SEMICIRCULAR CANALS AND THE VESTIBULE. 409 



canals or their sense cells are stimulated by sound waves and give 

 us the means of determining the direction of sound in accordance 

 with their position in three planes at right angles to one another. 

 This view has been revived from time to time by recent writers. 



2. Flourens himself believed that the impulses normally proceed- 

 ing from these organs serve to moderate, or, as we should say now, 

 to inhibit the movements of the head. As soon as the canals are cut 

 the movements that have been kept under control by their influence 

 are unrestrained. On this ... view the semicircular canals are 

 organs which reflexly inhibit or restrain the voluntary movements, 

 and thus take an essential part in the proper co-ordination of such 

 movements. He did not attempt to define the physiology of the 

 organs in terms of the sensations aroused. 



3. The view that the stimulus to the hair-cells is to be found in 

 the varying pressure of the endolymph. As first proposed by Goltz 

 (1870), it was assumed that the endolymph exerts a hydrostatic 

 pressure upon the hair cells which in any given position varies in the 

 different ampullas and varies with different positions of the head. 

 The sensory impulses thus aroused give us a knowledge of the posi- 

 tion of the head and enable us, therefore, to control its movements 

 and also those of the body. On this view these organs act as sense 

 organs in maintaining body equilibrium and may be designated as 

 peripheral sense organs of equilibrium. Later observers (Mach, 

 Breuer, Brown, etal.) modified this view by the assumption that the 

 hair cells are stimulated not so much by the hydrostatic pressure of 

 the endolymph as by the pressure changes developed during move- 

 ments of the head, making the organs, therefore, a means of 

 appreciating especially the movements of the head, a dynamic 

 rather than a hydrostatic organ of equilibrium. It was assumed 

 that rotation movements of the head in the plane of a canal set up 

 a movement or pressure of the endolymph in the opposite direc- 

 tion, just as, to use a rough comparison, when one twirls a pail of 

 water in one direction the water lags behind and exerts a pressure 

 in the opposite direction. According to this hypothesis, which 

 in some form or other is the view usually taught, the hair cells in 

 each ampulla are stimulated chiefly by movements in the plane of 

 that canal toward the ampulla, the pressure of the endolymph be- 

 ing in the opposite direction, that is, from utriculus toward the 

 canal. Moreover, the vertical canals act in pairs (see Fig. 179), the 

 superior or anterior vertical of one side acting with the posterior or 

 inferior vertical of the other side, the two canals lying in parallel 

 planes. Movements in this plane forward would stimulate the 

 anterior ampulla on one side chiefly, movements in the same plane 

 backward, the posterior ampulla of the opposite side. The horizon- 

 tal canals also act together, being stimulated chiefly by rotational 



