114 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



very incomplete, since in the train of gunpowder the material 

 is entirely consumed, whereas in the nerve an indefinite series of 

 impulses may be transmitted and with a strength varying with 

 the intensity of the originating 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 conduct- 

 ing nerve there are any detectible waste products formed. There is 

 no rise in temperature, 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 electronegative 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 conductor. This view rests upon the fact that no con- 

 sumption of material can be demonstrated in the acting nerve fiber, 

 and that apparently the fiber can conduct indefinitely without show- 

 ing 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 assumed 

 that this electrical charge constitutes the nerve impulse. To ex- 

 plain the physics of the conduction it is supposed that the nerve 

 fiber has a structure essentially similar to the "core conductor" 

 (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 perifibrillar substance. 

 The point of importance is that, with a core model (see Fig. 46) 

 consisting of a glass tube with a core of platinum wire and a sheath 

 of solution of sodium chlorid, 0.6 per cent., electrical phenomena 

 can be obtained similar to those shown by the stimulated nerve. 



