B 



1084 PHYSIOLOGY 



cardiometer so as to measure the degree of its contraction and relaxa- 

 tion with systole and diastole, and also its output at each beat. 

 Artificial respiration being kept up, the carbon dioxide tension in the 

 blood sinks considerably. It is possible to administer by artificial 

 respiration gaseous mixtures containing any desired quantity of carbon 

 dioxide. On pumping in a mixture containing a certain moderate 



amount of carbon dioxide the 

 heart will be observed to dilate 

 (Fig. 445), but the dilatation 

 affects the diastolic volume even 

 more than the systolic volume, 

 so that the stroke of the heart 

 at each beat, and therefore its 

 output and efficiency, are in- 

 creased. In Fig. 446 are shown 

 the cardiometer records obtained 

 on administration of a gaseous 

 mixture containing 7-9 per cent, 

 carbon dioxide. The full effect is 

 produced in the tracing D, where 

 it will be noticed that although 

 the heart is considerably dilated, 

 the output at each beat is in- 

 creased to such an extent that 

 the contraction becomes more 

 effective than before. There is 

 an optimum tension of carbon 

 dioxide in the blood, lying be- 



JL 



JL 



FIG. 446. Cardiometer records of ventri- 

 cular output (description in text). 

 (JERUSALEM and STARLING.) 



tween 5 and 10 per cent, of an 

 atmosphere, at which the output 

 of the ventricles is at a maximum. 

 The essential factor in the pro- 

 duction of these results is probably 



the concentration of hydrogen ions in the blood. When very weak 

 acids are transfused through the frog's heart there is a gradual diminu- 

 tion of tonus, and the same relaxation may be obtained as a direct 

 result of the action of carbon dioxide or of weak acids on the muscular 

 wall of the blood-vessels. 



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 blood between the fibres. In the mammalian heart the 

 muscular tissue is nourished through the coronary arteries, which 



