228 THE INTELLIGENCE SERVICE 



a nerve during the supernormal period may cause the motor end- 

 organ to act. Such a phenomenon is called summation. 



11. Fatigue. Nerve fibres can apparently act as conductors of 

 the nervous impulses for very long periods without showing any 

 signs of fatigue. It is generally said that nerves cannot be 

 fatigued. While this is true of the conducting power of the fibre 

 it is not applicable to the neuron as a whole. (1) The nerve cell 

 loses something in the process. Granules which are apparent while 

 the cell is at rest diminish slowly during activity. Then (2) 

 changes take place at the synapses, the junction between neuron 

 and neuron, and also at the " end plate " or junction between 

 nerve fibre and organ. These potential junctions lose their power 

 to cause the impulse of one neuron to act as stimulus to the next 

 neuron or to the end-organ. They become fatigued. 



12. Metabolism. This leads one to infer that the energy 

 exchanges during the conduction of ihipulses are small. There is 

 no doubt of the need for oxygen for the metabolic changes of the 

 nerve cell, but the extra amount necessitated by the passage of a 

 nervous impulse has not been estimated. Ingenious methods have 

 been devised by Waller, by Tashiro, and by Hill for the measure- 

 ment of the COo evolved during activity. Hill has found that the 

 heat and CO., liberated in nerve activity represent an appreciable 

 amount of metabolic change. 



13. Temperature coefficient of the nervous impulse. When a 

 length of nerve is cooled its power to conduct an impulse is 

 decreased ; that is, nerve-conduction has a positive temperature 

 coefficient. It was pointed out by Van't Hoff that the velocity 

 of chemical reactions is increased twofold or more for each ten 

 degrees in temperature (Centigrade scale), i.e.. the temperature 

 coefficient for chemical reactions is greater than 2. On the 

 other hand, the temperature coefficient for physical processes 

 is less than 2. The temperature coefficient, i.e. ratio of velocity 

 of propagation of nervous impulse at (T + 10)° to its rate at 



T° = — . has been estimated hy Lucas as approximately 



V ait T 



1-8. This value has been proved to be right by later workers. 

 Therefore, physical factors, as well as chemical reactions, are 

 involved in the propagation of a nervous impulse. 



11-. Polarisation, {a) Negative polarisation. A disturbing arte- 

 fact is produced when a medullated nerve is stimulated and leads 

 are taken to a galvanometer from two points on its length. In 

 Fig. 56 we have a diagram of such a circuit. The upper circuit is 

 the polarising one, i.e. at the anode, positive charges develop in the 

 sheath (due supposedly to the dissociation of electrolytes by the 



