164 THE BLOOD AND ITS CIRCULATION 



animals some of the ceUs lining the inside of the food cavity take 

 up or engulf minute bits of food. Later, this food is changed into 

 the protoplasm of the cell. Metchnikoff beUeved that the colorless 

 corpuscles of the blood have somewhat the same function. This 

 has been found to be true, for, hke the amoeba, they feed by en- 

 gulfing their prey. This fact has a very important bearing on the 

 relation of colorless corpuscles to certain diseases caused by bacteria 

 within the body. If, for example, bacteria get into a wound, 

 inflammation may set in. Colorless corpuscles called phag'ocytes 



at once surround the spot and 

 attack the bacteria which cause 

 the inflammation. The blood 

 contains certain anti-bodies 

 called op^sonins which, when 

 ^~ , ^ 1 ^. 1 • present, enable the corpuscles 



Colorless corpuscle attacking germs. ^ ' ^ 



to enguK and digest the bac- 

 teria. If the bacteria are few in number, they are quickly 

 destroyed. If bacteria are present in great quantities, they may 

 prevail and kill the phagocytes. The dead bodies of the pha- 

 goc>i;es thus killed are found in the pus, or matter, which ac- 

 cumulates in infected wounds. In such an event, we must come 

 to the aid of the colorless corpuscles by washing the wound with 

 some antiseptic, as weak carbohc acid, lysol, hydrogen peroxide, 

 or zonite. 



Plasma and the Clotting of Blood. — If fresh beef blood is 

 allowed to stand over night, it wiH be found to have separated into 

 two parts, a dark red, almost sohd clot, and a thin, straw-colored 

 Hquid called se'rum. 



If fresh beef blood is poured into a pan and briskly whipped with 

 a bundle of httle rods (or mth an egg beater) , a stringy substance 

 will stick to the rods. This, if washed carefully, is seen to be 

 almost colorless. Tested with nitric acid and ammonia, it is found 

 to contain a protein substance. This is called fi'hrin. 



In blood within the circulatory system of the body, the fibrin 

 is held in a fluid state called fibrin'ogen. Blood plasma, then, is 

 made up of serum and of fibrinogen, which coagulates when blood 

 is removed from the blood vessels, entangles blood corpuscles, and 

 thus forms a blood dot. The clotting of blood is of great physio- 



