TEE SIGNIFICANCE OF TROPISMS 109 



ganic oxidation, will also be more accelerated in one retina than in the 

 other, and accordingly more intense chemical changes will take place in 

 one optic nerve than in the other. S. S. Maxwell and C. D. Snyder have 

 demonstrated, independently, that the rate of the nerve impulse has 

 a temperature coefficient of the order of magnitude which is charac- 

 teristic for chemical reactions. According to this we must conclude 

 that when two retinas (or other points of symmetry) are illuminated 

 with unequal intensity, chemical processes, also of unequal intensity, 

 take place in the two optic nerves (or the sensory nerves of the two 

 points). This inequality of chemical processes passes from the 

 sensory to the motor nerves and eventually into the muscles con- 

 nected with them. We conclude from this that with equal illumina- 

 tion of both retinas the symmetrical groups of muscles of both halves 

 of the body will receive equal chemical stimuli and thus reach 

 equal states of contraction, while when the rate of reaction is unequal, 

 the symmetrical muscles on one side of the body come into stronger 

 action than those on the other side. The result of such an inequality of 

 action of symmetrical muscles of both halves of the body is a change in 

 the direction of movement on the part of the animal. 



This change in the direction of movement can result either in a 

 turning of the head toward the source of light and the accompanying 

 movement of the whole animal toward the source of light, or in a turn- 

 ing of the head in the opposite direction and the accompanying move- 

 ment of the whole animal in the opposite direction. In order to show 

 that the choice between the two possibilities has to do with purely phys- 

 ico-chemical conditions, we should have to discuss, one by one, a whole 

 series of topics upon the physiology of the central nervous system. It 

 may suffice to call to mind briefly first that the structure of the central 

 nervous system is segmental and that the head segments generally 

 determine 5 the behavior of the other segments with their accessory 

 parts; and secondly that chemical processes in any single element can 

 cause an increase in the tonus of certain muscle groups as well as 

 causing just the opposite effect under other conditions. 



In the winged aphids the relations are as follows: Suppose that a 

 single source of light is present and that the light strikes the animal 

 from one side. As a consequence the activity of those muscles which 

 turn the head or body of the animal towards the source of light will be 

 increased. 6 As a result the head, and with it the whole body of the 



5 Loeb, "Comparative Physiology of the Brain and Comparative Psychol- 

 ogy," New York and London, 1900. 



8 If two equally powerful sources of light are present at equal distances 

 from the animal, the animal will move in a line at right angles to a line con- 

 necting the two sources of light, because in this case both eyes are similarly 

 influenced by the light. Herein, as Bohn has rightly said, the machine -like 

 heliotropic reaction of animals differs from the movement of a human being 



