THE BLOOD AND ITS CIRCULATION 



367 



to every 100 c.c. (See Chapter XXVII.) Some waste products, 

 to be spoken of later, are also found in the plasma. 



Clotting of Blood. If fresh beef blood is allowed to stand overnight, 

 it will be found to have separated into two parts, a dark red, almost solid 

 clot and a thin, straw-colored liquid called serum. Serum is found to be 

 made up of about 90 per cent water, 8 to 9 per cent proteid, and from 

 1 to 2 per cent sugars, fats, and mineral matter. 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 proteid substance called fibrin. 



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

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

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

 It is this coagulation which aids in the formation of 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 physi- 

 ological importance, for otherwise we might bleed to death from the 

 smallest wound. 



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 proteid 

 in the blood, causes it to change 

 to an insoluble form, the fibrin of 

 the clot. 



The Red Blood Corpuscle; its 

 Structure and Functions. The red 

 corpuscle in the blood of the frog 

 is a true cell of disklike form. The 

 red corpuscle of man, however, 

 lacks a nucleus. Its form is that 

 of a biconcave disk. So small and 

 so numerous are these corpuscles 

 that over five million are found in a drop of normal blood. 

 The color, which is found to be a dirty yellow when separate 

 corpuscles are viewed under the microscope, is due to a proteid 



Human blood as seen under the 

 high power of the compound 

 microscope ; at the extreme right 

 is a colorless corpuscle. 



