134 



THE PHYSIOLOGY OF NERVE 



of collaterals are placed zinc electrodes which in turn are connected with the wires 

 leading to a corresponding number of galvanometers. Thus, the central wire is 

 made to represent the axis-cylinder, and the surrounding zinc solution the less 

 conductile myelin sheath, but it may also be said that the former corresponds to 

 one of the fibrilla comprising the axis-cylinder and the latter to the perifibrillar 

 substance investing it. If the end of this conductor is now stimulated with induction 

 shocks, the galvanometers along its course will indicate the passage of an electrical 

 wave in a direction away from the point of stimulation. This model also gives rise 

 to electrotonic alterations similar to those encountered in normal nerve. 



In accordance with the second theory, which assumes that the nerve 

 impulse consists in progressive chemical changes, it is held that con- 

 duction necessitates the destruction of some of the constituents of the 

 nerve. If gun powder is spread out upon a flat surface in the form of a 

 narrow band and a spark is applied to it at one end, an explosive chem- 

 ical reaction ensues during which this material is progressively con- 

 sumed. Very obviously, conduction in nerve is not associated with 

 changes of this intensity, but it can no longer be doubted that nerve 

 tissue undergoes certain metabolic alterations in consequence of its 



FIG. 77. SCHEMA TO SHOW THE ACTION OF THE CORE-MODEL. 



p, The polarizing current; g' and g, the galvanometers showing the anelectrotonic 

 and catelectrotonic currents, respectively. (Howett.) 



activity which differ from those of other tissues only in a quantitative 

 way. This point will be proved with absolute certainty by the suc- 

 ceeding discussion. Consequently, a nerve impulse may be regarded 

 primarily as a wave of chemical change which is accompanied by a 

 liberation of chemical energy. In addition, the ensuing electro- 

 lytic dissociation also permits of the generation of electrical energy. 

 Under ordinary conditions, the latter is the only means at our disposal 

 to recognize the Herve impulse as it sweeps over a nerve. But while 

 this phenomenon may be proved to possess a distinct chemico-physical 

 basis, its true character has not been established as yet. For the 

 present it must suffice to characterize it as a chemico-physical disturb- 

 ance, the most evident product of which is an electrical change, com- 

 monly called the wave of negativity. 



The Liberation of Energy by Nerve. In accordance with the 

 preceding statement it must be evident that we cannot ascribe a 

 chemico-physical basis to the nerve impulse unless it can be shown 

 that it is actually accompanied by chemical changes such as ordinarily 

 serve as indications of metabolism and fatigue. We have previously 

 seen that the contraction of muscle is associated with a liberation of 



