250 VETERINAEY PHYSIOLOGY 



But while this is the case the strength of stimulus necessary 

 to call forth a contraction varies at different periods. To 

 produce another contraction while the muscle is already in the 

 period of contraction is difficult, but as it relaxes it reacts more 

 and more readily to stimuli. In cardiac muscle, perhaps more 

 than in any other, the staircase increase in the extent of con- 

 traction with a series of stimuli is manifested. 



In cardiac muscle the greater the resistance to contraction the 

 stronger the force of contraction. Hence when extra blood is 

 poured into the heart from the veins, or when the outflow from 

 the ventricles into the arteries is impeded, the increased strain 

 put upon the heart muscle is met by increased contraction, and 

 the additional work thrown upon the organ is effectually 

 performed. Not only is this the case when temporary dis- 

 turbances of the circulation occur, but when these disturbances 

 are permanent, the heart adapts itself to them, and, if it has 

 continuously to perform extra work, its muscular wall hyper- 

 trophies, just as the skeletal muscles grow by continual use. Of 

 course, to allow such compensation to be established, the blood 

 supply to the heart muscle must be sufficient, and hence, when 

 the coronary arteries are diseased, heart failure rapidly ensues. 



If the coronary arteries are clamped and then relaxed, a 

 peculiar fibrillar contraction of the heart muscle occurs. 



The maintenance of this rhythmic contraction and relaxation 

 seems to depend greatly upon the presence of certain kations in 

 the circulating blood. A due admixture of salts of sodium, 

 potassium, and calcium is essential. For the frog's heart 

 Ringer finds that the proportions which give the best results 

 are 



NaCl . 070 per cent. 



KC1 . . . 0-03 



CaCl .... 0-025 ,, 



Since an excess of calcium salts leads to tonic contraction, and 

 since an excess of sodium or of potassium leads to relaxation, 

 it has been concluded that these two phases are determined 

 by the presence of these ions. 



How is the Rhythmic Contraction of the Heart maintained? 



The mechanism is in the heart itself, for the excised heart 

 continues to beat. 



