THE SENSES. 563 



(Fig. 150, ,), will experience a similar variation. Thus, although the 

 membranous semicircular canals be not themselves sensitive to press- 

 ure, they may serve as channels for conducting an impulse to the 

 sensitive organs in their ampulla. The configuration of the nervous 

 expansions in the ampulla seems especially adapted for this purpose, 

 since they are arranged in the form of transverse crescentic folds. 

 In the sacculus and utricle, on the other hand, they are simply flattened 

 prominences on the surface of the membrane. 



If it be asked, why an apparatus for appreciating equilibrium should 

 be associated with the organ of hearing, it may be remarked that in 

 the auditory labyrinth alone there are sensitive nerve fibres distributed 

 to an epithelium provided with hair cells, and surrounded by a watery 

 fluid; conditions which are especially suitable for the perception of 

 variations in pressure, and consequently for that of changes in posi- 

 tion. 



Cochlea. The cochlea, so named from its resemblance to a snail- 

 shell, is a spiral bony canal making two or three turns about a central 



FIG. 151. 



BONY COCHLEA OF THE HUMAN EAR, right side; opened from its anterior face. (Cruveilhier.) 



axis, with its apex directed forward, downward, and outward. It is 

 divided longitudinally by a thin, bony partition, the spiral lamina, 

 which winds round its axis, following the spiral turns, but presenting 

 externally a free border. 



From this border a fibrous membrane, the membrana basilaris, 

 extends outward to the external wall of the cavity ; thus forming two 

 parallel passages, one above the other. The upper passage, which 

 communicates at its base with the vestibule, is the scala vestibuli. 

 The lower reaches to the fenestra rotunda, where the membrane, 

 stretched across this opening, separates its cavity from that of the tym- 

 panum ; it is accordingly known as the scala tympani. At the apex 

 of the cochlea a minute orifice of communication between the two canals 

 has been described by some writers, and doubted by others. Accord- 



