332 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the terms " blood-serum " and " defibrinated blood." Blood, after it escapes 

 from the vessels, usually clots or coagulates ; the nature of this process is 

 discussed in detail on p. 352. The clot, as it forms, gradually shrinks and 

 squeezes out a clear liquid to which the name blood-serum is given. Serum 

 resembles the plasma of normal blood in general appearance, but differs from 

 it in composition, as will be explained later. At present we may say, by way 

 of a preliminary definition, that blood-serum is the liquid part of blood after 

 coagulation has taken place, as blood-plasma is the liquid part of blood before 

 coagulation has taken place. If shed blood is whipped vigorously with a rod 

 or some similar object while it is clotting, the essential part of the clot 

 namely, the fibrin forms differently from what it does when the blood is 

 allowed to coagulate quietly ; it is deposited in shreds on the whipper. Blood 

 that has been treated in this way is known as defibrinated blood. It consists 

 of blood-serum plus the red and white corpuscles, and as far as appearances 

 go it resembles exactly normal blood ; it has lost, however, the power of clot- 

 ting. A more complete definition of these terms will be given after the sub- 

 ject of coagulation has been treated. 



Reaction. The reaction of blood is alkaline, owing mainly to the alka- 

 line salts, especially the carbonates of soda, dissolved in the plasma. The 

 degree of alkalinity varies with different animals : reckoned as Na 2 CO 3 , the 

 alkalinity of dog's blood corresponds to 0.2 per cent, of this salt; of human 

 blood, 0.35 per cent. The alkaline reaction of blood is very easily demon- 

 strated upon clear plasma free from corpuscles, but with normal blood the red 

 color prevents the direct application of the litmus test. A. number of simple 

 devices have been suggested to overcome this difficulty. For example, the 

 method employed by Zuntz is to soak a strip of litmus-paper in a concentrated 

 solution of NaCl, to place on this paper a drop of blood, and, after a few 

 seconds, to remove the drop with a stream of water or with a piece of filter- 

 paper. The alkaline reaction becomes rapidly less marked after the blood has 

 been shed ; it varies also slightly under different conditions of normal life 

 and in certain pathological conditions. After meals, for instance, during the 

 act of digestion, it is said to be increased, while, on the contrary, exercise 

 causes a diminution. In no case, however, does the reaction become acid. 

 For details of the methods used for quantitative determinations of the alka- 

 linity of human blood, reference must be made to original sources. 1 



Specific Gravity. The specific gravity of human blood in the adult male 

 may vary from 1041 to 1067, the average being about 1055. Jones 2 made 

 a careful study of the variations in specific gravity of human blood under 

 different conditions of health and disease, making use of a simple method 

 which requires only a few drops of blood for each determination. He found 

 that the specific gravity varies with age and sex, that it is diminished after 

 eating and is increased by exercise, that it falls slowly during the day and 

 rises gradually during the night, and that it varies greatly in individuals, " so 



1 Peiper : Virchow's Archiv, vol. cxvi., 1889, p. 337. 



2 Journal of Physiology, vol. xii., 1891, p. 299. 



