660 THE SENSES. 



Somewhat similar conditions are present in the semicircular canals. 

 Whenever the head is rotated from side to side in a horizontal plane, a 

 momentary increase of pressure must take place in the fluid of the 

 horizontal semicircular canal (Fig. 211, *), either toward or from the 

 ampulla at one end; and this increase or diminution of pressure may be 

 preceptible by the nervous expansions which are situated there. If the 

 head be moved upward or downward, a similar variation of pressure 

 will take place in the inferior vertical canal (Fig. 211, a) ; and if it be 

 inclined laterally, toward the right or left shoulder, the superior vertical 

 canal (Fig. 211, 2), will experience a variation of the same kind. Thus, 

 although the membranous semicircular canals be not themselves sensi- 

 tive to pressure, they may serve as channels for conducting an impulse 

 to the sensitive organs in their ampullae. Even the peculiar configura- 

 tion of the nervous expansions in the ampullae seems especially adapted 

 for this purpose; since they are arranged in the form of transverse 

 crescentic folds, while in the sacculus and utricle they are simply flat- 

 tened prominences on the inner surface of these cavities. 



If the question be asked, why an apparatus for appreciating changes 

 of equilibrium should be especially associated with the organ of hearing, 

 it may be remarked that in the auditory labyrinth alone there is to be 

 found a terminal distribution of sensitive nerve fibres in an epithelium 

 provided with hair cells, and surrounded by a fluid of watery consis- 

 tency ; all of which conditions are suitable for the perception both of 

 sonorous vibrations and of the variation in pressure due to changes of 

 position. 



Cochlea. The cochlea, named from its external resemblance to a 

 snail-shell, is a bony canal rolled spirally about a central axis, and 



Fig. 212. 



BONY COCHLEA OF THB HUMAN EAR, right side ; opened from its anterior face. 



(Cruveilhier.) 



making between two and three turns upon itself. Owing to the gradual 

 rise of the turns, it has a slightly conoidal form, the extremity of which, 

 or cupola, is directed forward, downward, and outward. The canal of 



