FORCED MOVEMENTS 25 



contract more powerfully than those turning it in the 

 opposite direction. The outcome of this greater rudder 

 action of the tail when moving to the right is that the 

 fish instead of swimming in a straight line moves in a 

 circle to the right. 290 



It is often the case that the body of such a fish even 

 when quiet is no longer straight but bent in a circle, the 

 left side forming the convex side ; and when such a fish 

 dies and rigor mortis sets in it may become stiff in this 

 position. These latter observations prove that the circus 

 movements to the right are due to the lowering of the 

 tension of the lateral muscles of the body on the left side 

 of the fish. This is the fundamental fact for the theory 

 of forced movements — namely, that a lesion in one side 

 of the brain lessens the tension of the muscles on one side 

 of the body; as a consequence the motions of the animal 

 become difficult or impossible in one direction and become 

 easy in the opposite direction. 



In many cases the motions of an animal depend upon 

 a cooperative activity of two sets of appendages, e.g., 

 the pectoral fins of a fish or the legs of an animal. Such 

 cooperative or associated action is determined by the 

 fact that the same nerve center supplies antagonistic 

 muscles of the two organs {e.g., the lateral fins). Thus 

 the same nerve impulse causes both our eyes to move 

 simultaneously to the right or to the left. When we look 

 to the right, the same impulse which causes the contrac- 

 tion of the rectus externus muscle in the right eye causes 

 a contraction of the rectus internus muscle in the left 

 eye. These two muscles then are associated. 



In a fish like the shark the position and innervation 

 of the eyes differ from that of the human being. In the 

 shark the eyes are not in front but on the side, and the 



