NERVE 123 



which the otoliths press at any time as a result of the force 

 of gravity. 



Such a static mechanism accounts for the adjustment 

 of the optic axis in the blind according to the position of the 

 head. 



(2) That movements of the head play an important part 

 is sho^vn by the fact that, when an animal, deprived of its 

 labyrinths, attempts to move, the most marked disturbances 

 of muscular action occur. It is still more clearly indicated 

 by a consideration of how the system of semicircular canals 

 is influenced by any acceleration or retardation of move- 

 ments of the head. 



If the head is moved in any plane, certain changes are 

 set up in the ampulla or ampullae towards which the head is 

 moving, and converse changes in the ampulla or ampulla3 at 

 the other end of the arc of the circle. 



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

 the inertia of the endolymph and perilymph tends to make 

 them lag behind. Thus the endolymph in the ampulla of 

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

 but the canal is so small that the fluid will merely accumulate 

 in the ampulla, and thus a high pressure will be produced 

 (fig. 55++). The perilymph will tend to lag behind, 

 and a low pressure will result outside (fig. 55 — ). 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. 



In forward nodding movement of the head, the two 

 superior canals have the pressure of endolymph increased in 

 their ampulige — in backward 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. 



B. Labyrintho- Cerebral Arcs. — Modifications of Conscious- 

 ness.— These changes, which can be studied only in man, act, 

 not only in bringing about an adjustment of the balance 

 through the cerebellar arc, but also on account of 

 the connections with the cerebrum, in modifying con- 

 sciousness, giving a sensation of movement in the 



