188 THE INTELLIGENCE SERVICE 



or from carbonates from the action of acids set free electrolytically 

 by either the impulse or the stimulus. 



Hill has found that when a nerve is placed in a carefully shielded 

 thermopile, the registering galvanometer shows unaccountable 

 fluctuations corresponding to temperature changes of the order 

 of 7 xlO ~ 6 of a degree C. The passage of some 600 impulses does 

 not produce any larger variation. Therefore, the amount of 

 heat produced must be less than this figure. 



11. Electrical changes during conduction. Just as in muscle, 

 so in nerve, an electrical wave accompanies the nervous impulse. 

 The part excited becomes electro-negative to the rest (see Muscle, 

 p. 143), and this negative wave passes along the nerve in the 

 direction of and at the same rate as the nervous impulse. It is 

 followed by an electro-positive wave of greater potential. The 

 combined electrical changes are thus said to be diphasic. The 

 electrical wave complex must not be confused with the nervous 

 impulse. In other words the nervous impulse is not electrical 

 in nature but produces localised differences of potential in nerve 

 as it passes. The rate of conduction of an electrical impulse along 

 frog's nerve is about 300 metres per second, while the nervous 

 impulse travels at about 30 metres per second. 



12. Electrical changes during stimulation. This wave of 

 negativity is the current of action (or of injury) of the nerve. 

 It may be made manifest as indicated in Fig. 35, by rapidly 



s*^ p\ Polarising 



ft\ \ 



a/ -* k 



/ 

 * Positive polarisation 



FIG. 35. Diagram to show direction of the positive polarisation current, due 

 to a break excitation at the anode. 



connecting anodal and cathodal parts of a nerve through a gal- 

 vanometer. If a .stimulating current is passed through a section 

 of a nerve so that a is the positive electrode and k is the negative 

 electrode, then on breaking the circuit and connecting a and k 

 through a galvanometer (circuit 2), it will be obvious that a 

 current flows momentarily along the nerve from a to k, i.e. in 

 the same direction as the previous stimulating current. This is 

 sometimes termed a positive polarisation current. 



Under certain conditions it is possible to observe an electrical 

 change in the opposite direction after cessation of the stimulus 



