228 



PHYSIOLOGY 



FIG. 84. 



Diagram showing diphasic variation 

 of uninjured muscle. 



in the capillary. This polarisation causes an electromotive force which exactly balances 

 the E.M.F., setting up the polarisation so no current passes the surface. Hence the 

 use of non-polari sable electrodes is not so essential in experiments with this instrument 

 as when we make use of the galvanometer. 



In the D'Arsonval galvanometer (Fig. 82) the current is sent through a coil of fine 

 wire hung between the poles of a permanent magnet. The same principle is made use 

 of in the string galvanometer of Einthoven (Fig. 83). In this a very delicate thread 

 of silvered quartz or of platinum is stretched between the poles of a strong magnet. 

 The poles of the magnet are pierced by holes so that the thread may be illumined by 

 an electric light from one side, and from the other may be observed by means of a 

 microcope ; or a magnified image of the thread may be thrown upon a screen. When- 

 ever a current passes through the thread it moves laterally, and the lateral 

 movement may be photographed on a moving photographic screen. Owing to the 

 minute dimensions of the thread the instrument is one of extreme delicacy. It will 

 detect very minute currents and will respond accurately to very rapid changes in 

 potential. 



If a perfectly uninjured regular muscle (Fig. 84), such as the sartorius, 



be stimulated with a single in- 

 duction shock at one end, x, 

 and two points, a and 6, be 

 led off to a capillary electro- 

 meter, each stimulus applied 

 at x gives rise to an excursion 

 of the meniscus of the electro- 

 meter, known as a * spike,' and 



shown in Fig. 85. Knowing the constants of the instrument used, we 



can analyse this spike, and we find that it represents a diphasic change. 



Our study of the mechanical 



changes in muscle has shown 



that, when the muscle is stim- 

 ulated at x, a contraction wave 



commences which travels down 



the muscle through a and 6. 



The electrical investigation of 



the muscle shows that exci- 

 tation of x arouses an electrical 



change which also passes down 



the muscle at the same rate as 



the mechanical change which 



it precedes. If we are leading 



off from x and a, the electrical 



change ensues immediately 



upon stimulus, i.e. there is no 



latent period to the electrical 



change. On leading off from a FIG. 85. A typical electrometer record from a sar- 

 3 -i , -i i , n torius muscle excited by a single induction shock. 



and 6 there is a latent period Time . marking fc 200 D .V. (KEITH LUCAS.) 



between the stimulus and the 



first change, representing the time taken from the change to travel from 



x to a. When the change reaches a this becomes the seat of an electro- 



