138 THE PHYSIOLOGY OF NERVE 



have also been taken of the negative variations in the depressor nerve on increasing 

 the blood-pressure in the aorta 1 and of those occurring in the vagus nerve synchro- 

 nously with the respiratory movements. 2 



The Relation of the Nerve Impulse to the Wave of Negativity 

 and the Action Current. The preceding discussion must have satis- 

 factorily proven that the wave of negativity and the nerve impulse 

 are practically synonymous phenomena, because they advance with the 

 same velocity and cannot be dissociated by any known means. 



A nerve impulse may be generated by mechanical, electrical, ther- 

 mal, photic and chemical means, and may be the result of either director 

 indirect (reflex) stimulation. If regarded as a purely physical phe- 

 nomenon, it will be seen immediately that the impulse must consist 

 solely of a wave of negativity, while if considered as a chemical phe- 

 nomenon, it must be the product of certain chemical changes. In 

 accordance with the second view, which is the more widely accepted 

 at the present time, the nerve impulse consists of a progressive chem- 

 ical process entailing catabolism and anabolism. One of the results 

 of these changes is the wave of negativity which thus assumes the 

 character of a true current of action. This relationship having been 

 established, the negative wave is to be regarded as an associative phe- 

 ' nomenon of the chemical changes. Hence, the phenomena of conduc- 

 tion in nerve are very similar to those taking place in muscle whenever 

 a wave of contraction sweeps over its constituent fibers. The evidence 

 favoring this chemico-physical explanation of the nerve impulse, is 

 chiefly derived from the fact that the conduction in nerve entails 

 certain metabolic changes, which will be more fully discussed in the 

 succeeding paragraphs. 



The Metabolism of Nerve During Activity. In accordance with 

 the observation that contracting muscle yields lactic acid, carbon di- 

 oxid and other fatigue substances, efforts have repeatedly been made 

 to show that these bodies are also formed in active nerves. Inasmuch 

 as the functional capacity of nerve varies directly with the carbon 

 dioxid content of the air surrounding it, A. D. Waller 3 assumed at an 

 early date that this gas is actually liberated in the course of the activity 

 of this tissue. It has recently been proved by Tashiro 4 that this as- 

 sumption is correct. By employing an extremely delicate indicator 

 it co,uld be shown that even the resting nerves of frogs produce a 

 measurable quantity of carbon dioxid, and besides, it was found that 

 this amount may be greatly increased by stimulation. Positive evi- 

 dence of nerve metabolism has also been furnished by Bayer 5 and 

 Frohlich, 6 because these investigators have shown that oxygen is abso- 



1 Tschermak, Pfltiger's Archiv, xciii, 1903, 24. 



2 Lewandowsky, Pfliiger's Archiv, Ixxiii, 1898, 298; also see: Einthoven, Quart. 

 Jour, of Exp. Physiol., i, 1908, 243. 



3 Brain, Ixxvi, 1897, 569, and Proc. R. Soc., London, Ixii, 1897, 80. 



4 Am. Jour, of Physiol., xxxii, 1913, 137. 

 6 Zeitschr. fur allg. physiol., ii, 1903, 169. 

 Ibid., iii, 1904, 131. 



