THE CAUSATION OF THE HEART BEAT 



1009 



fluids, which will represent the optimum reaction for the beat of the frog's 

 heart. Mines has shown that this optimum reaction differs for the different 

 cavities of the heart, and also for the hearts from various animals, a 

 shifting of the reaction of the transfusing fluid to the acid side always 

 bringing about a diminished contraction ' and tonus, while the opposite 

 effects are produced by an increase in the alkaline reaction. In the 

 mammal under normal conditions, the chief factor affecting the reaction of 

 the blood is the tension of carbonic acid in this fluid ; and an increase in the 

 carbonic acid of the blood, when sufficiently pronounced, always brings 

 about dilatation of the heart. The resistance of the hearts of different 

 animals is however not of the same strength. Thus, a dog's heart is much 



FIG. 453. Volume curve of ventricles (cat) (lower curve). The upper curve is the 

 arterial pressure, maintained by an adjustable resistance at 130 mm. Hg. 

 Between the arrows the air used for artificial respiration was replaced by a 

 mixture containing 20 per cent. C0 2 and 25 per cent, oxygen. Note the 

 dilatation with impaired contraction, followed by increased amplitude of 

 contraction. 



more susceptible to the presence of a small excess of carbonic acid in the 

 blood than is the cat's heart (cp. Fig. 453). There is probably an optimum 

 tension of carbon dioxide in the blood, varying between 5 and 6 per cent, 

 of an atmosphere, at which the physiological condition of the ventricular 

 muscle is at an optimum, but the tension may be reduced considerably 

 below this without causing any marked change in the action of the heart. 



Yandell Henderson found that vigorous artificial ventilation of the lungs brought 

 about a condition in which the heart's contraction was very forcible and the heart's 

 cavities almost empty. He ascribed this condition to the hypertonicity of the heart 

 muscle produced by washing the carbonic acid out of the blood. These results were 

 however probably due to the mechanical influence of the respiratory movements 

 on the venous filling of the heart; and there seems no reason to believe that the 

 condition of * acapnia ' (deficiency of carbon dioxide in the blood) had anything to 

 do with the results observed. The improving effects of administration of carbon 

 dioxide, described in the first edition of this work, have not been confirmed by more 

 recent and accurate experiments. 



THE NUTRITION OF THE HEART 



In the frog's heart the muscle fibres are supplied directly by the blood 

 within the cavities, the spongy ventricular wall permitting the access of 

 64 



