THE BLOOD. 167 



The Osmotic Pressure of Blood Plasma. It has already been pointed 

 out that the osmotic pressure of the plasma is the same as that of 

 the corpuscles, so that it is also the same as that of the blood 

 as a whole. It has been found by experiment that the proteins of 

 plasma have a slight osmotic pressure, but the osmotic pressure is 

 chiefly due to the inorganic salts. One method of ascertaining the 

 osmotic pressure of blood is to determine how much lower its freezing 

 point is than that of water, and in the case of blood it is found 

 to be O56 C., which is equivalent to that of a 0'9 per cent, solution 

 of sodium chloride. 



If the osmotic pressure of the blood is higher than that of the 

 tissues, water will pass from the tissues into the blood in the capillaries, 

 and salts will diffuse from the blood into the tissues. If the osmotic 

 pressure of the blood is lower than that of the tissues, the reverse 

 processes will occur, water passing from blood to tissues, and salts from 

 tissues to blood. This interchange is an important factor in the main- 

 tenance of the balance between the intake and output of water and salts- 



PROTECTIVE AND OTHER SUBSTANCES IN THE PLASMA. 



A large number of substances, when introduced under the skin or 

 directly into the circulation (but not when given by the mouth), give 

 rise to the formation by the tissues and the setting free in the blood- 

 stream of products which tend to destroy or to precipitate the substance 

 introduced, or to neutralise its action. The products thus formed are 

 called antibodies, those which excite their formation being called 

 antigens. Antigens are colloidal, and almost any protein, including 

 harmless bodies such as egg-white and caseinogen, can act as an 

 antigen. Crystalloid substances of small molecular weight, such as 

 sugar, seem unable to give rise to antibodies. 



If a little human blood serum is injected into a rabbit on several 

 occasions at intervals of a week, the blood serum of the rabbit acquires 

 the power, when tested in vitro, of precipitating the proteins of human 

 serum, but not those of the serum of other animals. The substance 

 thus formed in the rabbit's blood is called a precipitin. If the rabbit 

 is injected with sheep's serum, the precipitin formed will precipitate 

 sheep's serum in vitro, but not that of any other animal. The precipitin 

 acts, therefore, only on the serum of an animal of the same species as 

 that from which blood is taken for injection into the rabbit ; and the 

 reaction is said to be specific. Since this reaction is not only one of 

 the most delicate known tests for the presence of blood, but also makes 

 it possible to ascertain the species of animal from wliich the blood was 



