4:10 THE BLOOD [oil. XXVI. 



under normal conditions bears a fairly constant relation to the body- 

 weight. The methods employed for estimating it are not so simple 

 as might at first sight have been thought. For example, it would not 

 be possible to get any accurate information on the point from the 

 amount obtained by rapidly bleeding an animal to death, for then an 

 indefinite quantity would remain in the vessels ; 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 

 tinder-estimate, and in the latter over-estimate, the total amount of 

 the blood. 



The method usually employed is 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 saline solu- 

 tion until the washings are no longer coloured, and these are added 

 to the previously withdrawn blood ; lastly, the whole animal is finely 

 minced with 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 com- 

 parison of the colour of the diluted blood with that of standard solu- 

 tions of blood and water of a known strength, until it is discovered 

 to what standard solution the diluted blood corresponds. 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 experiments 

 performed in this way showed that the quantity of blood in various 

 animals differs a good deal, but in the dog averages T V 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 follow- 

 ing data. A criminal was weighed before and after decapitation ; 

 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 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. (Weber and Lehmann.) 



