128 



THE BLOOD 



FIG. 77. A white blood-cor- 

 puscle sketched at successive 

 intervals of a few seconds to 

 illustrate the changes of form 

 dne to its ameboid move- 

 ments. 



independent life. They make their way through minute 

 openings in the walls of the blood-vessels and wander 



through into the spaces between 

 the cells of the tissues without 

 doing them any harm. By eat- 

 ing up microbes and small par- 

 ticles of foreign matter, they act 

 as scavengers in the body. 

 They are developed in the 

 lymph glands 1 and spleen, 

 from which when mature they 

 are set free in the lymph and 

 blood. 



The white blood corpuscles are present in the blood 

 in the proportion of one white to five hundred red. 

 In certain diseases, however, their number is often in- 

 creased and as a result they are better able to combat 

 the invasion of the harmful microbes to which the 

 disease is due. In abscesses and pustules, many white 

 corpuscles die, either overwhelmed in their attempt to 

 destroy the microbes or smothered as a result of their 

 own overcrowding in the inflamed tissue. The ' ' matter ' ' 

 from abscesses is made up almost entirely of these 

 corpuscles. 



Platelets. A third kind of blood corpuscle, the 

 platelet^ resembles the red blood corpuscle in form but 

 it is smaller, more transparent and so exceedingly deli- 

 cate that it breaks up almost instantly when blood is 

 shed. Practically nothing is known of its functions, ex- 

 cept that it assists in causing the blood to clot. 



Plasma. When the corpuscles are removed, the fluid 

 of the blood, the plasma, remains as an almost colorless 

 sticky liquid, composed of ninety per cent water and 

 1 See pp. 132, 150. 



