THE BLOOD. 57 



tant varieties. The lymphocytes are the mono-nucleur elements, and 

 may be large or small, according as the nucleus more or less than 

 half fills up the space of the cell. The polynucleur leucocytes pos- 

 sess more than one nucleus, sometimes four or five nuclei. The 

 eosinophilous leucocytes are those which take the eosin stain. They 

 do not act as scavengers in the system. The phagocytes comprise all 

 leucocytes which are not stained by eosin, or about seventy-five per 

 cent. They serve as scavengers in the body, removing fat and for- 

 eign substances from the blood and attacking and destroying ob- 

 noxious microbes. The leucocytes also assist in the process of anab- 

 olism, migrating through the stigmata of the capillaries to build up 

 worn-out tissues and going to the relief of diseased structures. Leu- 

 cocytes have no cell-wall. 



The plasma is the liquid part of the blood and consists of the 

 fibrinogen and the serum. The fibrinogen is a proteid compound 

 \\hich under the stimulus of the fibrin-ferment forms the fibrin. 

 The fibrin is formed when the blood is exposed to air, heat, etc., 

 solidifying in slender fibres which collect in their meshes the cor- 

 puscles, forming a clot. This process is known as coagulation. The 

 liquid which oozes from the clot is the serum. The serum contains 

 the fibrin-ferm.ent, the serum-globulin, the serum-albumen, and the 

 serosity. The first three of these are proteids and contain nutritive 

 material for the growing cells. The serosity consists of the water 

 and the mineral salts. The mineral salts commonly found in the 

 blood are sodium carbonate, sodium chlorid, sodium phosphate, mag- 

 nesium phosphate, calcium phosphate, and a small amount of sul- 

 phates. 



The microscopic study of the blood is an important aid in de- 

 termining the condition and relative number of its structural ele- 

 ments and the presence of invading parasites. The serum of the 

 blood is believed to be germicidal, but under abnormal conditions 

 it becomes infested with certain species of bacteria. A species of 

 Vermes, Distoma Hwmatobium, and a protozoan, Plasmodium 

 malariw, are two important animal parasites which infest the blood. 



Plasmodium malariae. This parasite is considered by some au- 

 thorities to be a plant; by others it is placed (with other organisms) 

 in a separate kingdom, called Protista. The view here adopted (that 



