QUANTITY OF BLOOD. 57 



tain amount always remains in the bloodvessels. In cases of 

 less rapid bleeding, on the other hand, when life is more pro- 

 longed, and when, therefore, sufficient time elapses before 

 death to allow some absorption into the circulating current of 

 the fluids of the body (p. 76), the whole quantity of blood that 

 escapes may be greater than the whole average amount natur- 

 ally present in the vessels. 



Various means have been devised, therefore, for obtaining 

 a more accurate estimate than that which results from merely 

 bleeding animals to death. 



Welcker's method is the following. An animal is rapidly 

 bled to death, and the blood which escapes is collected and 

 measured. The blood remaining in the smaller vessels is then 

 removed by the injection of water through them, and the mix- 

 ture of blood and water thus obtained, is also collected. The 

 animal is then finely minced, and infused in water, and the 

 infusion is mixed with the combined blood and water pre- 

 viously obtained. Some of this fluid is then brushed on a 

 white ground, and the color compared with that of mixtures 

 of blood and water whose proportions have been previously 

 determined by measurement. In this way the materials are 

 obtained for a fairly exact estimate of the quantity of blood 

 actually existing in the body of the animal experimented on. 



Another method (that of Vierordt) consists in estimating 

 the amount of blood expelled from the ventricle, at each beat 

 of the heart, and multiplying this quantity by the number of 

 beats necessary for completing the "round" of the circulation. 

 This method is ingenious, but open to various objections, the 

 most conclusive being the uncertainty of all the premises 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 bloodvessels ; 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 cer- 

 tain 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 bloodvessels 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 repre- 

 sented was calculated, by comparing the proportion of solid 

 matter contained in it, with that of the first blood which 



