SEMICIRCULAR CANALS AND THE VESTIBULE. 409 



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 equililDrium after injuries to 

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

 that false sensations are experienced and false compensator}' move- 

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

 rotation may be attributed to false sensations aroused by the 

 after-action of the sense cells of the ampullae. 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.* 

 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- 

 circular canals, are too indistinct to be recognized and named 

 by an appeal to consciousness, and it would seem to be wiser 

 to designate them after the analogy of the muscle sensations 

 simply as semicircular canal or labyrinthine sensations. Our 

 perceptions or ideas of space and direction are possibly founded 

 in part upon these reactions and in part upon the muscular, 

 visual, and tactile sensations. Our reasoning with regard to 

 the semicircular canal sensations would be more satisfactory 

 if it could be shown that the vestibular nerve, after ending in 

 the pons, was continued forward by sensory paths to the cortex 

 of the cerebrum. As a matter of fact, such paths have not 

 been demonstrated, and if we assume that conscious sensations 

 are mediated only through the cortex of the cerebrum we have 

 no anatomical proof that the semicircular canals give us any 

 * Magnus and Ivlijn, "Pfluger's Archiv," 1912, 145, 455. 



