64 HAND-BOOK OF PHYSIOLOGY. 



bleeding an animal to death, for then an indefinite quantity would remain 

 in the vessels, as well as in the tissues; nor, on the other 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-estimate, 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 solution 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 colored, and these are added to the previously withdrawn blood; lastly 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. Ths 

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

 with that of standard solutions of blood and watsr 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 with- 

 drawn 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 -fa to T ^ of the 

 total body weight. 



An estimate of the quantity in man which corresponded nearly with 

 the above, was made some years ago from the following data. A crim- 

 inal 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 in- 

 jection 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 decapitation. 

 Two experiments of this kind gave precisely similar results. (Weber and 

 Lehmann.) 



It should be remembered, however, in connection with these estima- 

 tions, that the quantity of the blood must vary, even in the same animal, 

 very considerably with the amount of both the ingesta and egesta of the 

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





