444 BLOOD AND LYMPH. 



place normally, but is vastly accelerated under pathological con- 

 ditions. As to the function or functions fulfilled by the leucocytes, 

 numerous suggestions have been made, some of which may be 

 stated in brief form as follows: (1) They protect the body from 

 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. 424). 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 " (ydfew, 

 to eat; /curor, 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. The leucocytes which seem especially adapted to 

 attack bacteria are the polymorphonuclear variety, designated 

 by Metchnikoff as microphags. This power of the leucocytes to 

 ingest bacteria depends, according to Wright, upon the presence 

 in the plasma of certain substances designated as opsonins (from 

 opso'no, I prepare food for), which sensitize or in some way prepare 

 the bacteria so that they are attacked by the leucocytes. These 

 opsonins, like the cytotoxins, belong to the group of antibodies, 

 and may be called into existence or increased in amount by the 

 injection into the body of suitable bacteria or their products.* 

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

 aid in the absorption of peptones from the intestine. (4) They take 

 part in the process of blood coagulation. (5) They help to main- 

 tain the normal composition of the blood-plasma in proteins. 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 disc- 

 shaped bodies which appear as short rods or as circular or elliptic 

 plates, according as they are seen on edge or on the flat face. 

 They vary in diameter on the flat face, but are in all cases much 



* For a brief general discussion of opsonins, see Hektoen, "Science," 

 Feb. 12, 1909. 



