THEOEIES OF CLOTTING 



169 



develop from bone marrow; the origin of lymphocytes is 



uncertain. The leucocytes have various functions. Some, 



called phagocytes, absorb disease-causing bacteria as their 



food, and thus protect the body 



from disease. Others aid in the 



absorption of fats and proteids 



from the digestive tract. They 



also aid in coagulation or clot 



forming and finally help to 



maintain the normal supply of 



proteid in the blood plasma. 



Blood plates. — These are cir- 

 cular or elliptical bodies smaller 

 than red corpuscles. They are 

 more numerous than the white 

 corpuscles, and have been shown 

 to be capable of amoeboid move- 

 ments . They disintegrate rapidly 

 when removed from circulation, 

 and hence little is known of 

 their structure. Their principal 

 function appears to be their 

 action in clotting, which we will discuss under that head. 



Fig. 60 — A phagocyte (white cor- 

 puscle), A, engulfing a bacte- 

 rium, £; 1, 2, 3, steps in tlie 

 process. 



Theories of clotting. — In the beginning of the chapter we 

 noted that blood removed from circulation tended to solidify 

 into a clot. We noted also that the foundation of this clot was 

 the coagulated fibrinogen or fibrin, of the blood plasma, and that 

 the failure of whipped blood to clot was due to the removal of 

 this fibrinogen by whipping. These experiments, however, merely 

 give us an explanation of what the clot is made of, namely, of 

 fibrinogen. The fibrinogen is made to coagulate and forms fibers 

 of solid proteid miatter called fibrin, between whose strands are 

 caught the corpuscles, and to whose shrinking is due the squeezing 



