GENERAL PROPERTIES I THE CORPUSCLES. 441 



form or variety of parent cells that has the characteristics of a 

 large or small lymphocyte. Most authors perhaps beheve that the 

 various forms as they exist in the blood are from an anatomical 

 standpoint permanently differentiated. Such a view implies, on 

 the physiological side, that each form has some special functional 

 activity of its own. Little or no progress has been made, how- 

 ever, in discovering the specific physiology of the various leu- 

 cocytes. 



Variations in Number. — Under normal conditions the tota! 

 number of leucocytes may show considerable variation; the aver- 

 age number in health varies usually between 5000 and 7000 

 per cubic millimeter. A distinct increase in number is designated 

 as a condition of leucocytosis, a marked diminution as a condition of 

 leiicopenia. Leucocytosis occurs under various normal conditions, 

 such as digestion, exercise or cold baths, pregnancy, etc. The 

 variations, relative or absolute, under pathological conditions, have 

 been studied with exhaustive care as an aid to diagnosis and classi- 

 fication. 



Functions of the Leucocytes. — Perhaps the most striking 

 property of the leucocj^tes as a class is their power of making 

 ameboid movements, — a characteristic which has gained for them 

 the sobriquet of "wandering" cells. By virtue of this property 

 some of them are able to migrate through the walls of blood capil- 

 laries into the surrounding tissues. This process of migration takes 

 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. 423). 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 "(faj'^'J^, 

 to eat; kuto^, 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 



