MAINTENANCE OF BALANCE. 



97 



nerves ; excitation of the renal nerves arrests the secre- 

 tion. 



Regulation of Locomotion (Maintenance of Balance}. 

 The motions of the body in walking and other modes of 

 progression, although under the influence of the will, are 

 regulated by centres which act in obedience to impressions 

 of which the will takes no cognizance, received from the 

 retinae, from the semi-circular canals, and other sensory 

 end-organs. These impressions have to do chiefly either 

 with the relation of the head to the plummet line, or to 

 changes of speed or direction in the motions of the head 

 or body. With reference to impressions of the latter 

 class, it is to be noted, that all felt motions give rise, on 

 their cessation, to a subjective sensation of motion in the 

 opposite direction. 



After injury of either of the crura-cerebri, animals have 

 a tendency to rotation of the body round an axis, which 

 usually lies on the side of the body opposite to that in- 

 jured. This tendency, when strong, manifests itself in 

 rolling ; when weaker, in manage motion. Similar effects 

 follow injury of the cerebellum, but in this case the axis 

 of rotation is often on the same side of the body as the 

 injury. After injury, or irritation of the semi-circular 

 canals, birds walk as if they had lost their balance, and 

 the head oscillates. The oscillation varies in direction 

 according as. the vertical or horizontal canals are inter- 

 fered with : if the former, the head moves backwards and 

 forwards ; if the latter, it is rotated from side to side. At 

 the same time, the head assumes an unnatural attitude, 

 and the body tends to fall backward or to the side. The 

 sensation of vertigo, in which the body of the affected 

 person seems to rotate round its vertical axis, is produced 

 by passive rotation in the opposite direction : when in- 

 tense, it expresses itself in actual rotation, the direction 

 of which is always opposed to that of the subjective 

 motion. Vertigo may be also produced by the passage 



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