112 THE BLOOD AND ITS CIRCULATION. 



Welcker. The head of an animal is cut off; all the blood 

 which flows is collected, and its coloring power measured. 

 The body is then cut in pieces, and, being thoroughly washed, 

 all the blood is withdrawn. By comparing the coloring 

 power of the bloody water thus obtained with that of the 

 blood which was first extracted, it is easy to calculate the 

 proportion of blood contained in the water, and to compute 

 the quantity in the whole body. But there are objections 

 to this method also, among which we need only mention 

 that by washing we obtain not only the blood, but also the 

 coloring matter of the muscles, of the marrow, of the spongy 

 bones, of the spleen, etc. ; these all being derived from that 

 of the blood, and included in this liquid, would give it more 

 than its proper value. 



It is, however, generally agreed, as the result of experi- 

 ments made in this way, that the total weight of the blood 

 is at least one-thirteenth part of the total weight of the 

 body, which would give for man five kilogrammes of blood, 

 his mean weight being sixty-five kilogrammes. 



The quantity of blood varies also according to circumstan- 

 ces : the state of fasting or digestion has the greatest effect 

 in influencing the quantity, and the difference in these states 

 even may be twofold. This has been directly proved by 

 Cl. Bernard. He killed two dogs, one of which was fasting, 

 and the other in the midst of the process of digestion. He 

 proves it indirectly by showing that, to kill an animal in 

 which digestion is going on, a dose of poison is required 

 (strychnine, for instance) double that which would suffice to 

 kill the same animal while fasting. It is true we must re- 

 member that in the former case, not only the system in gen- 

 eral is glutted with liquids, but the anatomical elements 

 themselves are saturated, and thus much less fitted for the 

 absorption of poison. Collard de Martigny mentions a still 

 more significant fact, which is this : in order to kill a rabbit 

 in its ordinary state by bleeding, thirty grammes of blood 

 must be taken from it ; but, after a three days' inanition, the 

 taking of seven grammes will produce the same result. We 

 can easily see how important this fact is to the physician, in 

 regard to bleeding a patient at the beginning of an illness, or 

 after several days' restricted diet. 



Composition of the Blood. If we examine the blood 

 from an anatomical point of view (as a tissue), we find that 

 it is composed of two distinct parts : the cruor, which com- 

 prehends the solid part, the globules / and the liquor, which 



