I 1 6 READINGS IN BIOLOGICAL SCIENCE 



white cells than for the reds. Counts anywhere from about 5,000-9,000 

 per cu. mm. are found in normal adult individuals and are said, therefore, 

 to lie within the range of the normal. Even in this same person fluctua- 

 tions of this magnitude may occur from time to time. Attempts to cor- 

 relate these changes with other normal physiological activities have not 

 always been very successful. It has been said by some, for example, that 

 during digestion or muscular exercise or exposure to cold there is a tem- 

 porary increase in numbers. Other investigators have been unable to con- 

 firm these findings. To estabHsh relationships of this sort would be most 

 important, for it might yield suggestions as to the possible functions of 

 the white cells in the normal individual. 



There is no question about the increases in the white count which 

 attend certain infections. In pneumonia, appendicitis, tonsillitis, and many 

 other infectious diseases, the white count is elevated to 12,000, 15,000, 

 25,000, or perhaps even to 50,000. This we call a "leucocytosis." With the 

 aid of the white count, therefore, it is possible for physicians to detect the 

 presence of infections in internal structures like the appendix, which are 

 hidden from view. Within limits, the degree of the leucocytosis parallels 

 the severity of the infection, so that counts taken at frequent intervals 

 often give evidence as to whether the infectious process is increasing in 

 severity or is subsiding. 



THE BLOOD PLATELETS 



The third and last kind of formed elements are the blood platelets. They 

 are roughly disk-shaped, far smaller than red cells, and show none of the 

 special internal structural difi"erentiation characteristic of cells. Their 

 origin is obscure, although it is suggested that they too arise partly in the 

 red bone marrow because bone-marrow injury often markedly reduces 

 their numbers. Recent observations indicate that they are also formed by 

 phagocytic cells in the lungs. A platelet count is difficult to make because 

 of the rapidity with which these bodies disintegrate in abnormal sur- 

 roundings. Such counts as have been made indicate that the normal varia- 

 tions cover a \vide range. Though averaging about 250,000 per cu. mm., 

 counts anywhere from 200,000 to 600,000 have been considered normal. 



CIRCULATION OF THE BLOOD 



The heart is a muscular organ lying within the thorax, inclosed in a 

 sac of fibrous connective tissue (the pericardium). In mammals it is com- 

 pletely divided by a partition into two parts, the so-called "left heart" 

 and "right heart." Leading from the left heart is a large vessel, the aorta, 

 which arches upward, backward, and then downward, extending to the 

 lower abdominal cavity. All along its course it gives off" arteries, which 

 branch more and more profusely into smaller and smaller vessels, the 

 capillaries, whose walls are but one cell in thickness, whose internal call- 



