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 characteristic 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 connected with them. We conclude from this that with equal illumination 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 turning of the head in the opposite direction and the accompanying movement of the whole animal in the opposite direction. In order to show that the choice between the two possibilities has to do with purely physico-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 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. As a result the head, and with it the whole body of the
- Loeb, "Comparative Physiology of the Brain and Comparative Psychology," New York and London, 1900.
- 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 connecting 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