608 PHYSIOLOGY 



the position of our head in space. " One canal can be affected by and trans- 

 mit the sensation of rotation about one axis in one direction only : and for 

 complete perception of rotation in any direction about any axis six semi- 

 circular canals are required in three pairs, each pair 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 

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

 These reflex movements of head and eyes are the invariable result of move- 

 ments set up in the endolymph, and occur equally well in the absence of 

 the cerebral hemispheres. If 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 is 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 volun- 

 tary 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 development of 

 the otolith organ. The primitive part of this organ is represented by th 

 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 once that 

 the incidence of the weight of the otoliths on the hairs of the macula will vary 

 according to the position of the head. Thus in the diagram (Fig. 200, p. 399) 

 in A (normal position) the chief weight of the otolith falls on the hairs from 

 b to c, whereas, when the head has been rotated round a right angle so that 

 the man, for instance, is lying on his right side, the chief weight of the otoliths 

 will fall on the hairs at c. The nerve- endings stimulated by the weight of the 

 otoliths will therefore vary according to the position of the head. The 

 cerebellum and its associated structures represent a mechanism for the 

 regulation of the movements of the trunk as a whole and the position of its 

 centre of gravity in relation to the position of the head. 



