122 PRACTICAL PHYSIOLOGY 



CHAPTER XV 



THE EFFECT OF A CONSTANT CURRENT UPON MUSCLE 



AND NERVE 



Muscle and nerve consist of complex chemical substances, and 

 contain about 70 per cent, of water in which various salts are 

 dissolved. Moreover, they are bathed in lymph. 



The passage of a constant current through a liquid produces 

 electrolysis ; thus, in the case of water, oxygen is given off at one 

 plate, hydrogen at the other. Animal tissues, containing, in addition 

 to a large percentage of water, salts and proteins, are also the seat 

 of electrolysis during the passage of a constant current ; the ions 

 are probably of a complex nature. These changes in nerve and 

 muscle are shown by alterations in excitability and conductivity. 



FIG. 123. Diagram of the frog's heart to show the effects of the 

 make and break of a constant current upon muscle. 



In A the ventricle is represented as pale and contracted, with a small shaded area to 

 represent the flushed and uncontracted [portion of the ventricle ; that is, a local 

 diastole during general systole. This condition can be produced by the make of the 

 anode or the break of the kathode of a constant current. In B the ventricle is 

 dilated and flushed, with a small pale area of contracted muscle ; that is, a local 

 systole during general diastole. This condition can be produced by the make of 

 the kathode or the break of the anode. 



These it is necessary to consider in relation to the changes which 

 occur at the anode and kathode during the make and break of the 

 constant current. The simplest experiment can be made upon the 

 frog's heart. 



The Effects of Anode and Kathode upon the Frog's Heart. The 

 brain and spinal cord of a frog are pithed and then the heart is 

 exposed. Care should be taken to avoid the severance of large 

 blood vessels in order that the vascular system may be well filled 

 with blood. The pericardium is opened and the heart is observed ; 

 the ventricle during systole is pale owing to the contraction of its 

 muscle fibres forcing out the blood from its spongy walls ; during 

 diastole, when the muscle is relaxed the ventricle is flushed owing to 



