378 PRINCIPLES OF GENERAL PHYSIOLOGY 



in general, since it appears that many protoplasmic processes may IK- compared 

 rather to the utilisation of fuel in a petrol motor, where the fuel does not I income 

 built up into a chemical complex with the mechanism, but gives up its energy by 

 means of the mechanism. The mechanism acts upon it from without, in a certain 

 sense. Further discussion of this question will be necessary later. 



The name "excitation" is usually given to the increasing or setting into action 

 of a process, and that of "inhibition" to the opposite phenomenon of stopping a 

 process or decreasing its activity. 



In the strict sense, all living protoplasm is "excitable," that is, it is capable 

 of being affected by external forces, as was clearly pointed out by John Jirown 

 (1788, p. 3 of 1795 edition) and, in more detail, by Claude Bernard (iss.\ I, 

 p. 242), who defines "irritabilite," which is equivalent to the name " excitability " 

 as used above, as "la propriete que possede tout element anatomique (c'est a dire 

 le protoplasma qui entre dans sa constitution) d'etre mis en activite et de reagir 

 d'une certaine maniere sous 1'influence des excitants exterieurs." 



Nevertheless, it is usually the custom to apply the name especially to such 

 tissues as respond to stimuli by a rapid change of some kind and more particularly 

 to nerve and muscle. 



THE PROCESS OF EXCITATION IN NERVE 



As animals in the course of evolution increased in size and complexity, means 

 of communication between different parts became more and more necessary. 

 To a certain extent, such intercommunication is effected in a chemical way, 

 through the blood, or similar fluid. But this is not sufficiently rapid for many 

 purposes, the fact of contact of a solid object must be conveyed to the muscles of 

 locomotion, so that the organism may react rapidly enough to avoid it. Hence 

 we find the presence of nerves at a very early stage of evolution of multicellular 

 animals. Even in Crelenterates, the complexity at the nervous channels is 

 considerable. The effect of something happening at one end of such a thread is 

 conveyed with great rapidity to the other end of the nerve, wherever it may be. 

 Our study of the phenomena of excitation will begin with that of the nerve fibre. 

 In some ways, it is the simplest case ; in others, more difficult. Nerve fibres ha\e 

 no other function than that of conveying excitations. When left alone, they are, 

 as far as we know, in complete rest, so that their activity does not require 

 inhibitory influences to quell their state of excitation. When set into activity 

 by some influence, called a " stimulus," the disturbance set up disappears 

 spontaneously after a certain very short time, if the stimulus ceases to act. 



If we take what is known as a "nerve-muscle preparation," that is, the 

 gastrocnemius muscle of the frog, with the nerve, the sciatic, supplying it, we find 

 that if we lightly pinch the end of the nerve distant from the muscle, the latter 

 enters into contraction, and, as it seems, simultaneously with the stimulus. Nothing 

 to be seen has happened in the nerve, yet something must have passed along it 

 from the point at which it was pinched, otherwise the muscle would have been 

 unaware of anything having taken place at the other end of the nerve. It is 

 usual to speak of a " propagated disturbance " passing along the nerve, or 

 sometimes a " nerve impulse." But how are we to detect it and investigate it in 

 the nerve itself, apart from the indicating muscle 1 



The most careful investigation with the most sensitive apparatus has only In en 

 able to detect with certainty one kind of change accompanying the passage of the 

 " propagated disturbance," namely, an electrical effect. 



The production of heat in any quantity that would have any significance at 

 all is definitely excluded by the experiments of A. Y. Hill (1912). By the use 

 of a method by which changes in temperature of six-millionths of a degree could 

 be detected, no effect was obtained by twenty-five seconds continuous stimulation. 

 This result means that a single propagated disturbance does not result in 

 the production of more than 1 x 10~ 8 C., that is, a hundred-millionth of a 

 degree. Hill calculates that heat of this amount would be afforded by the 

 consumption of 1 molecule of oxygen by a volume of nerve of 3'7 p. cubic measure, 



