THE BLOOD 115 



There may also be present in the blood sub- 

 stances called agglutininS) that arrest the 

 movements of bacteria and throw them 

 into masses or clumps, and in this con- 

 dition they are more readily devoured by 

 leucocytes. Lastly, there are substances 

 known as opsonins, that also appear to 

 increase the phagocytic power of leucocytes. 

 The true nature of these chemical sub- 

 stances is unknown. They are probably 

 proteins, but whether they are different 

 substances or modifications of one substance 

 s a question to be answered by further re- 

 search. The history of this obscure subject 

 is a striking illustration of the complexity 

 of the physiological processes that may 

 possibly occur in the blood. 



59. The blood is rich in proteins, especially 

 in the form of a variety of albumen and of a 

 protein substance known as serum globulin 

 or fibrinogen. It contains traces of many 

 other substances. If we examine the blood 

 as it circulates in the capillaries, under 

 the microscope, we see that the fluid, liquor 

 sanguinis, is an almost colourless fluid, and 



