58 HANDBOOK OF PHYSIOLOGY. 



hand, would it be possible to obtain a correct estimate by less rapid 

 bleeding, as, since life would be more prolonged, time would be allowed 

 for the passage into the blood of lymph from the lymphatic vessels and 

 from the tissues. In the former case, therefore, we should under-esti- 

 mate, and in the latter over-estimate the total amount of the blood. 



Of the several methods which have been employed, the most accurate 

 appears to be the following. A small quantity of blood is taken from an 

 animal by venesection; it is defibrinated and measured, and used to make 

 standard solutions of blood. The animal is then rapidly bled to death, 

 and the blood which escapes is collected. The blood vessels are next 

 washed out with water or saline solutions until the washings are no longer 

 colored, and these are added to the previously withdrawn blood; lastly 

 the whole animal is finely minced with water or saline solution. The 

 fluid obtained from the mincings is carefully filtered, and added to the 

 diluted blood previously obtained, and the whole is measured. The next 

 step in the process is the comparison of the color of the diluted blood 

 with that of standard solutions of blood and water of a known strength, 

 until it is discovered to what standard solution the diluted blood corre- 

 sponds. As the amount of blood in the corresponding standard solution 

 is known, as well as the total quantity of diluted blood obtained from 

 the animal, it is easy to calculate the absolute amount of blood which 

 the latter contained, and to this is added the small amount which was 

 withdrawn to make the standard solutions.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 T ^ to ^ of the total body weight. 



An estimate of the quantity in man which corresponded nearly with 

 this proportion, was made some years ago from the following data. A 

 criminal was weighed before and after decapitation; the difference in the 

 weight representing, of course, 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 color. This fluid was then also weighed; and the amount of blood 

 which it represented was calculated by comparing the proportion of solid 

 matter contained in it with that of "the first blood which escaped on de- 

 capitation. Two experiments of this kind gave precisely similar results. 

 (Weber and Lehmann.) 



It should be remembered, in connection with these estimations, that 

 the quantity of the blood must vary, even in the same animal, very con- 

 siderably with the amount of both the ingesta and egesta of the period 

 immediately preceding the experiment; and it has been found, indeed, 

 that the amount of blood obtainable from the body of a fasting animal 

 rarely exceeds a half of that which is present soon after a full meal. 



Coagulation of the Blood. One of the most characteristic prop- 

 erties which the blood possesses is that of clotting or coagulating, when 



