

THE ORGANS OF SENSE. 153 



for the co-ordination of muscular movements and for the maintenance 

 of equilibrium. Each movement of the head sets up a stimulus in 

 one or other of the canals, which passes along the vestibular nerve 

 to the cerebellum ; these impulses serve to co-ordinate and restrain 

 the movement. In the absence of these guiding impulses the move- 

 ments become completely uncontrolled, and the sense of the position 

 of the head is lost. The existence of three pairs of canals in three 

 different planes makes it possible for impulses to be aroused in what- 

 ever plane the head is moved. 



These impulses originate in the movements of the endolymph, 

 which take place when the head is moved and cause the pressure on 

 the hair cells of the cristse to be increased or diminished. This was 

 shown by Ewald, who connected a small tube with one canal in such 

 a way that he was able suddenly to blow into the canal so as to cause 

 the endolymph to flow towards the ampulla. Every time this was 

 done, the animal moved its head and eyes in the plane of the stimu- 

 lated canal and in the direction of the current. 



The afferent impulses arising in the semicircular canals maintain 

 equilibrium and muscular co-ordination, even after removal of the 

 cerebral hemispheres. In normal circumstances, however, they also 

 affect consciousness, and we are aware of the movements of the head. 

 Interference with these impulses in man by disease of the canals 

 brings about sensations of giddiness and disturbance of equilibrium. 



The semicircular canals are also concerned with the maintenance of 

 muscular tone, and more particularly with the variations in tone of 

 different muscles according to the position of the head and body. In 

 the rabbit, for example, when the head is raised the tone of the 

 muscles of the forelimbs is increased, the effect being abolished by 

 destruction of the semicircular canals. 



These reflex variations of muscular tone, which are known as postural 

 reflexes, play a part in the muscular adjustment of the animal to 

 changes of position, and assist in* the maintenance of equilibrium. 



The maculae acusticae are subject to constant stimulation by the 

 otoliths. The part of the macula upon which the otoliths exert 

 pressure varies according to the position of the head, and the stimu- 

 lation of the hair cells by the pressure of the otoliths is also variable. 

 The impulses to which these stimuli give rise serve to give informa- 

 tion as to the position of the head when at rest, or in progressive, as 

 distinct from rotatory, movement. 



The Muscle Sense. Even when the eyes are closed we are conscious 

 of the position of the body and limbs when at rest, and of movement 

 of the limbs, whether this be active or passive, ^his consciousness 



