THE CIRCULATION OF THE BLOOD 703 



up of amoeboid corpuscles, the other with a coagulation effect in the plasma. The 

 papers by Hardy (1892) and by Tait (1910) may be consulted. 



The existence of something which prevents or retards the process of coagulation 

 has been referred to in speaking of the extract of the heads of leeches (page 360). 

 A similar " anti-thrombin " can be obtained from the liver, as shown especially by 

 Doyon (1912). 



On account of the difficulty and expense of procuring hirudin, it would seem worth while 

 to attempt to prepare an anti-thrombin from the liver by Doyon's method. It would require 

 removal of the toxic impurities present in the crude mixtures hitherto obtained. The 

 substances in question seem to make the colloidal system more stable, so that the 

 coagulation process induced by rough surfaces is prevented. 



Zak (1912) shows that the " lipoids " of the plasma play a considerable part 

 in the phenomena of coagulation, a fact which points to the intervention of surface 

 action. This investigator shows that the hypothesis of a " thrombokinase " is 

 superfluous. 



The object of the circulation of a fluid through the larger organisms is to supply 

 food, especially oxygen, to the tissues, and to bring about effective interchange of 

 chemical products. 



The function of the heart as a pump to drive blood into the arteries was 

 shown by Leonardo da Vinci, but the actual fact of the movement of the blood 

 in a circle back to the heart was first demonstrated by Harvey. The passage of 

 the blood through the peripheral capillaries from arteries to veins was first seen 

 by Leeuwenhoek. 



A high arterial pressure is necessary, and was shown to exist by Stephen 

 Hales. This is in order to ensure a sufficiently rapid flow through the fine 

 branching tubes of the various organs. 



In the higher vertebrates there are two pumps in series, having the lungs 

 between them ; one is to drive the aerated blood from the lungs to the organs in 

 general ; the other to drive the venous blood, returning from these organs, through 

 the lungs, in order that it may take up oxygen and lose carbon dioxide. The two 

 pumps are combined in one organ, the heart, but their cavities are separate. 



The greater part of the work done by the heart muscle is expended in raising 

 the pressure of the blood driven out, and may be measured by the product of the 

 volume and pressure.. 



Determination of the time course of the pressure curve in the ventricle shows 

 that no blood is expelled until the maximum tension is developed. The muscle, 

 therefore, works at its best efficiency. During the expulsion of blood into the 

 aorta, the pressure in the ventricle remains nearly constant, the curve showing 

 a flattened top. 



The inflow from the veins is, practically, the determining factor in the work 

 done by the heart. The human heart, indeed, can deal with as much as 21 litres 

 per minute. Thus it is the length of the fibres that determines the energy 

 given out. 



A brief account is given of the heart sounds. 



The oxygen consumed by the heart is in direct proportion to the energy of the 

 tension developed, and is the same at 15 as at 36. The actual amount of oxygen 

 used depends on whether the "reserve-stuff" of the heart itself is oxidised, or the 

 glucose of the solution perfused ; but the relation between the energy produced 

 by oxidation and that of the tension developed is constant. The effect of excess 

 of carbon dioxide is to prevent the conversion of chemical to mechanical energy ; 

 similarly, the presence of calcium is necessary for the due conversion of this 

 chemical energy into that of tension. 



