GENERAL PROPERTIES: THE CORPUSCLES. 417 



(6) Polynuclear or polymorphonuclear leucocytes. The nucleus is seg- 

 mented into lobes connected by narrow strands. The cytoplasm 

 is especially ameboid and is granular. The granules in most cases 

 are neutrophilic and small in size. The typical cells of this kind 

 form the bulk of the white corpuscles of the blood, 60 to 75 per 

 cent. Eosinophilic leucocytes form a subgroup of this variety. They 

 have a similar segmented nucleus, but the cytoplasm contains nu- 

 merous coarse granules that stain in acid dyes, such as eosin, whence 

 the name. 



(c) Mast cells. These peculiar cells exist in very small numbers under 

 normal conditions, less than 1 per cent, of the total number of 

 white corpuscles. They have a polymorphic nucleus like the pre- 

 ceding group, but differ in the fact that the granules in the cyto- 

 plasm are strongly basophilic, that is, will stain only with basic 

 dyes, such as thionin. 



According to some authors the small lymphocytes are cells 

 that have an origin and function different from those of the granular 

 leucocytes. While the latter are supposed to originate from cells 

 (leucoblasts, myeloblasts) in the bone marrow, the lymphocytes 

 are produced in the nodules of the lymph glands and lymphoid 

 tissue, and enter the blood through the lymph circulation. Others, 

 however, lay stress on the fact that lymphocytes occur in the bone- 

 marrow and hold that it is possible or probable that the lympho- 

 cytes of the blood may be derived from the marrow tissue as well 

 as from the lymphoid tissue. The subject is one that at present is 

 discussed chiefly in connection with the pathology of blood dis- 

 eases.* 



Variations in Number. Under normal conditions the total 

 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 

 leucopenia. 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 leucocytes 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 

 *See Emerson, 'Clinical Diagnosis," 1906, p. 483. 



