562 THE NERVOUS SYSTEM. 



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by a singular affection of the posture and voluntary movements, indi- 

 cating disturbance of equilibrium. These phenomena were first made 

 known by Flourens in 1825,* and have been witnessed by many sub- 

 sequent observers. They are not explained by all in the same way, 

 but there is little discrepancy in regard to their character and details. 

 The exposure of the semicircular canals during life is impracticable, as 

 a rule, in the mammalia, owing to the density of the petrous bone ; 

 but it can be done without much difficulty in birds, where they are 

 surrounded by spongy osseous tissue. The pigeon has been most fre- 

 quently used for this purpose. 



The most striking and constant effect from injury of the semicircular 

 canals consists of abnormal oscillatory movements of the head, with 

 imperfect balancing of the body. These phenomena vary according 

 to the canal which has been divided. If it be a vertical canal, the 

 oscillation of the head is upward and downward ; if it be a horizontal 

 one, the oscillations are from left to right, and vice versa. If the cor- 

 responding canal on both sides be divided, the abnormal movements 

 are more rapid and continuous than if the injury be inflicted on one 

 alone. The animal is still capable of preserving his equilibrium when 

 at rest ; but any attempt at movement brings on a disordered action, 

 which makes walking, running, or flying difficult or impossible. The 

 most simple interpretation of these results is that the animal can no 

 longer appreciate the position of the head, and that the sense of equi- 

 librium is consequently impaired for the body and limbs. 



The manner in which the semicircular canals may contribute to the 

 sense of equilibrium is as follows : If a goblet, filled with water, be 

 turned round its vertical axis, it will be seen that the water does not 

 readily turn with it ; and any small objects suspended in it, or floating 

 upon its surface, will remain in nearly the same position, while the 

 goblet revolves through an entire circle. The adhesion of the fluid to 

 the glass surfaces is not sufficient to communicate to it at once the 

 motion of the vessel. Consequently -the water lags behind the glass ; 

 and if any projecting object were cemented to the inside of the goblet, 

 so as to turn with it, it would be subjected to a backward pressure, 

 whenever the goblet was put in rotation. 



Somewhat similar conditions are present in the semicircular canals. 

 Whenever the head is rotated from side to side in a horizontal plane, a 

 momentary increase of pressure must take place in the fluid of the 

 horizontal semicircular canal (Fig. 150, 4 ), either toward or from the 

 ampulla at one end ; and this increase or diminution of pressure may 

 be perceptible by the nervous expansions there situated. If the head 

 be moved upward or downward, a corresponding change of pressure 

 will take> place in the inferior vertical canal (Fig. 150, 3 ) ; and if it be 

 inclined laterally, toward the right or left, the superior vertical canal 



* Recherches Experimentales sur les Proprites et les Fonctions du Systeme 

 Nerveux, 2me edition. Paris, 1842, pp. 452, 454. 



