154 COMPARATIVE PHYSIOLOGY OF THE BRAIN 

 ments have also been noticed in dop;s and rabbits 



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after an operation on one side of the medulla. 



If a fish whose progressive movements are deter- 

 mined by the sculling motions of the tail turns to the 

 right, the tail moves with greater force toward the 

 right than toward the left. This condition might be 

 made permanent if it were possible to weaken the 

 muscles on the left side of the spinal column. This 

 occurs when the right side of the acoustic segment of 

 the medulla is destroyed. The fish moves in a circle 

 toward the right. We also obtain circus-motions to- 

 ward the right if we destroy the ventral portion of the 

 left optic lobe. Hence, fibres must pass from the 

 ventral portion of the left optic lobe to the right 

 acoustic segment of the medulla. After such an 



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operation, an increase in the tension of the skeletal 

 muscles occasionally shows itself, for the fish may lie 

 permanently bent into a circle without being able to 

 straighten itself out again. 1 Such a fish can no longer 

 swim straight ahead. The difference in the tension 

 of the muscles on the two sides of the animal is, how- 

 ever, usually not so great, in which case the circus- 

 motions will appear only spasmodically, for example, 

 when the animal is excited. 



One-sided division of the spinal cord and of the 

 medulla behind the acoustic segment produces no 

 forced movements (2 and 3). On the other hand, roll- 



1 If such a fish be decapitated the curvature of the body remains. It may 

 even remain after death. We have to deal with an organic change in the 

 muscles, caused by the operation. 



