418 BLOOD AND LYMPH. 



pathogenic bacteria and other foreign cells or organisms. In 

 explanation of this action it has been suggested that they may 

 either ingest bacteria, and thus destroy them directly, or they 

 may form certain substances, bacteriolysins, that destroy the 

 bacteria. The wonderful protective adaptation of the body des- 

 ignated by the term "biological reaction" has already been referred 

 to (p. 399). The formation of immune substances in the blood is 

 attributed, in part at least, to the leucocytes. Leucocytes that 

 act by ingesting the bacteria are spoken of as " phagocytes " (y-dfeev, 

 to eat; proc, cell). This theory of their function is usually 

 designated as the "phagocytosis theory of Metchnikoff " ; it is 

 founded upon the fact that the ameboid leucocytes are known to 

 ingest foreign particles, including bacteria, with which they come 

 in contact. One of the most interesting recent developments in 

 pathology in this connection is the discovery (Wright) that this 

 power of the leucocytes to ingest bacteria depends upon the presence 

 in the plasma of certain substances designated as opsonins (from 

 oyovsev, to prepare food for). While the nature of the opso- 

 nins is not known, their amount can be determined relatively 

 for different bacteria by the degree of phagocyting activity of the 

 leucocytes. Whether . or not there is a specific opsonin for each 

 pathogenic organism is a subject still under discussion. (2) They 

 aid in the absorption of fats from the intestine. (3) They aid in 

 the absorption of peptones from the intestine. It may be noticed 

 here that these theories apply to the leucocytes found so abundantly 

 in the lymphoid tissue of the alimentary canal, rather than to those 

 contained in the blood itself. (4) They take part in the process 

 of blood coagulation. A complete statement with reference to 

 this function must be reserved until the phenomenon of coagula- 

 tion is described. (5) They help to maintain the normal composi- 

 tion of the blood-plasma in proteins. It may be said for this view 

 that there is considerable evidence to show that the leucocytes 

 normally undergo disintegration and dissolution in the circulating 

 blood, to some extent at least. The blood proteins are peculiar, 

 and they are not formed directly from the digested food. It is 

 possible that the leucocytes, which are the only typical cells in 

 the blood, aid in keeping up the normal supply of proteins. From 

 this standpoint they might be regarded in fact as unicellular glands, 

 the products of their metabolism serving to maintain the normal 

 composition of the blood-plasma. The formation of granules 

 within the substance of the eosinophiles offers a suggestive analogy 

 to the accumulation of zymogen granules in glandular cells. 



Physiology of the Blood Plates. The blood plates are small, 

 circular or elliptical bodies, nearly homogeneous in structure and 

 variable in size (0.5 to 5.5 //), but they are always smaller than the 

 red corpuscles. Less is known of their origin, fate, and functions 



