THE BLOOD CELLS 



73 



cells are removed from the circulation in the s]>lcen and liver and 

 new cells arising in the blood-forming centers take their places. 



The reasons given for the segregation of hemoglobin in special cells 

 or corpuscles, a condition characteristic of vertebrates and rarely 

 found in animals of the lower phyla, is that capillary walls are so 

 constructed that if the hemoglobin were free in the i)lasma it would 

 leave the capillaries and be eliminated from the circulation, as 

 occurs in cases where red cells are destroyed, as in malaria. ?\u-ther- 

 more, the retention of hemoglobin within the ca])illaries, should it 

 be free in the plasma, w'ould necessitate the capillary walls being 

 so constructed as to prevent its passage. If this were so, various 



Erythrocyte 



Lymphocytes 



Monocyte 



Neutrophil 



Eosinophil- 



Basophil 



Fig. 41. — Diagram showing types of leukocytes. The size is showTi in comparison 

 with that of an erythrocyte. 



other substances whose ])assage through such walls is essential 

 would also be retained. In its present arrangement the hemoglobin 

 held in the red cells is siu'roimded by a fluid stroma whose constantly 

 balanced ])roperties ])ermit it more efficient functioning than would 

 be possible if it were free in the ])lasma of the blood, where the 

 sodium chloride concentration would interfere with its function. 



Leukocytes.— These elements of the blood are called white l)lood 

 cells and all are nucleated. Special stains make possil^le a differ- 

 entiation of five distinct types, which can be separated into two 

 classes. One class lacks granules in the cytoplasm and its cells are 

 know^i as agranulocytes; the other class includes those cells with a 

 distinctly granular cytoplasm, the granulocytes. (Fig. 41.) 



