26 TROPISMS 



muscles which lift the eye on one side are associated 

 with those which lower it on the other side of the head. 

 A similar association exists in regard to the pectoral fins, 

 the muscles which lift the right pectoral fin are associated 

 with those which lower the left one, and vice versa. When 

 a normal shark swims the two pectoral fins work equally 

 and the fish swims without rolling over to the right or 

 to the left. 



If we destroy in a shark the left side of the medulla 

 oblongata forced changes in the position of the two eyes 

 and the two pectoral fins will follow. 290 (There are in 

 addition correlated changes in the other fins and the rest 

 of the body which we will omit in order to simplify the 

 presentation of the subject.) When a shark, whose left 

 medulla is cut, is kept in a horizontal position, its left eye 

 looks down and the right eye looks up. This change of 

 position of both eyes indicates that the relative tension 

 between the muscles of the eyes has changed. In the left 

 eye the tension of the lowering muscles predominates over 

 that of their antagonists, in the right eye the reverse is 

 the case. The pectoral fins likewise show associated 

 changes of position. The left fin is raised up dorsally, 

 the right is bent down ventrally. Since we know that 

 the destruction of the central nervous system causes a 

 paralysis of muscles and not the reverse we must con- 

 clude that the destruction of the left side of the medulla 

 in a shark causes a weakening or partial paralysis of the 

 muscles which lower the left fin and of those which raise 

 the right fin. Hence the muscles which press down on the 

 water will press harder in the right than in the left fin. 

 When such an animal swims rapidly, it will come under 

 the influence of a couple of forces which must produce 

 a rolling movement around the longitudinal axis of its 



