THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 97 



assume that the process is essentially a physical change (prob- 

 ably of an electric nature) which involves no chemical altera- 

 tions, no consumption of material, no metabolism. 



But by means of recently devised apparatus of extreme deli- 

 cacy Tashiro has shown very clearly and quantitatively that 

 the resting nerve-fiber eliminates CO 2 and that during functional 

 activity caused by stimulation the amount of CC>2 is increased 

 to about double that of the resting nerve. The same investiga- 

 tor subsequently showed that the amount of CC>2 given off by 

 nerve-fibers is quite as great per unit of weight as that given off 

 by the nerve-cell bodies of the ganglia. Tashiro has shown, 

 moreover, that the rate of C02 production is greater in that por- 

 tion of a nerve-fiber which lies nearer to the source of the stimu- 

 lus than in a similar portion of the same nerve-fiber farther from 

 the receptive end and nearer to the discharging end. This ap- 

 plies to both sensory and motor fibers. Child has confirmed this 

 by showing that different parts of the nerve-fiber show differ- 

 ences in susceptibility to certain poisons corresponding to the 

 differences in rate of oxidation of their substance. .There is, 

 accordingly, a physiological gradient in the nerve-fiber, the physi- 

 ological activity diminishing in the direction of the normal con- 

 duction of the nervous impulse. The neuron is thus seen to 

 have an intrinsic physiological polarity of its own quite apart 

 from that occasioned by the irreversible character of the 

 synapse (see p. 53). 



It is, therefore, probable that the transmission of a nervous 

 impulse involves a wave of chemical change throughout the 

 length of the nerve-fiber, though a change of a quite different 

 character from that occurring in the cell body during its func- 

 tional activity. That the nervous conduction is not a simple 

 electric discharge through a free conductor, nor any other 

 sort of simple ethereal or molecular vibratory wave motion, is 

 evident from the fact that its velocity of propagation through 

 the nerve-fiber, which is easily measured, is slower than any 

 known wave movement of this character. 



In the unmyelinated nerves of vertebrates the rate of pro- 

 gression of the nerve impulse varies from 0.2 to 8 meters per 

 second; in the myelinated sciatic nerve of the frog it varies 

 from 24 to 38 meters per second; and in human myelinated 

 7 



