SEMICIRCULAR CANALS AND THE VESTIBULE. 377 



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. Cyon has advocated the view that the semicircular canals con- 

 stitute an organ for the perception of space in its three dimensions. 

 Each canal or pair of canals gives us the sense of direction in its own 

 plane, and the fact that we have three pairs in planes at right angles 

 to one another gives the physiological foundation of our conception 

 of three dimensional space. On this fundamental conception of 

 space is projected the additional space conceptions derived from our 

 visual, tactile, and muscle senses. This author is not specific in 

 stating by what means the sensory cells in the three canals are 

 stimulated. In addition to the sensations of direction and of space 

 furnished by the canals, the nerve impulses from them are supposed 

 to co-ordinate the action of the motor centers concerned in move- 

 ments of the head and body. 



5. 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. On this view, therefore, the semi- 



* Consult Lee, loc. dt. 



