THE CIRCULATION OF THE BLOOD 673 



these arrangements may be found in the textbooks of comparative anatomy. 

 In the schema, the blood which has lost its oxygen to a great extent, and taken 

 up excretory products, is represented black. As is well known, this blood appears 

 blue by reflected light through the skin. "Blue blood" is that which is of 

 comparatively little use for the demands of the organism. 



THE HEART 



As remarked above, Leonardo da Vinci realised that the heart is an organ 

 which, by its active muscular contraction, decreases periodically in its volume, 

 and thus drives out the blood which has run into it during the time in which 

 it was relaxed. Owing to the high pressure necessary to drive the blood through 

 the peripheral arterioles, it is clear that, unless there were valves at the origin 

 of the aorta to prevent the blood flowing back, this pressure could only last for 

 a moment, during the contraction of the ventricle, and also that no -blood, or 

 very little, could run in from the veins, since the ventricle would fill up from 

 the aorta. In fact, there would be a very inefficient circulation. Valves are 

 obviously necessary between the auricles and ventricles also, to enable the energy 

 of the ventricular contraction to drive the blood into the aorta or pulmonary 

 artery against the pressure existing there, and not backwards to the veins. 



Owing to its great importance, the physiology of the heart and the circulation has 

 probably attracted more attention than any other branch of the science. It is clearly 

 impossible to refer to the whole of this work, so that I must confine myself to facts which 

 seem to be of the most general interest. Further details may be found in the book by 

 Starling (1912). 



The general properties of the muscle of the heart have been described on pages 45 1-454 

 above. 



THE WORK OF THE HEART 



The energy given out by the muscular contraction of the ventricles is, 

 apart from the heat produced, used in raising the pressure in the aorta and in 

 giving to the mass of blood a certain velocity. The former is mainly expended, 

 to begin with, in stretching the elastic walls of the arteries. The kinetic 

 energy of the latter is only a small fraction of the whole work when the 

 output is small. According to the data given in Starling's book (1912, 

 p. 1032), in the human heart the kinetic energy only amounts to 0'7 gram- 

 metre per beat ; whereas the former, measured by the product of the volume of 

 the blood driven out by the pressure to which it is raised, amounts to about 81-6 

 gram-metres. On the other hand, when the output is large, as in muscular work, 

 the kinetic energy of the blood current is an appreciable fraction of the total 

 external work of the ventricular contraction. Evans finds, for example, that 

 with an output of two litres per minute, the kinetic energy amounts to as much 

 as 16 to 18 per cent, of the whole. In the case of the right ventricle, owing to the 

 low pressure in the pulmonary artery, the product of volume and pressure is 

 probably a much smaller fraction of the whole work than in that of the left 

 ventricle. The pressure here is taken as the mean between the aortic pressure 

 at the moment of opening of the aortic valves and that when they close again 

 as the ventricle begins to relax. It is obvious that, for an accurate estimation, 

 we need to know the time course of the pressure and to determine the integral of it. 



INTRA-VENTRICULAR PRESSURE 



In order to determine the time curve of the pressure change, a manometer 

 is necessary which is capable of following exactly the changes of pressure 

 without distortion by inertia of moving parts, and so on. The first approxima- 

 tion to such an instrument was made by Starling and myself (1894), and the 

 curves we obtained were very similar to those which Piper (1912, 2, and 

 1913, 2 and 3) has published as the results of a much more perfect method. 

 This method, also an optical one, is described in the first of the papers 



43 



