CH. xxvi.] BLOOD COAGULATION. 407 



are taken and allowed to fall into fluids of known specific gravity. 

 When the drop neither rises nor sinks in the fluid it is taken to 

 be of the same specific gravity as that of the standard fluid. The 

 reaction of blood is faintly alkaline and the taste saltish. Its 

 temperature varies slightly, the average being 3 7 - 8 C. (100 F.). 

 The blood stream is warmed by passing through the muscles, 

 nerve centres, and glands, but is somewhat cooled on traversing 

 the capillaries of the skin. Recently drawn blood has a distinct 

 odour, which in many cases is characteristic of the animal from 

 which it has been taken. It may be further developed by adding 

 to blood a mixture of equal parts of sulphuric acid and water. 



Quantity of the Blood. The quantity of blood in any 

 animal 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 under- 

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

 blood. 



Of the several methods which have been employed the most 

 accurate 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 

 solution 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 added to the previously withdrawn blood ; lastly the whole 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 comparison of the colour of the diluted 

 blood with that of standard solutions 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 withdrawn to 



