TROPISMS AND RELATED PHENOMENA 139 



the current curves of radiating energy, e.g. light rays, strike an animal 

 on one side only, or on one side more strongly than on the symmetrical 

 side, the velocity or the kind of chemical reactions in the symmetrical 

 photosensitive points of both sides of the body will be different. The 

 consequence will be in a positively heUotropic animal a stronger ten- 

 sion or tendency to contract in the muscles connected with the photo- 

 sensitive points of the one side of the body than in those connected 

 with the opposite side. It seems that in animals the region at the oral 

 pole is, as a rule, more sensitive than the rest of the body. Consequently 

 the tension of the muscles determining the position of the head or oral 

 pole is more intensely affected by differences in the intensity of light 

 than that of the muscles of the rest of the; body. The head is conse- 

 quently bent until its symmetrical photosensitive points are again 

 struck at the same angle by the rays of light. The tension of the sym- 

 metrical muscles of the head then again becomes equal, and the head 

 must remain in this position unless other forces disturb its orientation. 

 The rest of the body follows the orientation of the head, a point which 

 is more fully discussed in my book on Brain Physiology. 



Aside from the data given in the previous lecture on this subject, 

 two more facts support this view. The one-sided section or destruc- 

 tion of certain parts of the brain causes a diminution in tone in the 

 muscles which turn the body toward one side. The consequence is 

 that animals in which such an operation has been performed, no longer, 

 or only with difficulty, are able to move in a straight Une, moving instead 

 constantly in a circle or spiral.* We speak in such cases of forced 

 movements. The same condition which is brought about in a more 

 permanent way by certain one-sided lesions of the brain can be pro- 

 duced transitorily by a one-sided illumination of the photosensitive 

 surface of a highly heliotropic animal, with this difference only, that 

 the very difference in the tension of the muscles and the forced move- 

 ment resulting therefrom leads to a remedy of the evil by bringing the 

 symmetrical points of the animal back into a position where they are 

 struck at the same angle by the lines of force. 



The second fact in support of this conception is that when the photo- 

 sensitive elements on one side of the body are ehminated, the animal is 

 compelled to move in a circle. S. J. Holmes f and Parker \ have 

 indeed found that such is the case. I will quote Parker's observation 

 on the subject. His experiments were made on a butterfly (Vanessa 



* Loeb, Comparative Physiology of the Brain, p. 150. 

 t S. J. Holmes, Am. Jour. Physiology, Vol. 5, p. 211, 1901. 



X G. H. Parker, The Phototropism of the Mourning-cloak Butterfly {Vanessa Antiopa), 

 Mark Anniversary Volume, 1903. 



