146 HISTOLOGY. 



slower peripheral part of the blood stream, near the endothelium. The 

 leucocytes may be divided into three classes according to nuclear charac- 

 teristics, namely into lymphocytes, large mononuckar leucocytes, and poly- 

 mor phomiclear leucocytes. 



Lymphocytes are large and small. The ordinary small ones are 

 about the size of red corpuscles, 4 to 7.5 [i in diameter. Large ones may 

 double this diameter. Their protoplasm forms a narrow rim, sometimes 

 almost imperceptible, about the dense round nucleus (Fig. 170, A). The 

 chromatin is arranged in a network associated with coarse chromatic 

 masses such as cause a characteristic checkered appearance. Some of 

 the masses rest against the nuclear membrane. Lymphocytes are capable 

 of amoeboid motion but not to the extent of the polymorphonuclear type. 

 They form from 22 to 25% of all leucocytes. 



Large mononuclear leucocytes, sometimes 20 n in diameter, possess 

 round, oval, slightly indented, or crescentic nuclei, which are vesicular 

 and usually eccentric in position. Their chromatin occurs in a few large 



granules; as a whole the nu- 

 cleus is clear and pale (Fig. 

 170, B). The protoplasm, 

 which is abundant, usually 

 lacks coarse granules or other 



FIG. 170. LEUCOCYTES AS SEEN IN A SECTION OF rlictinrtiVp fpatiirpc 



HUMAN TISSUE PRESERVED WITH ZENKER'S FLUID. UlSlincme ICaiUrCS. 



A, Lymphocyte; B, large mononuclear leucocyte; C, three t j mes j t contains a few 



polymorpnonuclear neutropniles. 



philic granules to be described 



presently. These cells are notably phagocytic. They form only from 

 i to 3% of the leucocytes. In certain respects they are intermediate 

 between lymphocytes and polymorphonuclear cells. 



Polymorphonuclear leucocytes are cells somewhat larger than red 

 corpuscles, being from 7.5 to 10 /* in diameter. They are characterized 

 by having nuclei with irregular constrictions leading to an endless variety A 

 of shapes (Fig. 170, C). The nodular subdivisions may be connected by 

 broad bands or by slender filaments. It is said that in degenerating cells 

 the nucleus is divided into several separate masses. Such unusual forms 

 can properly be called 'polynuclear,' an abbreviated term which is a mis- 

 nomer as applied to the ordinary cells; 'mononuclear' as designating the 

 preceding types is also unfortunate since it implies that others have several 

 nuclei. The irregular shape of the polymorphous nuclei has been ascribed 

 to degeneration, comparable with irregularities in the erythroblast nuclei, 

 and also to amoeboid changes associated with those of the cell body. 

 It has been asserted that the nuclei become rounded when the cells are at 

 rest. The latter explanation appears improbable. In the protoplasm a 



