424 PRINCIPLES OF GENERAL PHYSIOLOGY 



substance. To exclude this, the experiments were repeated on preparations in 

 which the afferent fibres from the muscle had been cut. No difference in behaviour 

 could be detected. The other point is that if we accept the results of various 

 observers on " all-or-nothing " in the excitatory process, increase or decrease in the 

 height of the reflex contraction must mean a greater or smaller number of cells in 

 the centre in a state of activity. It would appear, therefore, that an increased 

 response, following a period of inhibition, must be due to the inhibitory stimulus 

 having made some synapses accessible to excitation which were previously 

 inaccessible. 



Drainage or Diversion Theories. Another set of theories is based on tin- 

 idea of a stream of excitation, flowing in a particular direction, being diverted to 

 a different course by the presentation to it of an easier way. Apart from the 

 "animal spirits" of Descartes, about which something will be said later, the first 

 form in which the theory was expressed seems to be that of William James (1890, 

 1, p. 585, footnote). It was given more definite expression to by von Uexkull 

 (see 1909, p. 185), who uses the names "tonus" and "excitation," which flows 

 from one part to another of the neuro-muscular mechanisms, and by McDougall 

 (1903), who speaks of a stream of "energy," to which he, originally, gave the name 

 "neurin." 



The simplest illustration we can take is that of a water tank feeding a fountain 

 at a lower level than itself ; there is a continual stream of water, possessing energy, 

 passing along the pipe to the fountain. If the gardener opens a large tap on the 

 course of this pipe, in order to fill his watering can, the fountain stops for the 

 time, since the pressure in it falls to zero ; we may say that we have " inhibited " 

 the fountain. 



Now, to begin with, I find some difficulty in discussing this view of the 

 mechanism of the nerve centres, because the conception does not readily fit in 

 with what we know of the nature of the excitatory process in nerve. There arc. 

 however, certain assumptions made which cannot be accepted in the form stated. 

 It must be admitted that von Uexkiill (1909, p. 58) defends himself from the 

 imputation of using a misleading image, in speaking of a liquid flowing about in 

 the nervous system, on the ground that the object of science is not " truth " but 

 " order," so that we must accept his assurance that he uses such expressions as 

 " quantity " and " pressure " of excitation and " varying capacities of reservoirs " 

 merely to facilitate the description of experimental facts. At the same time, 

 there appears to me to be a fundamental misconception at the base of all theories 

 of this nature, namely, that the nerve " energy " in a given organism is a definite, 

 limited quantity. When a part of it is diverted into some channel, it must, 

 therefore, be drained away from some other place. But there is no reason to 

 suppose that, when a nerve fibre divides into two, the magnitude of the propagated 

 disturbance is diminished to half in each of the branches. Indeed, Adrian's results 

 show that 'the excitation is "all-or-nothing" in both branches; since it cannot be 

 nothing in both, it must be " all " in both. Similar conclusions are forced upon us 

 by consideration of the electrical organ of Malapterurus (see Gotch and Burch, 1896, 

 p. 387), in which the single efferent nerve fibre divides into about 1,800 branches, 

 one to each plate of the organ. If any diminution occurred, the whole excitatory 

 process must be frittered away to practical non-existence. If there is no diminution 

 on branching of the nerve fibre, it is clear that adding or removing a branch will 

 have no effect on the disturbance in the main fibre. The process is more analog >us 

 to the propagation of an explosion along a train of gunpowder, which can be made 

 to branch as many times as desired without affecting the intensity of the explosion 

 along the main track. In the nerve, of course, the process is reversible and perhaps 

 unaccompanied by evolution of energy, thus differing from that of an explosion 

 (see page 396 above). 



The use of the words "stream of energy" is also inappropriate. The actual 

 energy involved in the propagation of a nerve impulse is quite infinitesimal, as 

 we have seen. If it be said that the words are used metaphorically, it is 

 misleading to take a word which has a definite quantitative, mechanical meaning. 



The application of the drainage theory to explain " reciprocal innervation," 



