320 PHYSIOLOGY 



there must be a decrement of the change as it is transmitted away 

 from its seat of origin, a decrement for the existence of which there 

 is no evidence in a nerve fibre or other excitable tissue.* Moreover the 

 phenomenon of propagation of an excitatory process is equally well 

 marked in tissues, such as muscle and non-medullated nerve fibres, 

 which show very little of the electrotonic effects described in the last 

 section. The absence of decrement in the excitatory process has been 

 taken as an indication that the axis cylinder of the nerve is the seat 

 of energy changes which may be let loose under the influence of chemical 

 or electrical changes, just as the energy of a contracting muscle is set 

 free by the exertion of an infinitesimal force applied as a stimulus. 

 The nerve on this view does not simply transmit the energy which is 

 imparted to it, like a telegraph wire, but itself furnishes the energy 

 of the descending nerve-process. 



Against this view might be urged the absence of phenomena of 

 fatigue in nerve, as showing that nervous activity is not accompanied 

 by any expenditure of energy or using up of material. But it must be 

 remembered that this absence of fatigue holds good only for medullated 

 nerve fibres and is not found in non-medullated nerves, and even in 

 medullated nerves the persistence of irritability is dependent on the 

 continual supply of a certain small amount of oxygen. It may there- 

 fore possibly be explained by a continual process of restitution taking 

 place at the expense of the sheath. Fatigue is absent, not because 

 nothing is used up, but because the assimilative changes exactly 

 balance and make good the dissimilation involved in the propagation 

 of a nervous impulse. 



There is thus a certain amount of justification in the comparison 

 of a nerve fibre to a chain of gunpowder, though in the nerve fibre the 

 impetus to disintegration, imparted from each particle to the next in 

 order, consists, not in a rise of temperature at the point of ignition, but 

 in all probability in an electrical change ; and the total evolution of 

 energy is so small that it cannot be measured as heat by the most 

 sensitive methods at our disposal. The excited condition at any 

 segment of a nerve is associated with a development of electromotive 

 forces at the junction of the segment with the adjacent resting seg- 

 ments. The current of action thereby produced can pass by the 

 sheath of the nerve, so that it must enter the axon at the excited spot, 

 and leave it at the adjacent unexcited segment. Hermann has sug- 

 gested that in this way the current of action at any excited spot may 

 excite the adjacent segments or molecules,, causing them to become 

 negative and thus setting up a current of action which in its turn 

 excites the molecules in order. In this way the excitatory process 



* It might be urged, on the other hand, that one would not expect to find any 

 appreciable decrement in a cable only 1 to 3 inches long. 



