THE BLOOD PLASMA. 145 



diseases their number may vary greatly. The number is also in- 

 creased after meals. A marked increase over normal is known 

 as a leucocytosis. 



THE FUNCTION OP THE LEUCOCYTES. In acute infections, as 

 in appendicitis, pneumonia, and localized or general septic con- 

 ditions in which pus is formed, there is usually a great increase 

 in the number of the polymorphonuclear leucocytes. In more 

 chronic infections, as in tuberculosis, the lymphocytes are found 

 in greater number. In the parasitic diseases of animal origin, as 

 tapeworm and hookworm, in some skin diseases, and in scarlet 

 fever, the eosinophile leucocytes are more abundant. In the 

 disease leueocythsemia the lymphocytes may be present in such 

 great numbers that they impede the movement of blood by in- 

 creasing its viscosity or thickness. The above observations sug- 

 gest that leucocytes play an important role in the protection of 

 the body from infective processes. This function will be dis- 

 cussed later. Another important function they may have is the 

 preparation of the peculiar proteins which are found in the 

 blood plasma. 



THE BLOOD PLATELETS. These bodies are smaller than the 

 erythrocytes, and number about 300,000 in a cubic millimetre 

 of blood. When blood is shed they disintegrate very rapidly, 

 and set free a substance which plays a part in the coagulation of 

 the blood. Little is known concerning their chemical constitution 

 or their physiological function. 



The Blood Plasma. 



The blood plasma is a very complex fluid containing all the va- 

 ried substances associated with the function of the blood. Water 

 composes 90 per cent of the plasma. The plasma proteins consti- 

 tute the largest solid constituent (7 per cent), and include_SOUiL 

 _jy]jjbn1in j serum albumjn, and fibrinogen, There are a number of 

 bodies which contain nitrogen which are not proteins. These 

 may be grouped into two classes, the first, represented by the 

 amino acids and other nitrogenous bodies derived from the pro- 

 tein of the food and from which the tissue cells are built, and the 

 second group, represented by waste materials given off by the 



