CHAPTER V. 



THE NATURE OF THE NERVE IMPULSE AND THE 



NUTRITIVE RELATIONS OF NERVE FIBER 



AND NERVE CELL. 



The question of the nature of the nerve impulse has always 

 aroused the deepest interest among physiologists. It has consti- 

 tuted, indeed, a central question around which have revolved vari- 

 ous hypotheses concerning the nature of living matter. The impor- 

 tance of the nerves as conductors of motion and sensation was 

 apparent to the old physiologists, and the nature of the conduction 

 or the thing conducted was the subject of many hypotheses and 

 many different names. For many years the prevalent view was 

 that the nerves are essentially tubes through which flows an ex- 

 ceedingly fine matter, of the nature of air or gas, known as the 

 animal spirits. Others conceived this fluid to be of a grosser struc- 

 ture like water and described it as the nerve juice. With Galvani's 

 discovery of electricity the nerve principle, as it was called, became 

 identified with electricity, and, indeed, this view, as will be ex- 

 plained, prevails in modified form to-day. Du Bois-Reymond, 

 after discovering the demarcation current and action current in 

 muscle and nerve, formulated an hypothesis according to which the 

 nerve fibers contain a series of electromotive particles, and by 

 this hypothesis and the facts upon which it was based he thought 

 that he had established that " hundred-year-old dream" of phys- 

 icists and physiologists of the identity of the nerve principle 

 and electricity. His theory to-day has fallen into disrepute, but 

 the facts upon which it was based remain, as before, of the deepest 

 importance. In the middle of the nineteenth century those who 

 were not convinced of the identity of the nerve principle with 

 electricity believed, nevertheless, that the process of conduction 

 in the nerve is a phenomenon of an order comparable to the trans- 

 mission of light or electricity, with a velocity so great as to defy 

 measurement. But in this same period a simple but complete 

 experiment by Helmholtz demonstrated that its velocity is, rela- 

 tively to light or electricity, exceedingly slow, 27 meters per 

 second. To many minds this slow movement and the absence 

 of a complete circuit made the electrical theory impossible. Modern 

 views have taken, therefore, divergent directions; the movement 

 or excitation that is conducted along the fiber has been named 



104 



