The regulation of the supply of oxygen to the tissues 153 



Another possible cause of the changes in the rate of flow through 

 the coronary system is the rhythmic contraction of the cardiac 

 muscles, which may be held to promote a more rapid flow of blood 

 along the coronary system. 



in 



FIG. 83. Record of the movements of the heart of a small cat perfused from the circulation 

 of a large cat. Upstroke = systole. Period I = normal. In period II the signal mark 

 represents the time of vagus stimulation. The third period corresponds to the after 

 effect and in this period the third sample of blood was taken. 



At first sight it seemed difficult to analyse the complex of 

 processes into the elements necessary for a decision whether the 

 variations in the activity of the heart produce their effect on the 

 blood flow by the mechanical method of squeezing the blood inter- 

 mittently along the vessels, or by the chemical method for producing 

 dilator substances. Fortunately nature has performed the analysis 

 for us. The output of C0 2 , as I have already indicated in Chapter VI, 

 lags to some extent behind the actual variations of the frequency and 

 power of the contractions. The time relations of the activity and the 

 C0. 2 output are shown in Fig. 84, also those of the changes in blood 



