44 CONTRACTILITY 



injured a difference of potential is set up between the 

 healthy and the injured part. 



A similar difference of potential exists between uncon- 

 tracted and contracted parts of a muscle, the part which 

 is in contraction becoming, hke the injured part, electro- 

 negative. 



Now a single contraction takes the form of a wave of 

 contraction which passes from one end of the muscle to 

 the other. If the two electrodes are placed one near each 

 end of the muscle there is first a deflection of the galvano- 

 meter, indicating negativity at the proximal electrode. 

 When the wave is equidistant between the electrodes, these 

 are at iso-potential. At this moment there is no current 

 flowing. As the wave approaches the distal electrode this 

 becomes negative; a current now passes through, the 

 galvanometer in the reverse direction. The total effect 

 is a diphasic variation and is known as the current of action. 

 It is represented graphically in Fig. 6. 



For detecting these electrical changes two instruments 

 are used — the capillary electrometer and the string 

 galvanometer. 



The capillary electrometer consists of a glass tube drawn 

 out into a fine point. The tube is filled with mercury, 

 which does not run as far as the point. The point of the 

 tube dips into dilute sulphuric acid in a wider vessel. 

 Beneath the acid is a layer of mercury. The mercury in 

 the capillary and in the vessel become the terminals of the 

 electric circuit. When the mercury in the vessel becomes 

 negative the lower end of the mercury in the capillary 

 moves downwards, and vice versa. The movements of the 

 mercury are recorded photographically. 



In the string galvanometer a fine quartz fibre is suspended 

 in the field of a powerful electro-magnet. When a current 

 passes along the string the latter is deflected to one side or 

 the other according to the direction of the current. Oppo- 

 site the middle of the string a hole is bored through the 



