382 BLOOD AND LYMPH. 



in suspension in this liquid. The proportion by bulk of plasma to 

 corpuscles is usually given, roughly, as two to one. 



Blood-serum and Defibrinated Blood. In connection with the 

 explanation of the term "blood-plasma " just given it will be con- 

 venient to define briefly the terms " blood-serum " and " defibrin- 

 ated blood." Blood, after it escapes from the vessels, usually clots 

 or coagulates; the nature of this process is discussed in detail on 

 page 415. The clot, as it forms, gradually shrinks and squeezes out 

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

 sembles the plasma of normal blood in general appearance, but dif- 

 fers 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 clotting. A more complete definition of 

 these terms will be given after the subject of coagulation has been 

 treated. 



Reaction of the Blood. When tested with litmus or lakmoid 

 paper blood gives an alkaline reaction. This reaction is attributed 

 to the sodium carbonate in solution in the plasma, and the amount 

 of the alkalinity has been determined, therefore, by titration with 

 a weak acid, such as tartaric acid. The acid is employed in 

 a known strength one two-hundredth or one four-hundredth 

 normal solution, ^ or ~ } that is the solution contains in each 

 liter i or of the number of grams represented by the molecular 

 weight of tartaric acid (C 4 H 6 6 = 150). A solution of this strength 

 is added to a known quantity of blood until the sodium carbonate 

 is all neutralized, the end of the reaction being determined usually 

 by one of the recognized indicators, such as litmus or lakmoid. 

 Tested in this way, it has been found that the alkalinity of the blood 

 corresponds to that of an aqueous solution containing from 0.2 to 

 0.3 per cent, of sodium carbonate. Much attention has been paid 

 to the variations in the alkalinity of the blood in different diseases, 

 as also under varying conditions of normal life, and in consequence 

 many methods for determining this alkalinity have been suggested 

 with reference to their clinical application, these methods being 

 characterized by the fact that but little blood is employed. The 



