THE SENSE OF MOVEMENT DYNAMIC SENSE 793 



in the direction of the rotation. These progressive deviations of the 

 hairs evoke those sensations which inform us regarding the direction 

 and extent of the movement executed by us. It should be emphasized, 

 however, that the endolymph does not move about in a circle through 

 the entire canal, but undergoes simply the slightest possible oscilla- 

 tions in the manner just indicated with the help of the preceding 

 schema. This must necessarily be so, because (a) the internal diameter 

 of the semicircular canals of man measures only 0.1 mm. (0.04 mm. in 

 the pigeon), (6) because their course is not absolutely circular, and 

 (c) because the endolymph possesses a relatively high viscosity. 



Naturally, only those hair cells can be affected by a certain gen- 

 eral movement which lie in this particular plane. It has previously 

 been mentioned that the semicircular canals act in pairs, i.e., the 

 anterior of one side is stimulated simultaneously with the posterior 

 of the opposite side. Both together control movements along vertical 

 planes. The horizontal canals also act in unison, but are chiefly 

 concerned with movements along the horizontal plane. Intermediate 

 movements always stimulate two adjoining pairs of canals but in an 

 unequal degree. There is this to be remembered, however, that 

 the primary sensation arises at the beginning, when the movement of 

 the canal is toward the ampulla and hence, when the pressure of the 

 endolymph is exerted in the direction from the utricle toward the 

 other extremity of the canal. Psychically, therefore, all movements 

 are interpreted correctly, although in a manner opposite to the position 

 of the hair-like processes. The secondary dynamic effect, producing 

 the deviation of the hairs at the end of the rotation, does not stimulate 

 unless excessive. In the latter case, a sensation of rotation is produced 

 in a direction opposite to the primary. 



Naturally, the labyrinthine sensations of movement are augmented 

 by others to form the sense of equilibrium. Chief among these are 

 the sensations of position, the muscle-sense and the sensations of sight 

 and touch. Ewald believes that all these unite in regulating the tonus 

 of the musculature which forms the basis of the stability of our body. 

 If the body sways toward one side, the stimulation of the hair cells 

 then ensuing, gives rise to an increase in the tonus of the muscles ordi- 

 narily counteracting this movement. In this way, the labyrinthine 

 reflexes are utilized, together with others, in evoking those compensa- 

 tory reactions which are directly responsible for our orientation in space. 

 This point has found substantiation in the experiments of Magnus 

 and Klijn, 1 made upon cats during the condition of decerebrate 

 rigidity. The muscles of the extremities having been rendered rigid 

 by the removal of the cerebrum, the mere tilting of the head of the 

 animal sufficed to produce perfectly definite changes in the position 

 of its limbs. Besides, these compensatory reactions disappeared 

 immediately after the destruction of both labyrinths. 



1 Pflliger's Archiv, cxlv, 1912, 455. 



