THE LEUCOCYTES 



173 



The white cells frequently change their shapes by means of 

 ameboid movements, that is, like the ameba, they thrust out portions 

 of their substance and draw them back. They can send out little 

 prolongations and draw floating particles to themselves, or they 

 can wrap themselves around foreign substances. 



They possess also the power of slipping (squeezing) through the 

 walls of capillary vessels. This is called diapedesis (Fig. 117). 



Of the several varieties of leucocytes the percentage of polymorpho- 

 nuclear cells (nuclei of many shapes) is the largest. 



The polymorphonuclear cells (oftenest called polynuclear) and 

 the lymphocytes are called phagocytes, because they destroy 

 bacteria by absorbing and digesting them. This process is called 

 phagocytosis (to be referred to later on) (pp. 214 and 220). 



The origin of the white cells is from two sources : the lymphocytes 

 originate in lymph glands and other lymphoid tissues; the poly- 

 nuclear leucocytes and others are developed from cells in the marrow 

 of long bones. A third form of colorless cell is called a blood plate 

 or platelet. 



The platelets are very small, being barely one-half the size of 

 the ordinary cells. They are similar to leucocytes; their origin 

 is not understood, but they take an important part in the coagu- 

 lation of blood. 



The plasma is a thin watery saline fluid in which the corpuscles 

 float. It contains both nutritive and waste matters in solution, 

 and certain elements from which fibrin is derived, also enzymes 

 (and certain extractives). Fibrin is essential to the production 

 of a blood-clot, without which hemorrhage would never cease of its 

 own accord. (Fibrin and corpuscles removed serum remains.) 



The substances dissolved in the watery portion of the plasma are : 



Nutritive (derived from food) . . 



Waste products (derived from 

 tissue changes). 



Mineral salts. Chiefly salts of 



Proteins (chiefly) 



Sugars 

 Fats 



Extractives 



Sodium 



Potassium 



Calcium 



( Serum-albumin 

 Paraglobulin 

 Fibrinogen 

 Prothrombin (or 

 Thrombogen) 



Urea 



Uric acid, etc. 



