THE BLOOD AND ITS CIRCULATION 315 



other organic foods, and 1 per cent mineral substances. In these 

 respects it very closely resembles the fluid food that is absorbed from 

 the intestines. 



If another jar of fresh beef blood is poured into a pan and briskly 

 whipped with a bundle of little rods (or with an egg beater), a stringy sub- 

 stance will be found to 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 which is called fibrin. 



Blood plasma, then, is made up of a fluid portion of serum, and 

 fibrin, which, although in a fluid state in the blood vessels within 

 the body, coagulates when blood is removed from the blood vessels. 

 This coagulation aids in making a blood clot. A clot is simply a 

 mass of fibrin threads with a large number of corpuscles tangled 

 within. The clotting of blood is of great physiological importance, 

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



Blood Plates. In blood within the circulatory system of the 

 body, the fibrin is held in a fluid state called fibrinogen. An 

 enzyme, acting upon this fibrinogen, the soluble protein in the 

 blood, causes it to change to an insoluble form, the fibrin of the 

 clot. This change seems to be due to the action of minute bodies 

 in the blood known as blood 

 plates. Under abnormal 

 conditions these blood 

 plates break down, releas- 

 ing some substances which 

 eventually cause this en- 

 zyme to do its work. 



The Colorless Corpuscle; 

 Structure and Functions. 

 A colorless corpuscle is a 



... . .. , A small artery (A) breaking up into capillaries 



Cell irregular in Outline, the (c) which unite to form a vein (F). Note 



Shape Of which is Constantly at ( P) several colorless corpuscles, which are 



_. fighting bacteria at that point. 



changing. These corpuscles 



are somewhat larger than the red corpuscles, but less numerous, 

 there being about one colorless corpuscle to every three hundred 

 red ones. They have the power of movement, for they are found 

 not only inside but outside the blood vessels, showing that they 



