Jfnnjngs, UexhiUl on Physiology of Behavior. 325 



Tlu' fundamental concept is tonus. Just what arc we to under- 

 stand by tliis ? V. Uexkiill at first defines it merely as the sum of 

 those manifestations of the life of the cell that produce effects on 

 external things (as distinguished from the internal energy used in 

 metabolism, etc.) (19, p. 78). As his work develops, he finds need 

 for a more precise idea of tonus. It is defined as a "form of 

 energy-' which has the property of flowing in certain ways (20, p. 

 4Y4). The concept of tonus gradually becomes more and more 

 definite. For purposes of handling and imaging it with ease, it 

 becomes convenient to think of tonus as a fluid, which flows through 

 a set of tubes (the nerves). This fluid becomes at last identified 

 with Bethe's "Fibrillensiiure," — an actual chemical, visible under the 

 microscope (27, p. 31). V>\\\ this identification is not held to uni- 

 formly. 



This fluid tonus is contained in a system of tubes, the nerves. 

 "The structure of the nervous system may then be conceived as an 

 aggregate of peculiar vessels united one with another, which inter- 

 change and equalize each other's contents with relation both to 

 pressure and quantity" (25, p. 305). From the nerves the tonus 

 either j)asses into the muscles, or causes in them the production 

 of a fluid with similar properties, giving rise to either tension 

 ("Sperrung") or contraction ("Vcrkiirzung''). In dealing with 

 tonus, eith(n' in the nerves or the muscles, we must distinguish its 

 quantity from its pressure; these may varj' independently, so that 

 any given quantity may have high or low pressure. On the quan- 

 litij of tonus depends the contraction of muscles; on the pressure, 

 the tension of muscles. 



There are certain general laws iv r the movements of tonus. In 

 simj)le uerve nets it always flows into muscles that are extended 

 (causing them to contract again). This is attributed to a change 

 in the capacity of the muscles ; extended muscles have greater capac- 

 ity than contracted ones, so in extending they suck, as it were, the 

 tonus out of the nerves. This property gives a remarkable degree 

 of self-regulation to the action of the nerves and muscles. 



In most animals, further, the tonus shows a marked tendency to 

 flow toward a -certain part of the body, — usually the anterior end, — 



