58 THE BLOOD. 



Quantity of Blood. 



Only an imperfect indication of the whole quantity of 

 blood in the body is afforded by measurement of that 

 which escapes, when an animal is rapidly bled to death, 

 inasmuch as a certain amount always remains in the blood- 

 vessels. In cases of less rapid bleeding, on the other 

 hand, when life is more prolonged, and when, therefore, 

 sufficient time elapses before death to allow some absorp- 

 tion into the circulating current of the fluids of the body 

 (p. 84), the whole quantity of blood that escapes may be 

 greater than the whole average amount naturally present 

 in the vessels. 



Various means have been devised, therefore, for obtain- 

 ing 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 col- 

 lected and measured. The blood remaining in the smaller 

 vessels is then removed by the injection of water through 

 them, and the mixture 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 com- 

 bined blood and water previously obtained. Some of this 

 fluid is then brushed on a white ground, and the colour 

 compared with that of mixtures of blood and water whose 

 proportions have been previously determined by measure- 

 ment. 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. 1 



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 uncer- 



