THE NATURE OF THE EXCITATORY PROCESS 285 



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 ex- 

 penditure 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 therefore 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 develop- 

 ment of electromotive forces at the junction of the segment with the adjacent 

 resting segments. 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 suggested 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 succeeding segments. In this 

 way the excitatory process may travel the whole length of the nerve. Propa- 

 gation would thus involve the successive setting up of an excitatory process 

 all along the nerve or excitable tissue, though it is difficult to see why on this 

 theory every excitatory state should not give rise to a propagated change. 



We are as yet a long way from a comprehension of the changes involved 

 in the process of excitation, though we are able to form some idea of many of 

 the factors which must be involved. Any theory of the excitatory process 

 must take into account the following phenomena : 



(1) The excitatory state is attended with an electrical change of such 



* This statement is based chiefly on experiments on the olfactory nerve of the pike. 

 Halliburton and Brodie found no signs of fatigue in the non-medullated fibres of the 

 sympathetic supply to the spleen, even after several hours' stimulation. 



