408 THE BLOOD. [CH. xxvi. 



make the standard solutions. This gives the total amount of 

 blood which the animal contained. It is contrasted with the 

 weight of the animal, previously known. 



The result of many experiments shows that the quantity of blood 

 in various animals averages -^ to -~ of the total body-weight. 



An estimate of the quantity in man which corresponded nearly 

 with this proportion has been more than once made from the 

 following data. A criminal was weighed before and after decapi- 

 tation ; the difference in the weight represented the quantity of 

 blood which escaped. The blood-vessels of the head and trunk 

 were then washed out by the injection of water until the fluid 

 which escaped had only a pale red or straw colour. This fluid 

 was then also weighed ; and the amount of blood which it repre- 

 sented was calculated by comparing the proportion of solid matter 

 contained in it with that of the first blood which escaped, on 

 decapitation. Two experiments of this kind gave the same 

 results. (Weber and Lehmann.) 



Haldane and Lorrain Smith have recently investigated the 

 question by another method. A man breathes a certain measured 

 volume of carbonic oxide ; the percentage saturation of the haemo- 

 globin is then determined in a drop of blood colorimetrically ; 

 from this, the total capacity of the subject's blood for absorbing 

 carbonic oxide can easily be calculated ; this is the same as the 

 'oxygen capacity.' The total volume of the blood is then 

 calculated from the total and percentage oxygen capacities, and 

 the total weight obtained by multiplying the volume by the 

 specific gravity (about 1*05 5). These experiments show that the 

 commonly accepted estimate of the mass of 'the blood is too 

 high ; the average in fourteen healthy adults was -^ of the 

 body weight. 



Coagulation of the Blood. 



One of the most characteristic properties which the blood 

 possesses is that of clotting or coagulating. This phenomenon 

 may be observed under the most favourable conditions in blood 

 which has been drawn into an open vessel. In about two or three 

 minutes, at the ordinary temperature of the air, the surface of 

 the fluid is seen to become semi-solid or jelly-like, and this change 

 takes place in a minute or two afterwards at the sides of the 

 vessel in which it is contained, and then extends throughout the 

 entire mass. The time which is occupied in these changes is 

 about eight or nine minutes. The solid mass is of exactly the 

 same volume as the previously liquid blood, and adheres so closely 

 to the sides of the containing vessel that if the latter is inverted 



