256 



PHYSIOLOGY 



thread may be thrown upon a screen. Whenever a current passes through the 

 thread it moves laterally, and the lateral movement may be photographed on a 

 moving photographic screen. Owing to the extremely 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. 85), such as the 

 sartorius, be stimulated with a single induction shock at one end, x, 



FIG. 85. Diagram showing diphasic variation of uninjured muscle. 



and two points, a and b, be led off to a capillary electrometer, each 

 stimulus applied at x gives rise to an excursion of the meniscus 

 of the electrometer, known as a ' spike,' and shown in Fig. 86. 



FIG. 86. A typical electrometer record from a sartorius muscle excited by 

 a single induction shock. Time-marking = 200 D.V. (KEITH LUCAS.) 



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 stimulated at x, a contraction wave commences which travels down 

 the muscle through a and b. The electrical investigation of the 

 muscle shows that excitation of x arouses an electrical change which 



