98 THE NERVOUS SYSTEM 



chemical change of some sort passing along the nerve. As is well recog- 

 nized, there is a tendency in Physics today to unify so-called molecular, 

 electrical, and chemical changes. It becomes more certain continually, 

 therefore, that the nervous impulse is of a nature somewhat akin 

 to these. That it is any one of them alone, we cannot at present say. 

 Whatever be the nature of the impulse, it can be shown that it passes 

 progressively from one portion of a nerve to the next at a rate not very 

 rapid, and that it produces various changes on its way, especially of an 

 electrical nature. Whether there is an electrical variation during the 

 passage along a nerve which is absolutely normal in an unharmed 

 animal, we are not certain. It is probable that an exceedingly small 

 amount of heat also is produced by the process of nervous conduction, 

 although all attempts to measure it have so far proved futile, even 

 with very delicate electrical thermometers. 



Whatever its nature, the nervous impulse conveys exciting energy 

 rather than efficient energy. It is the protoplasm of the muscle or the 

 epithelium which provides the energy for its work the nervous impulse 

 only activates it and changes it from potential to kinetic. The nerve- 

 energy is many millions of times the smaller in amount. 



The speed of the nervous impulse varies in different conditions and in 

 different nerves. Thus, in general, heat somewhat accelerates this as it 

 does most other functions, and we find the rate higher in the so-called 

 warm-blooded animals (homotherms) than in cold-blooded animals 

 (poikilotherms). A frog's sciatic shows a rate varying between 10 and 

 27 meters per second, according to the time of year, etc. In the 

 afferent nerves of man the speed is 45 or even 50 meters per second. 

 In the efferent channels the rate of conduction is less, and ranges from 

 30 to 35 meters per second. It seems to vary not a little in different 

 nerves even of the same general sort. (See Appendix, Experiment 79.) 



Nerves conduct their currents in both directions, in the abnormal as 

 well as in the normal direction. This is readily shown in the common 

 laboratory experiments with the sartorius muscle split at its iliac end. 

 (See Appendix, Experiment 77.) Whether or not the nerves of the 

 uninjured animal ever transmit impulses in two directions is not known. 

 The ordinary presumption is that they do not, because of the arrange- 

 ment of their terminal connections, if for no other reason. 



Chemical agents effect the conductivity of nerves in various compli- 

 cated ways. (See the experiments in the Appendix.) For example, 

 carbon dioxide poured about an isolated frog's sciatic lessens the 

 excitability of the nerve but does not change its conductivity. On the 

 other hand, the vapor of ethyl alcohol stops the transmission of the ner- 

 vous impulse but does not lessen the irritability of the nerve-protoplasm. 

 From work by Overton and by Myer it appears likely that this action of 

 alcoholic vapor is the type of the action of all anesthetics on the nervous 

 system. 



Electricity in constant current acts in a more complicated way, 

 for at the anode the excitability is lessened (anelectrotonus), while at 



