114 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



since in the train of gunpowder the material is entirely consumed, 

 whereas in the nerve an indefinite series of impulses may be trans- 

 mitted and with a strength varying with the intensity of the origina- 

 ting stimulus. This general view implies that a disassimilation or 

 catabolism occurs in the nerve, a breaking down of complex material 

 with the liberation of the potential chemical energy; it assumes, 

 in other words, that the wave of chemical change that sweeps 

 along a nerve fiber is similar to the wave of chemical change, 

 contraction wave, that passes over a muscle fiber. As was stated 

 in preceding paragraphs, there is no evidence for this view. It 

 has not been shown that in the conducting nerve there are any 

 detectible metabolic products formed. There is no rise in tempera- 

 ture, no change in reaction, no formation of carbon dioxid. The 

 view rests entirely upon analogy with what is known to occur in 

 other tissues, especially muscle, during functional activity. The 

 electrical change that accompanies the nerve impulse is considered 

 as a by-action, so to speak, due probably to the liberation of electro- 

 negative ions (anions) in the reaction that constitutes the nerve 

 impulse. The second general view of the nature of the nerve 

 impulse assumes that it is a physical or physico-chemical process 

 transmitted along the fiber without involving a metabolism of the 

 living nerve substance. One may find an analogy for such a process 

 in the wave of pressure transmitted through a tube filled with 

 liquid or the electrical current conveyed through a metallic con- 

 ductor. This view rests upon the fact that no consumption of 

 material can be demonstrated in the acting nerve fiber, and that 

 apparently the fiber can conduct indefinitely without showing 

 fatigue. Various suggestions have been offered as to the character 

 of this physical change, but the one that is perhaps most worthy 

 of consideration identifies the nerve impulse with the negative 

 electrical charge that is known to pass along the fiber. It is as- 

 sumed that this electrical charge constitutes the nerve impulse. 

 To explain the physics of the conduction it is supposed that the 

 nerve fiber has a structure essentially similar to the "core con- 

 ductor" (see p. 102) in that it contains a central thread surrounded 

 by a liquid sheath of less conductive material. The central thread 

 may be supposed to be the axis cylinder and the less conductive 

 sheath the surrounding myelin, or perhaps, to follow another 

 suggestion that fits the non-medullated as well as the medullated 

 fibers, the central threads are represented by the neurofibrils 

 within the axis cylinder and the surrounding sheath by the perifi- 

 brillar substance. That the axis cylinder is a better conductor 

 than the myelin sheath has been demonstrated by the micro- 

 chemical researches of Macallum. This observer has shown that 

 in the axis cylinder the chlorids exist in greater concentration 



