756 NERVE 



electrical change accompanies the nerve-impulse travelling at the 

 same rate, and although this is to be distinguished from the impulse 

 itself, there is little doubt that the latter is essentially connected 

 with a disturbance of the electrical equilibrium of the nerve- 

 substance. 



An attempt has been made to settle the question by determining the 

 temperature coefficient of the velocity of conduction of the impulse 

 i.e., the quantity which measures the change of velocity for a given 

 change of temperature. For most physical processes the quotient 



velocity at Tn + 10 . 



i TI pp where Tn is any given temperature, is not greater 



than 1-2, while for frog's sciatic nerve the temperature coefficient for 

 the most part lies between 2 and 3 (Snyder). The mean value of a 

 large number of observations is 1-79, with Tn= 8 to 9 C. (Lucas). For 

 the pedal nerve of the giant slug the mean value of the temperature 

 coefficient is 1-78 (Maxwell). In other words, while for the majority of 

 physical processes an increase ol 10 C. increases the velocity of the 

 process by at most one-fifth, the same increase of temperature increases 

 the velocity of conduction of the nerve-impulse by four-fifths, or even 

 more. While it is true that it may not be entirely safe to apply such a 

 criterion to a biological process which need not be either entirely chemical 

 or entirely physical, and very likely is a complex one, the suggestion, 

 so far as it goes, is undoubtedly in favour of the chemical hypothesis. 

 That chemical changes go on in living nerve we need not hesitate to 

 assume; and, indeed, if the circulation through a limb of a warm- 

 blooded animal be stopped for a short time, the nerves lose their 

 excitability. Even the nerves of cold-blooded animals gradually 

 become inexcitable and incapable of conduction when placed in an 

 oxygen-free medium, as the oxygen already contained in the tissue is 

 exhausted. The excitability and conductivity of the nerve are restored 

 by oxygen. It is clear, then, that even a resting nerve requires oxygen, 

 and it ca.n be shown that the loss of function is acceleiated by stimulation 

 in the absence of oxygen. But the metabolism is very slight compared 

 with that in muscle or gland. Until recently even in active nerve no 

 measurable production of carbon dioxide had ever been observed, nor, 

 in fact, had any chemical difference between the excited and the resting 

 state ever been unequivocally made out. However, it has been 

 announced that by the aid of an extremely delicate method of estimating 

 small quantities of carbon dioxide, a measurable production of carbon 

 dioxide can be detected even in resting frogs' nerves, and that this pro- 

 duction is increased two to three times on stimulation (Tashiro). This 

 result is somewhat puzzling in view of the fact that neither in cold- 

 blooded nor in mammalian nerves is there any sensible rise of temper- 

 ture during stimulation. With the apparatus shown in Fig. 265 (an 

 electrical resistance thermometer or bolometer whose use depends upon 

 the fact that the electrical resistance of a metallic conductor varies 

 with its temperature) an increase even of 0-0003 C. in the temperature 

 of the sciatic nerves of dogs could not be detected during tetanization. 

 Rolleston failed to find evidence of a rise of even 0-0002 C. in frog's 

 nerves during stimulation. And according to the latest investigation 

 with a more suitable and much more sensitive thermo-electric arrange- 

 ment, the passage of a single nerve impulse along a frog's nerve cannot 

 be associated with an increase of temperature in the nerve of even the 

 hundredth million of a degree (A. V. Hill). The difficulty of inducing 

 fatigue in nerves under ordinary conditions has been considered a strong 



