CONDUCTION OF SOUND VIBRATIONS. 599 



or tympanum, and apply this to the teeth. This direct conduc- 

 tion of sound is, however, very valuable in determining the seat 

 of the disease in cases of deafness. So long as a clear sensation 

 of sound reaches the brain through the bones of the head, one 

 knows that the important nerve endings and their central con- 

 nections are unimpaired, and can then conclude that the disease 

 lies in the mechanical conducting parts of the hearing organ. 



fn fishes, where the labyrinth is the only part of the auditory 

 apparatus which exists, it is imbedded in the cranium, and the 

 sound waves arrive through the medium of water, and are 

 directly conveyed to the nerve endings by the bones of the head. 

 An air-containing tympanum would be a great impediment to 

 the hearing of these animals. 



CONDUCTION OF SOUND VIBRATIONS THROUGH THE OUTER EAR. 



The parts of the ear through which the sound commonly passes 

 before it reaches the nerve are naturally separated into three, 

 viz., (1) the external ear and the auditory canal ; (2) the middle 

 ear, tympanum or drum, which is shut off from the latter by the 

 tympanic membrane ; and (3) the fluid of the labyrinth. 



In man, the ear muscles are so poorly developed that he can 

 hardly move the external ear or pinna perceptibly, and the part 

 commonly called the ear is of little use. We know this, because 

 the outer ear may be quite removed without materially affecting 

 the power of hearing. Birds hear well without any outer ear, and 

 the sound reflected from the pinna may be excluded by placing a 

 little tube in the auditory canal without reducing the intensity of 

 the sound. But the movable ears of many animals are, no doubt, 

 useful in helping them to ascertain the direction from which a 

 sound arrives, by catching more of the vibrations coming to their 

 orifice. That the external ear may be of some use, even to man, 

 one is led to believe, by the natural readiness with which a person 

 with dull hearing supplements it by means of his hand. But in 

 this act the ear is commonly pushed away from the head to an 

 angle of about forty-five degrees, and thus its projection is con- 

 siderably increased. 



The auditory canal is a crooked and irregular passage, getting 



