PROPERTIES AND FUNCTIONS OF LEUKOCYTES. 101 



wall is weak and easily ruptured, as the granules often ap- 

 pear spreading out into the surrounding plasma without any 

 definite boundary. Intermediate forms between the eosino- 

 phile, the polymorphonuclear, and the mononuclear leukocytes 

 are conspicuously absent from the blood. 



These different varieties of leukocytes occur in normal 

 blood in tolerably constant proportions to one another, aver- 

 aging about as follows : 



Small mononuclear leukocytes 22 per cent. 



Large mononuclear and transitional leukocytes 6 " " 



Polymorphonuclear leukocytes 70 " " 



Eosinophile leukocytes 2 " " 



100 



Occasionally a stray mast-cell finds its way into the blood. 

 Variations from the normal occur in morbid conditions ; the 

 total number of leukocytes and the relative number of the 

 different varieties may be altered, or entirely new forms of 

 leukocytes (myelocytes) may appear. 



Vital properties and functions of leukocytes : Leukocytes are 

 typical cells, undifferentiated and independent, actively en- 

 dowed with all the essential attributes of vitality. They are 

 capable of undergoing division, direct and indirect. They 

 (especially the polymorphonuclears) exhibit active auioaboid 

 movements, which enable them to make their way through 

 the walls of the capillaries and wander about in the tissues. 

 By means of their amoeboid movements they also have the 

 power of surrounding and engulfing within their protoplasm 

 foreign particles and living parasites, bacteria and protozoa. 

 This process is called phagocytosis, and cells which engage in 

 it are called phagocytes. 



The act of phagocytosis can sometimes be observed under 

 the microscope, as in connection with the malarial protozoon, 

 and is a remarkable and interesting sight. The leukocyte 

 seems capable of recognizing the presence and whereabouts 

 of the parasite at a distance of many times its own diameter, 

 and moves upon it directly and with distinct purpose, pushing 

 the red corpuscles aside. By protruding pseudopodia toward 



