GEOTROPISM 123 



a lish is down the right pectoral fin is turned more ven- 

 trally, the left fin is turned more dorsally. This means, 

 the tension of the muscles causing the right fin to press 

 down and the left fin to press up is increased. This is 

 the mechanism by which the normal "equilibrium" or 

 more correctly the normal geotropic orientation of the 

 animal is maintained. If the animal should accidentally 

 roll to one side in its normal movements, the tension of 

 the muscles of the pectoral fins would automatically 

 change in such a way as to restore the normal orientation 

 of the animal, whereby the plane of symmetry becomes 

 vertical again. This "maintenance of equilibrium" is 

 therefore a case of automatic orientation by gravitation 

 comparable to the automatic orientation by light. 



Geotropic changes in the position of the eyes are not 

 confined to fishes, 320 they can be demonstrated in a rabbit 

 and in crustaceans as well. 



In vertebrates the reactions leading to the maintenance 

 of equilibrium are apparently produced in the ear, since 

 they disappear if the acoustic nerves are cut. Moreover, 

 those parts of the brain whose injury brings about such 

 changes in the position of the eye and the fins are parts 

 of the receiving fibers from the acoustic nerve. 290 



It seems that some change in the pressure upon the 

 endings of the auditory nerve is responsible for the effects. 

 There are fine grains of CaC0 3 — the otoliths — in the ear 

 of many species pressing on the underlying nerve end- 

 ings. If we put the median plane of a fish at an angle 

 of 45° with the vertical, the otoliths will no longer press 

 down equally in both ears. The idea first suggested by 

 Delage that it is the pressure of the otoliths upon the 

 nerve endings which is responsible for these reactions 

 receives some support by a well-known experiment by 



