228 



PHYSIOLOGY 



of a second, and polarises the surface of the mercury, so that it takes up a new position 

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

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

 the use of non-polarisable electrodes is not so essential in experiments with this instru- 

 ment 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 

 microscope ; 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 dimen- 

 sions 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 



FIG. 84. 



Diagram showing diphasic variation 

 of uninjured muscle. 



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 stimu- 

 lated at x, a contraction wave 

 commences which travels down 

 the muscle through a and b. 

 The electrical . investigation of 

 the muscle shows that excita- 

 tion 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- 

 nnrl 7> fhprp iq a Intpnt rjpriod toriu8 muscle excited by a single induction shock. 



Time-marking=200 D.V. (KEITH LUCAS.) 

 between the stimulus and the 



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



