680 PHYSIOLOGY 



and movement of endolymph away from, and therefore diminished 

 pressure on, the corresponding ampulla of the right side. In this 

 way, for movement in any given plane, the two corresponding semi- 

 circular canals of the two sides are synergic, and unite in sending 

 impulses which guide the equilibrating centres, and inform us of 

 the position of our head in space. '' One canal can be affected by 

 and transmit the sensation of rotation about one axis in one direction 

 only ; and for complete perception of rotation in any direction about 

 any axis six semicircular canals are required in three pairs, each 

 parr having its two canals parallel (in the same plane), and with 

 their ampullae turned opposite ways. Each pair would thus be 

 sensitive to any rotation about a line at right angles to its plane 

 or planes, the one canal being influenced by rotation in the one direc- 

 tion, the other by rotation in the opposite direction " (Crum Brown). 

 These reflex movements of head and eyes are the invariable result 

 of movements set up in the endolymph, and occur equally well in 

 the absence of the cerebral hemispheres. li an animal or man be 

 placed on a turntable and rotated, his first tendency will be to turn 

 his head and eyes in the opposite direction to that of rotation. If 

 the rotation be continued, the endolymph gradually takes up the 

 movement of the surrounding parts of the head, and if the eyes be 

 closed, no movement of head or eyes is observed. If now the rotation 

 be stopped, the endolymph will tend to go on moving, and the effect 

 will be the same as if a movement of rotation were suddenly begun 

 in the opposite direction. Head and eyes will now be turned, without 

 any voluntary impulse, in the direction of the previous rotation, and 

 in consciousness there will be an actual sensation of rotation in the 

 opposite direction. This sensation is in opposition to the sensations 

 derived from other parts, and hence the feeling of giddiness and the 

 actual disorders of equilibrium which are its concomitants. 



That this feeling of giddiness on rotation is due to impulses started 

 in the semicircular canals is shown by the fact that, in a large number 

 of deaf-mutes where these organs are imperfectly developed, it is 

 impossible to produce giddiness and the associated eye movements 

 by passive rotation. 



THE FUNCTION OF THE OTOLITHS 



The semicircular canals are, as we have seen, a higher develop- 

 ment of the otolith organ. The primitive part of this organ is repre- 

 sented by the maculae in the utricle and saccule. It is to these organs 

 that we must ascribe our powers of appreciating the static position 

 of the head, as well as, to a slight degree, movements, not of rotation, 

 but in one plane forwards or backwards. 



A consideration of the structure of the otolith organ shows at 



