FUNCTION OF BLOOD 129 



ten per cent solids. The solids consist chiefly of pro- 

 teids, together with fat and other organic substances, 

 salts, and a small amount of fibrin-forming substance. 



Fibrin. Blood plasma upon its escape from the blood- 

 vessels becomes very quickly a jelly-like mass, or clot, 

 by the rapid formation of a close tangle of slender 

 fibres, known as fibrin. When taken singly, these fibres 

 are so delicate that they can be seen only through a 

 microscope. When fresh blood is whipped with a bun- 

 dle of twigs, the fibrin is caught on the twigs, where it 

 forms very elastic white threads. Since the blood which 

 remains does not clot, it is proved that its coagulation 

 depends upon the formation of these threads, which, 

 although constituting but one two-hundredth part of 

 the blood bulk, are yet so numerous that they completely 

 entangle the water and other constituents of the blood 

 and hold it as a firm, fairly dry jelly. That the blood 

 should be able to coagulate is most important, since 

 thereby a vein or small artery when cut is quickly 

 plugged and severe bleeding prevented. If a larger 

 artery is cut, however, the force of the blood is so great 

 as to prevent plugging by the formation of a clot. 



Blood as a common carrier. The general work of the 

 blood is that of a common carrier and as such it fur- 

 nishes the only means of transport within the body. It 

 carries in solution to all the tissues their supply of 

 food, whioh has been absorbed from the digestive tract, 

 and gathers up from the special organs their secretions, 

 which it distributes to all parts of the body. It also 

 collects from the cells the waste products of oxidation 

 and carries them to such organs as the lungs and kidneys 

 for disposal. 



Blood changes. Because of these exchanges with the 

 various tissues of the body, the blood is constantly 



