56 EXPERIMENTAL PHYSIOLOGY. 



at the make whether the current be weak, medium or strong. 

 But with this position of the kathode, contraction is obtained at 

 the break when only a medium current is used. The break stimulus 

 always being less effective than the make, its effect, when a very 

 weak current is used, will be below the threshold of stimulation. 

 On the other hand when very strong currents are used, the break 

 stimulus being anodal, is blocked in the nerve by the development 

 of electrotonic changes, so that the impulse does not reach the 

 muscle. 



When the ANODE is NEAR THE MUSCLE, a weak current causes a 

 contraction at the make, but has no effect at break, not having 

 reached the threshold. With medium currents both make and 

 break are effective. In the case of strong currents, the break is 

 effective because it occurs at the anode, which is near the muscle 

 with nothing to block it. The make occurring at the kathode 

 has to traverse the whole region of the anode at the time when 

 the anode produces its greatest block to conduction, i.e., immedi- 

 ately after making the current. 



We do not know the nature of the nerve impulse, but whatever 

 it is, an electrical change always accompanies it. This can be 

 shown by connecting a sensitive galvanometer to the nerve. For 

 the same reasons as in the muscle, the galvanometer will show a 

 DIPHASIC VARIATION, that is a deflection, first in one direction and 

 then in the opposite direction. 



METABOLISM IN NERVE. 



The metabolism in nerve is so slight that it is very difficult to 

 obtain indications that such changes are going on. 



So far it has been impossible to demonstrate the production 

 of heat in nerve. Hill has used a thermo-electric couple sensitive 



to ioo^boiooo c without a posit!ve result ' 



Every living cell respires, i.e., it is constantly producing CO 2 but 

 cells manifesting increased activity such as a contracting muscle 

 show an increase in CO 2 production. Tashiro claims that the 

 quantity of CO 2 produced by a nerve in action is more than double 

 that produced by a resting nerve. If this is correct it indicates 

 metabolic changes during the passage of the nervous impulse. 



