388 FOODS CIRCULATED AND USED IN THE BODY 



of the blood tubes. Like the amoeba, the colorless corpuscles 

 feed by engulfing their prey. This fact has a very important 

 bearing on the relation of the corpuscles to certain diseases caused 

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

 wound, colorless corpuscles, called phagocytes (fag'6-sit), at once 

 surround the spot and attack the bacteria which cause the 

 inflammation. The blood contains certain antibodies called 

 opsonins (6p'sS-nin), which, when present, enable the corpuscles 

 to engulf and digest the bacteria. 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 phagocytes thus killed and the destroyed 

 tissue help form pus which also contains many dead and living 

 bacteria. When such an infection occurs, we must come to the 

 aid of the colorless corpuscles by washing the wound with an 

 antiseptic. 



Laboratory Exercise. What causes blood to clot ? Wash your finger 

 thoroughly with soap and water. Holding the finger down, prick it 

 with a sterilized needle. Draw off three drops of blood, placing each 

 drop on a clean microscopic slide. Place the first slide at once on ice. 

 To the second add a drop of 5 per cent sodium oxalate solution. 

 Leave the third drop exposed to the air of the room. At intervals of 

 one minute draw a clean hair through each drop. 



Note how long it takes the third drop of blood to clot. Compare 

 this drop with the drop on ice and the drop to which the sodium oxalate 

 was added. 



Laboratory Exercise. Let fresh beef blood stand over night. What 

 happens ? Whip fresh beef blood briskly with an egg beater. A stringy 

 almost colorless substance will stick to the beater. This, if washed care- 

 fully and tested with nitric acid and ammonia, is found to contain a 

 protein substance. It is called fibrin (fi'brin). 



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

 is held in a fluid state called fibrinogen (fi-brin'6-jen). Blood 

 plasma, then, is made up of a thin liquid, serum, and of fibrinogen 

 which coagulates under certain conditions, entangling the blood 

 corpuscles in a network of fine threads, thus forming the clot. 



The clotting of blood is of great physiological importance, for 

 otherwise we might bleed to death even from a small wound. A 

 substance called thrombin is the active agent in changing fibrin- 



