The Circulatory System 177 



which the respiratory pigment, hemoglohin, is carried. Hemoglobin is 

 an iron-containing protein which forms the greatest part of the soHd 

 portion of the erythrocyte, and also gives the typical red color to these 

 cells. It will be recalled that this pigment is capable of combining with 

 oxygen to form the compound oxyhemoglobin. 



The er}throcytes are produced principally by special cells in the 

 red marrow in the ends of the long bones, in the ribs, sternum, and 

 certain bones of the skull. When no longer useful, they are destroyed 

 by special cells, the macrophages, of the spleen and the red bone marrow^ 

 The hemoglobin thus made available is utilized in several ways. The 

 iron from it may be used to produce more hemoglobin or it may be 

 stored. Other portions are brought to the liver where they are utilized 

 for bile pigments. The average red blood cell remains in the circulation 

 for about four months. 



NEUTROPHIL 



Fig. 52. — Mammalian blood cells. 



The white blood cells or leucocytes (Fig. 52) are another of the 

 formed elements of the blood. They number considerably less than the 

 red blood cells, and a cubic millimeter of human blood contains from 

 5,000 to 9,000. The principal function of these cells is scavenging, that 

 is, they destroy bacteria and worn-out cells, engulfing them as if the 

 leucocytes were individual living units. In shape, these cells are ir- 

 regular and move by a motion known as amoeboid from the resemblance 

 to the movements of some of the protozoans. 



A number of different types of white blood cells are present in the 

 blood, each with a particular function. These various cells bear names 

 which are indicative of their staining reactions, general appearance, 

 or type of nucleus. Three types are known from their staining re- 



