EXCITATION AND INHIBITION 397 



a minimal consumption of material for energy purposes, although it seems to me 

 that the evidence on this question is not so decisive as it should be, and that 

 further investigation is necessary before it can be interpreted in the sense mentioned. 

 / A distinction must be made, as we saw, between the local process at the 

 spot excited, a process which is not propagated, and the propagated disturbance 

 set up when the former exceeds a certain magnitude. /A stimulus must, therefore, 

 possess a certain minimal amount of energy in order to excite, and it seems that 

 this is required to effect the local change. No further supply of energy appears to 

 be necessary in the progress of the wave along the nerve fibre. In the natural 

 connection of the nerves with their cell bodies, the energy required to start the 

 initial process is, no doubt, supplied by the cell body, in which oxidation processes 

 of a recognisable degree are known to occur. 



The conditions in the nerve fibre which are apparently concerned in the process 

 are the polarisable membranes, of colloidal structure, and electrolytes, present in 

 the complex, liquid, colloidal system of the axis cylinder. 



The account given by Macdonald (1905, pp. 331-350) will be found very 

 instructive. 



Although the evidence seems to preponderate on the side of the physico- 

 chemical theory, it must not be forgotten that certain phenomena are not easy to 

 explain on this view. Keith Lucas has pointed out that the propagation of 

 disturbances along wires or similar channels may be of two kinds : (1) Like that of 

 sound waves, or the passage of an electric potential difference along a condenser 

 system, such as a submarine cable ; this is purely physical, and does not involve 

 production of energy as it travels. (2) A chemical system, such as a train of 

 gunpowder. In this case, energy is evolved as the disturbance is transferred from 

 one point to another, and products of change are given off. Now, Adrian's 

 results, showing that a disturbance recovers its magnitude in a normal region, 

 after having been reduced in a narcotised one, suggest a process more like the 

 second one. If a sound wave, for example, is reduced in magnitude by passing 

 thi-ough a region in which sound is conducted badly, say cotton wool, the energy 

 of its vibration is diminished, and there is nothing to increase it again when it 

 returns to a medium conducting well. On the other hand, if the train of gun- 

 powder be made very narrow at one part, so that the energy of the disturbance 

 is much less as it traverses this part, the original energy is regained when the 

 dimensions of the train become similar to what they were original!}'. As long as 

 the disturbance passes through the narrowed part at all, the original magnitude is 

 regained in the normal part. Of course, the process in nerve cannot be regarded 

 as being so simple as this ; there are evidently physico-chemical processes connected 

 with the movement of ions and the presence of semi-permeable membranes, and 

 we are at present in the dark as to the way a reaction associated with the giving 

 off of chemical energy conies into relation with the former. It is evident, however, 

 that if, in excitation, the membrane ceases to be semipermeable, so that the 

 internal electrolytes diffuse out, some supply of energy may be necessary in order 

 to restore the original state of the system. 



THE PROCESS OF EXCITATION IN MUSCLE 



The chief function of the tissue known as muscular is that of producing move- 

 ments by shortening or change of tension. This aspect of its activity will be con- 

 sidered in the next chapter. 



Regarded as excitable tissues, muscles show very much the same characteristics 

 as nerves. They can be set into activity by the direct application of a stimulus, 

 which sets up a wave of excitation, which travels at a slower rate than that 

 in nerve. - 



The Refractory Period is longer than in nerve, and can be particularly well 

 seen in the muscle of the heart. 



Muscle shows the " all or none " phenomenon, as shown especially by the work 

 of Keith Lucas (1909). 



There is an electrical change similar to that in nerve. 



