494 



THE NERVOUS SYSTEM. 



mals the movements described affect the eyes rather than the 

 head. 



The explanation of the manner in which the canals are con- 

 cerned with equilibrium is that any change in the pressure of the 

 endolymph upon the hair-cells of the crista? acusticaB in the am- 

 pul Ia3 of the semicircular canals will produce a change in the sen- 

 sations which reach the center presiding over equilibration i. e. y 

 probably the middle lobe of the cerebellum. 



It will be seen by a reference to Fig. 284 that the canals are 



FIG. 284. Diagram of semicircular canals to show their positions in three planes 

 at right angles to one another. It will be seen that the two horizontal canals (H) lie 

 in the same plane, and that the superior vertical of one side (S) lies in a plane 

 parallel to that of the posterior vertical (P) of the other (after Ewald). 



at right angles to one another and thus occupy the three planes of 

 space : the horizontal canal of one side being in the same plane 

 with the corresponding canal of the other, and one anterior 

 vertical canal practically parallel with the opposite posterior 

 vertical canal. Thus movements of the head cause an increase 

 of the pressure of the endolymph in one ampulla and a decrease 



in that of its parallel canal on 

 the other side, and the sensory 

 impressions thus produced are 

 at once transmitted to the center. 

 When a canal is injured, its endo- 

 lymph is drained off, and at once 

 there is an interference with the 

 pressure upon the hair-cells, to- 

 gether with a consequent modifica- 

 tion of the normal impressions. 



Observation upon man confirms 

 these experiments upon the lower 

 animals. Thus a man with his eyes 

 closed lying upon a table which is 

 rotated, can tell that he is being 

 moved, in what direction, and to some extent through how great 

 an angle ; and when* the rotation ceases the sensation of rotation 

 in the opposite direction is experienced. In deaf-mutes, in whom 



FIG. 285. Diagrammatic hori- 

 zontal section through the head to 

 illustrate the planes occupied by 

 the semicircular canals : s, superior 

 canal ; P, posterior canal ; H, hori- 

 zontal canal (after Waller). 



