1 66 HUMAN PHYSIOLOGY 



If, for example, the head is suddenly turned to the right, 

 the inertia of the endolymph and perilymph tend to make 

 them lag behind. Thus the endolymph in the ampulla of 

 the right horizontal canal will tend to flow into the canal, 

 but the canal is so small that it will merely accumulate in 

 the ampulla, and thus a high pressure will be produced (Fig. 

 89, b + +). The perilymph will tend to lag behind, and a 

 low pressure will result outside (Fig. 89, b ). The converse 

 will take place in the opposite horizontal canal. 



When the movement is continued the pressures will be 

 readjusted, and, on stopping the movement, the opposite 

 conditions will be induced, and a sensation of moving in an 

 opposite direction will be experienced. 



In forward movement, the two superior canals have the 

 pressure of endolymph increased in their ampullae in back- 

 ward movement this occurs in the two posterior canals. In 

 nodding to the right the superior and posterior canals of the 

 right ear undergo this change. 



There is also evidence that the semicircular canals assist 

 in maintaining the tone of the skeletal muscles and that 

 destruction of the canals is followed by a loss of tone. This 

 might be expected from their intimate connection with the 

 cerebellum. 



When the information as to our relationship with our 

 surroundings derived from these various sources is not 

 concordant e.g. when through the semicircular canals we 

 have a sensation of movement, and through the eyes an 

 apparent absence of movement balancing becomes difficult, 

 and a feeling of giddiness results. This may be readily 

 demonstrated by setting a poker vertically on the floor, 

 holding it in the hand, placing the forehead on the top, 

 walking rapidly three times round it, then standing up and 

 trying to walk out of the room. The sudden stoppage of 

 the rotatory movement causes a disturbance in the semi- 

 circular canals giving a sense of rotation in the opposite 

 direction, while the eyes tell us that no rotation is taking 

 place. The feeling of giddiness is, however, not the cause 

 of the loss of balancing, but a mere accompaniment. 

 (Experiment.) 



