68 THE BLOOD. 



tainty of all the premisses on which the conclusion is 

 founded. 



Other methods depend on the results of injecting a 

 known quantity of water (Valentin) or of saline matters 

 (Blake) into the blood-vessels ; the calculation being 

 founded in the first case, on the diminution of the specific 

 gravity which ensues, and in the other, on the quantity of 

 the salt found diffused in a certain measured amount of 

 the blood abstracted for experiment. 



A nearly correct estimate was probably made by Weber 

 and Lehmann, 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 colour. 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. 



The most reliable of these various means for estimating 

 the quantity of blood in the body yield as nearly similar 

 results as can be expected, when the sources of error un- 

 avoidably present in all, are taken into consideration ; and 

 it may be stated that in man, the weight of the whole 

 quantity of blood, compared with that of the body, is from 

 about i to 8, to i to 10. 



It must be remembered, however, that the whole quan- 

 tity of blood varies, even in the same animal, very consider- 

 ably, in correspondence with the different amounts of food 

 and drink, which may have been recently taken in, and 

 the equally varying quantity of matter given out. Bernard 

 found by experiment, that the quantity of blood obtainable 

 from a fasting animal is scarcely more than a half of that 

 which is present soon after a full meal. The estimate above 



