HEARING. 505 



actually before us. It is this sense also which becomes most easily 

 and thoroughly excited in certain nervous disorders ; as, for exam- 

 ple, in delirium tremens, where the patient often sees passing before 

 his eyes extensive and magnificent landscapes, crowds of human 

 faces and figures, and series of towns and cities, which seem to be 

 depicted upon the imagination with a force and distinctness, much 

 superior to that of other delirious impressions. Since the sense of 

 sight, therefore, depends less directly than the other senses upon 

 the actual contact of material objects, it is also more easily thrown 

 into activity when withdrawn from their influence. 



HEARING. The sense of hearing depends upon the vibrations 

 excited in the atmosphere by sonorous bodies, which are themselves 

 first thrown into vibration by various causes, and which then com- 

 municate similar undulations to the surrounding air. These sono- 

 rous vibrations are of such a character that they cannot be directly 

 appreciated by ordinary sensibility, but the result of numerous and 

 well-directed physical experiments on this subject leaves no doubt 

 whatever of their existence ; and when such vibrations are commu- 

 nicated to the auditory apparatus, they produce in it the sensation 

 of sound. 



In the case of the aquatic animals, which pass their entire exist- 

 ence beneath the surface of the water, the water itself, which is 

 capable of vibrating in the same way, communicates the sonorous 

 impressions to the organ of hearing; but in terrestrial animals, 

 and particularly in man, it is the atmosphere which always serves 

 as the medium of transmission. 



The auditory apparatus, in man and in the quadrupeds, consists, 

 first, of a somewhat expanded and trumpet-shaped mouth, or ex- 

 ternal ear, destined to receive and collect the sonorous impulses 

 coming from various quarters. This external ear is constructed 

 of a cartilaginous framework, covered with integument, loosely 

 attached to the bones of the head, and more or less movable by 

 means of various muscles, which by their contraction turn its 

 expanded orifice in different directions. In man, the movements 

 of the external ear are almost always inappreciable, though the mus- 

 cles may be easily demonstrated ; but in many of the lower animals 

 these movements are exceedingly varied and extensive, and play a 

 very important part in the working of the auditory apparatus. 



At the bottom of the external ear, its orifice is prolonged into a 

 tube or canal, the external auditory meatus, partly cartilaginous and 



