



OF SENSATION, AND THE ORGANS OF THE SENSES. 



some of them. The functions of the Meatus appear to be threefold. The 

 sonorous undulations entering from the atmosphere are propagated directly, 

 without dispersion, to the membrana tympani : the sonorous undulations 



Fig. 229. 



Fig. 230. 



A view of the Left Ear in its natural state : 1, 2, 

 the origin and termination of the helix ; 3, the anti- 

 helix ; 4, the anti-tragus ; 5, the tragus ; 6, the lobus 

 of the external ear; 7, points to the scapha and is 

 on the front and top of the pinna; 8, the concha; 9, 

 the meatus auditorius externus. 



An anterior view of the External Ear, as well as 

 of the Meatus Auditorius, Labyrinth, Ac.: 1, the 

 opening into the ear at the bottom of the concha ; 

 2, the meatus auditorius externus or cartilaginous 

 canal; 3, the membrana tympani stretching upon 

 its ring; 4, the malleus ; 5, the stapes; 6, the laby- 

 rinth. 



received on the external ear are conveyed along the walls of the meatus to the 

 membrana tympani : the air which it contains, like all insulated masses of air, 

 increases the intensity of sounds by resonance. That, in ordinary hearing, 

 the direct transmission of atmospheric vibrations to the membrana tympani is 

 the principal means of exciting the reciprocal vibrations of the latter, is suffi- 

 ciently evident ; the undulations which directly enter the passage will pass 

 straight on to the membrane ; while those that enter obliquely will be reflected 

 from side to side, and at last will fall obliquely on the membrane, thus perhaps 

 contributing to the notion of direction. The power of the lining of the meatus 

 to conduct sound from the external ear is made evident by the fact that, when 

 both ears are closely stepped, the sound of a pipe having its lower extremity 

 covered by a membrane, is heard more distinctly when it is applied to the 

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

 surface of the head. The resonant action of the air in the tube is easily demon- 

 strated, by lengthening the passage by the introduction of another tube j the 

 intensity of external sounds, and also that of the individual's own voice, as 

 heard by himself, are then much increased. 



907. Many facts prove, however, that the fluid of the Labyrinth may be 

 thrown into vibration in other ways than by the Tympanic apparatus. Thus, in 

 Osseous Fishes, it is only by the vibrations transmitted through the bones of the 

 head that hearing can take place. There are many persons, again, who can 

 distinctly hear sounds which are thus transmitted to them; although, through 

 some imperfection of the tympanic apparatus, they are almost insensible to those 

 which they receive in the ordinary way. It is evident, where this is the case, 

 that the nerve must be in a state fully capable of functional activity; and, on 



