CHAPTER III 



WHEN we destroy or injure the brain on one side we 

 paralyze or weaken the muscles connected with this side. 

 As a consequence the morphological plane of symmetry 

 ceases to be the dynamical plane of symmetry and the 

 animal has a tendency to move in circles instead of in 

 a straight line. Suppose a fish swimming forward by 

 motions of its tail fin. Normally the stroke occurs with 

 equal energy to the right and to the left, and the rudder 

 action of the tail is equal in both directions, but after 

 the lesion of one side of the brain the stroke and the 

 rudder action cease to be the same in both directions, it is 

 weakened in one direction. Hence the animal instead of 

 swimming in a straight line is forced to deviate contin- 

 ually toward one side from the straight line of locomo- 

 tion. We speak in such a case of a forced motion. 



When we destroy the ventral portion of the left optic 

 lobe in a shark (Scyllium canicula), the fish no longer 

 swims in straight lines but in circles to the right (when the 

 right optic lobe is destroyed it swims in circles to the 

 left).. After the destruction of the left optic lobe, the 

 muscles on the left side of the tail are weakened or semi- 

 paralyzed, and they no longer produce the same rudder 

 action as the muscles on the right side. Hence the im- 

 pulses (or nerve processes) which flow in equal intensity 

 to the muscles on both sides will no longer produce equally 

 energetic rudder action of the tail to the right and to the 

 left, but the muscles turning the tail to the right will 

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