THE CIRCULATORY SYSTEM. 



105 





FIG 



129 



THE BLOOD. 



While, when physiologically considered, the blood is regarded, 

 with Bernard, best as an internal medium of exchange, histologically 

 it may be classed as a mesodermic tissue possessing a fluid inter- 

 cellular substance, the liquor sanguinis ; in the latter float the 

 cellular elements the blood-corpuscles. 



The morphological constituents of the blood are of two kinds, 

 the colorless or white corpuscles and the colored or red cells ; 

 to these must be added a third variety, the blood-platelets or 

 blood-plaques, which are probably constant and independent ele- 

 ments. 



THE COLORLESS CELLS OF THE BLOOD. 



The colorless blood-cells are not peculiar to the blood,, since 

 they originate in lymphoid tissues and bone- marrow and are car- 

 ried into the blood-current, in which fluid they usually are ob- 

 served. Critical examination 

 of suitably stained preparations 

 of human blood shows that the 

 colorless cells include several 

 varieties which are character- 

 ized by differences of size, nu- 

 cleus, and granularity. 



These varieties are : 



i. Small lymphocytes, 

 which are about the same size 

 as the red blood-cells, or some- 

 what smaller, and contain a 

 nucleus relatively so large that 

 almost the entire cell is occupied leucocyte; c> transition form; rf , po iymorphonu- 



by this body. The protoplasm clear cells: e, eosinophilic leucocyte; r.redcorpus- 



is confined to a thin inconspicu- 

 ous envelope and is usually devoid of granules. This variety may 

 be regarded as the youngest form, and probably originates solely 

 within lymphoid tissue. 



2. Large mononuclear leucocytes, two or three times larger 

 than the red cells, are frequently of an oval form and contain a nu- 

 cleus which stains less intensely than in the younger cells, owing to 

 its comparative^ poverty in chromatin. The protoplasm of these leu- 

 cocytes contains but few, if any, pale granules. 



3. Transitional leucocytes, which represent more advanced 

 stages of development, are characterized by a nucleus exhibiting vary- 

 ing degrees of indentation} so that the nucleus often appears kidney- 



Leucocytes in normal human blood, stained and 

 highly magnified : a, small lymphocytes ; b, large 







