410 THE SPECIAL SENSES. 



movements in the horizontal plane, the hair cells in one responding 

 chiefly to movements in one direction, the other to movements in 

 the same plane, but in the opposite direction. Rotational 

 movements in other planes sagittal, oblique, etc. would 

 affect two or more of the pairs of canals in proportion to the 

 degree that each is involved in the movement on the principle of 

 the parallelogram of forces.* By a mechanism of this sort it 

 may be supposed that we are informed regarding the plane, di- 

 rection, and extent of the movements of the head and are thereby 

 enabled to control these movements. The canals function es- 

 pecially as a dynamic organ of equilibrium, but may also give us 

 guiding sensations when the movements are progressive rather than 

 rotational, and also when the head is at rest, although, as is ex- 

 plained below, this last function is by some relegated to the hair 

 cells of the utriculus and sacculus. According to this view, the 

 loss of the power of maintaining exact equilibrium after injuries to 

 the canals or section of the nerves may be explained by supposing 

 that false sensations are experienced and false compensatory move- 

 ments are made. So, also, the vertigo experienced after continued 

 rotation may be attributed to abnormal stimulation of these sense 

 organs, a view that finds some support in the fact that many 

 deaf-mutes, whose internal ear is supposed to be deficient, do not 

 experience vertigo after rotation, and in animals with the labyrinth 

 destroyed rotational movements fail to give the symptoms of 

 vertigo. 



4. Ewald, while accepting the general view that the sense cells 

 are stimulated by the pressure of the endolymph, lays stress upon 

 the fact that the nerve impulses thus aroused have, as their main 

 result, a reflex effect upon the tonicity of the voluntary muscula- 

 ture. The constant flow of impulses from these organs serves to 

 maintain the muscles in a normal condition of tone. In animals 

 with the labyrinth destroyed on both sides the body musculature 

 is flabby and lacking in tonicity. This view has received consid- 

 erable support from the experiments of Magnus and Klijn.f 

 These authors made use of cats thrown into the condition of exag- 

 gerated tonus, known as decerebrate rigidity, which follows upon 

 removal of the cerebrum (section of the cerebral peduncles) . With 

 the animal in this condition they showed that different positions 

 of the head in space caused definite changes in the posture of the 

 extremities, and they demonstrated that these changes were due 

 to labyrinthine reflexes, since they disappeared upon destruction 

 of both labyrinths. 



Summary. The sensations, if any, aroused through the semi- 



* Consult Lee, loc. cit. 



t Magnus and Klijn, "Pfliiger's Archiv," 1912, 145, 455. 



