THE SENSE OF HEARING. 



J 69 



scillate from front to rear, with a tendency to fall forward. A sec- 

 tion of all the canals is followed by contortions of the most bizarre 

 lature. After a destruction of all the canals the animal cannot main- 

 in his equilibrium. 



Similar phenomena have been observed in man in disease of the 

 ;micircular canals, known as Meniere's vertigo. In the fixed position 

 >f the head there is equilibrium, but with each movement the vary- 

 ig tension of the liquid in the ampulla changes and irritates the 

 ur-cells. 



The horizontal, semicircular canals form the arc of a circle, with 

 ampulla at each end. In rotation of the head to the right, the 

 idolymph in the ampulla of the right horizontal canal will accumu- 



Fig. 319. Twisting of the Head of a Pigeon twenty days after 

 removal of all the semi-circular canals on the right side. (EWALD, 

 J. R.) (From Tigerstedt's "Human Physiology," copyright, 1906, by 

 D. Appleton and Company.) 



ite in the ampulla because the membranous canal is very narrow. 



lis will cause a high pressure in the ampulla. 



These ampullae and canals are, then, sensory organs, and give the 



limal an idea of the position of his head in space. Now, as the canals 

 ire at right angles to each other according to the three dimensions in 

 space, their section makes the animal unable to know the position 

 )f his head and thus produces vertigo. Cyon's theory that the semi- 

 circular canals give us a series of unconscious sensations as to the 

 sition of our heads in space. (See cerebellum.) 

 Ewald holds that all the muscles of the body are kept in a state of 



mus by means of the semicircular canals, and that injury to them 

 iffects those muscles whose movements are most delicate, such as 



lose of the eye and larynx. The loss of tonus may be explained for 



>me of the muscles by disturbances in the reflex arc of the vestibular 



irve, Deiters's nucleus, and the vestibulo-spinal tract. Here is a 



