50 PRACTICAL PHYSIOLOGY 



CHAPTER X. 



THE EFFECT OF A CONSTANT CURRENT UPON MUSCLE AND 



NERVE. 



MUSCLE and nerve consist of complex chemical substances, and con- 

 tain 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 electro- 

 lysis; 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 electro- 

 lysis 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. 



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 



FIG. 51. 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. 



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 its distension with blood. 

 There are no blood-vessels in a frog's cardiac muscle. 



