THE SENSE OF HEARING 739 



cochlea to 0.495 mm - at tne apex, and, according to Retzius, are about 24,000 

 in number. As the human ear usually cannot distinguish more than 11,000 

 tones, it is evident that there is a sufficient anatomic basis for this theory. 



The Functions of the Semicircular Canals. From the pronounced 

 disturbances of equilibrium and progression which follow injury or destruc- 

 tion of one or more of the semicircular canals or of the vestibular branch of 

 the acoustic nerve (phenomena which have been alluded to in connection 

 with a consideration of the functions of the cerebellum (see page 608), it is 

 apparent that they are among the peripheral sense-organs, the physiologic 

 action of which is the development of nerve impulses, which when trans- 

 mitted to the brain assist the equilibratory mechanism to maintain the equi- 

 librium of the body, both in the standing position and in the various modes 

 of progression. The character of the stimulus, however, and the manner in 

 which it acts on the specialized portion of the sense-organs (the hair cells) is 

 not entirely clear. 



In any explanation it must be recalled (i) that the membranous canals 

 are in direct connection with the utriculus, and that both are filled with 

 endolymph; and (2) that these canals act in pairs at least, in the varying 

 positions or movements of the head, e.g., the two horizontal or lateral canals 

 act together when the head is rotated around a vertical axis ; the two superior 

 and the two posterior verticals act together when the head is rotated around 

 a horizontal axis ; the superior and posterior vertical of one side act together 

 with the corresponding canals of the opposite side when the head is rotated 

 around an antero-posterior or sagittal axis; the superior vertical of one side 

 acts with the posterior vertical of the opposite side when the head is rotated 

 around an oblique axis. 



Goltz was the first to present the theory that the stimulus is to be found 

 in the pressure of the endolymph. He stated that in any given position of 

 the head, the endoiymph would gravitate to the most dependent portion of 

 the canals and thus press on the nerve endings in one or more of the ampullae, 

 and thus develop nerve impulses which, when transmitted to the brain, 

 would evoke more or less conscious sensations. These sensations would 

 indicate the position of the head, and lead to the necessary effort to control 

 the movement, to maintain equilibrium and to restore the head to the normal 

 position. If this mechanism is injured this knowledge and this control is 

 interfered with and a want of equilibrium is developed. 



Later Breuer propounded the view that the stimulus was to be found in 

 a movement of the endolymph in one or more canals when the head was 

 rotated, a movement which varied in rapidity in proportion to the rate of 

 head rotation. Mach subsequently presented the view that the stimulus was to 

 be found in a variation of pressure, positive or negative in one or more of 

 the ampullae rather than in an actual movement of the fluid. Crum Brown 

 made the further suggestion that in addition to the movement or pressure 

 of the endolymph there was a movement of the perilymph as well which 

 involved also the walls of the canal. This general theory is usually desig- 

 nated as the dynamic in contradistinction to the theory of Goltz which is 

 designated as the static. In the dynamic theory, the movement of the fluid 

 in the canal or canals, the plane of which corresponds to the plane in which 

 the head is rotated, though in the opposite direction, is over the hair cells at 

 the ampullae and thus acts as a stimulus. Thus, if the head is rotated to the 



