THE CIRCULATION 



are not harmful to man. For instance, there is a germ 

 which gets into milk and causes it to turn sour. The 

 yeast germ, used in bread making, is a microscopic one- 

 celled plant which obtains its nourishment from sugar. 



140. The germicidal power of the blood is believed by 

 many physiologists to rest, in part at least, in the white 



corpuscles. When the flesh 



is cut, or when bacteria lodge 



in the tissues, these little 

 scavengers may be seen 

 collecting at the danger 

 point in great numbers: 

 some of the germs are de- 

 voured bodily by the white 

 corpuscles : others are killed 

 probably by substances in 

 the plasma which were 

 formed by these little guardi- 

 ans. In no way is the provi- 

 sion for our welfare better 

 shown than by the existence 

 of these corpuscles (Fig. 82). 



141. Experiment on the 

 Parts of the Blood. If 

 some blood from an animal 

 is allowed to stand in a ves- 

 sel, it soon becomes a red, 

 jelly like mass. This change 



is called coagulation. If we let the coagulated blood stand, 

 it gradually separates into two parts, a light yellow liquid 

 called serum, which is colored by a few blood cells, and a 

 semi-solid mass called the clot, which contains most of 

 the cells together with some threadlike fibers (Fig. 83). 

 A substance called animal fibrin was in solution in the 

 plasma or liquid portion of the blood. It has solidified 



FIG. 82. Migration of White Corpuscles 

 through the Walls of a Vein. 



They are shown in different stages of migra- 

 tion. The red corpuscles remain in the 

 stream. 



