230 PHYSIOLOGY 



diphasic variation of such a direction that the point stimulated first becomes 

 negative to all other points of the muscle, and this ' negativity,' to use a loose 

 but convenient expression, passes as a wave down the muscle, preceding the 

 wave of contraction and travelling at the same rate. 



If one leading-off point be injured, e.g. at b, the change accompanying 

 excitation is absent at that point. A single stimulus applied at x will in 

 this case give only a monophasic variation in which a is relatively negative 

 to b. 



When we study the time relation of the electrical variation ensuing on a 

 single stimulus, we find that the electrical change under the electrodes 

 begins at the moment that the stimulus is applied. It takes about -CG25 

 sec. to attain its culminating point. At this point the mechanical .change 

 or contraction of the muscle begins. These time-relations vary with the 

 temperature of the muscle. We have alread}^ seen that the effect of lowering 

 the temperature is to increase that latent period of the contraction. In the 

 same way it slows the rise of the electrical change and the rate of propagation 

 of the wave of electrical change. This is shown in Fig. 86, in which are 

 given the diphasic response of the sartorius first at 8C. and secondly at 

 18C. ' We are therefore justified in regarding the electrical change as an 

 index to the chemical changes evoked in the muscle as the direct result of the 

 stimulus. The flow of material, which is responsible for the change in form 

 of each contracting unit, is secondary to these changes. As the result of 

 stimulation, a chemical change is. aroused at the point of excitation and 

 travels thence along the muscle fibres at a rate of about three metres per 

 second, i.e. the same rate as that of the following wave of mechanical change 

 and, like this, varying with the temperature. 



Under certain conditions an excitatory condition may be propagated without the 

 presence of a visible contraction. Thus, if the middle third of the sartorius be soaked 

 for a time in water, it passes into a condition known as ' water rigor,' in which it is 

 incapable of contracting, although capable of transmitting an excitation from one end 

 of the muscle to the other. 



The connection of a diphasic current of action with an excited condition 

 of the tissues passing as a wave from one end to the other is shown still more 

 clearly on a slowly contracting tissue, such as the ventricle of the frog or 

 tortoise. Fig. 87, A, is a photographic record of the variation obtained from 

 the tortoise ventricle, which is led off to a capillary electrometer, one (acid) 

 terminal being connected with the base of the ventricle, the other (mercury) 

 with the apex. Each part of the ventricle remains contracted for a period of 

 1 j to 2 seconds, and then the contraction passes off, first at the base and later 

 at the apex. The electrical events are an exact replica of the mechanical. 

 Directly after the stimulus has been applied, the base becomes negative and 

 the column of mercury moves up. A moment later the excitatory condition 

 extends to the apex. There is thus a sudden equalisation of potential 

 between the two terminals, and the mercury comes back quickly to the base 

 line. Here it stays for 1 J to 2 seconds. During this time the whole heart is 

 in an excited condition. Both base and apex are equally excited, and there 



