SECTION III 



EVENTS ACCOMPANYING THE PASSAGE OF A 

 NERVOUS IMPULSE 



IN muscle we saw that the passage of an excitatory wave was 

 accompanied or followed by electrical changes, production of heat, and 

 mechanical change, all pointing to an evolution of energy from the 

 explosive breaking down of contractile material. 



In nerve, however, which serves merely as a conducting medium, we 

 should not expect so much expenditure of energy, or in fact any 

 expenditure at all. All that is necessary is that each section of the 

 nerve should transmit to the next section just so much kinetic energy 

 as it has received from the section above it. And experiment bears 

 out this conclusion. The most refined methods have failed to detect 

 the slightest development of heat in a nerve during the passage of 

 an excitatory process, and we know already that there is no mechani- 

 cal change in the nerve. The only physical change in a nerve under 

 these circumstances is the development of a current of action. A nerve 

 becomes, when excited at any point, negative at this point to all other 

 parts of the nerve, and, just as in muscle, this ' negativity ' is pro- 

 pagated in the form of a wave in both directions along the nerve. 



That the excitatory process in nerves is probably accompanied 

 by certain small chemical changes is indicated by the facts that, in the 

 complete absence of oxygen, the nerve fibres lose their irritability, and 

 that this loss of irritability is hastened by repeated stimulation of the 

 nerve. When the irritability has been abolished by stimulation in 

 the absence of oxygen, it may be restored within a few minutes by 

 readmission of oxygen to the nerve. 



If we connect a galvanometer to two points of an uninjured nerve, 

 no current is observed, all points of a living nerve at rest being iso- 

 electric. On making a cross-section of the nerve at one leading-off 

 point, a current is at once set up, which passes from the surface through 

 the galvanometer to the cross-section. This is a demarcation current, 

 set up at the junction between living and dying nerve. This current 

 rapidly diminishes in strength and finally disappears, owing partly 

 to the fact that the dying process started in the nerve by the section 

 extends only as far as the next node of Ranvier and there ceases, 

 so that after a short time the electrode applied'to the cross-section is 



287 



