838 CENTRAL NERVOUS SYSTEM 



is the source of the energy expended in the conduction of a nerve im- 

 pulse. It was perhaps natural for physiologists to assume that this 

 energy was derived from the stimulus, much as the energy of a pro- 

 jectile is derived from an explosion. That this is not true has been 

 shown by important experiments by Adrian. 2 ' 3 The intangible nature 

 of the nerve impulse makes a measurement of its strength difficult. One 

 cannot use the muscular contraction produced by it as a measure of 

 its strength, because the degree of muscular response will be determined 

 by the number of nerve fibers brought into action as well as by any 

 variation which may occur in the intensity of the nerve impulse. Adrian 

 chose as a criterion of the strength of the nerve impulse its ability 

 to traverse a region of nerve fiber in which conduction had been rendered 

 difficult by the application of a narcotic. Whether a nerve impulse 

 can emerge from such a region depends on the distance which the im- 

 pulse must travel before coming to the normal part of the fiber. This 

 indicates that in passing through the narcotized area the impulse becomes 

 weaker and weaker, and if it fails to emerge in time it may become 

 completely extinguished. If the nerve impulse derives its energy from 

 the stimulus, it should be impossible for it to regain its strength after 

 dissipating its energy in a region in which conduction is difficult. This 

 point can be tested by passing the impulse through two equal narcotized 

 areas separated by a region of normal nerve. If the length of each of 

 these areas be not quite great enough to extinguish an impulse, but if 

 their combined length be sufficient for this purpose, and if no recovery 

 occurs in the intervening region of normal tissue, then the impulse 

 will be unable to pass through both areas. In passing the first area 

 the intensity will be reduced in part; on passing through the normal 

 region there will be no recovery, and in the second area of narcosis, the 

 strength of the impulse will be reduced to extinction. As a matter of 

 fact Adrian found that the nerve impulse does not behave in this man- 

 ner. Impulses which would be completely extinguished by a given area 

 of narcosis could pass through two areas one half as long if these were 

 separated by a short length of normal tissue. In passing through the 

 normal tissue the strength of the impulse recovered so that it was as 

 great on entering the second region as it was when it came to the first, 

 and consequently it was able to pass through one as well as the other 

 (Fig. 209). 



From this observation several important conclusions can be drawn. The 

 energy of the nerve impulse is derived not from the stimulus, but from 

 the nervous tissue through which it is passing. Consequently the strength 

 of the impulse depends on the condition of the tissue, and it will vary 

 only as the condition of the tissue varies. If a nerve conducts an im- 

 pulse at all, it will be of the maximum strength possible for the con- 



