CIRCULATION 1 1 5 



Structural features as well. They are always nucleated, even in the ma- 

 ture form circulating in the blood. 



Though fewer in number, the white cells are no less important than 

 the red cells. When they are markedly reduced in numbers, as occurs in 

 certain diseases, the individual becomes quite susceptible to infections, 

 especially around the mouth and throat. Very great reduction in numbers 

 (e. g. to 500 or 1,000) is fatal. 



The best known function of the white cells or leucocytes is in the pro- 

 tection of the body against infectious disease. These cells, though usually 

 spherical in the blood stream, possess the capacity for changing their 

 shape and moving about in the tissues, ameba-like. Furthermore, they dis- 

 play the primitive capacity for engulfing particles by phagocytosis and 

 for destruction and digestion of such particles as are of an organic nature. 

 However, it is only in emergency situations that these activities come into 

 play — that of invasion of the body by infectious organisms, or in other 

 cases of tissue destruction. 



In the immediate vicinity of infections by certain kinds of bacteria 

 a remarkable series of reactions occur. When bacteria lodge in the 

 deeper layers of the skin, they commence to destroy the tissues of the skin 

 partly by means of toxic products which they liberate in their metabolism. 

 Changes soon take place in the adjacent blood vessels. They dilate widely, 

 leading a great quantity of blood into the infected region, producing the 

 familiar reddening, and, because blood is warmer than skin, a character- 

 istic, localized warmth. Quantities of fluid enter the tissue from the blood 

 vessels, causing a swelling. 



In all this the blood leucocytes display a typical behavior. They seem to 

 adhere to the blood-vessel wall in the injured area, and, by ameboid move- 

 ment, they migrate through the vessel wall in great numbers. Free from 

 the circulating blood, they migrate toward the bacteria and phagocytize 

 them. Fragments of local cells killed by the bacteria are also engulfed 

 by the leucocytes. The ingested bacteria are usually killed and digested, 

 but in the course of this process numbers of the leucocytes themselves 

 may be destroyed by bacterial poisons. As they disintegrate, the leucocytes 

 liberate their digestive enzymes, which, in turn, act upon other nearby 

 dead cells and cell fragments. The net result is the local accumulation of 

 blood and tissue fluids, digestive enzymes, dead tissue cells, living and 

 dead leucocytes and bacteria, and cell fragments in all stages of disintegra- 

 tion. The whole conglomerate, thick, semifluid mass is called "pus," A 

 rather large collection of pus is called an "abscess." 



Fluctuations in the White Count — In the level of the white-cell con- 

 tent of the blood we encounter another example of a physiological con- 

 stant. Physiological constancy is relative. There are fluctuations, and the 

 normal is more often correctly expressed as a range, with certain upper and 

 lower limits, than as a fixed, dead level. The normal range is larger for the 



