BLOOD CELLS 173 



when very distended they are known as " foam cells." The 

 nucleus is usually well marked when unmixed with fat and only 

 surrounded with a narrow margin of protoplasm ; the presence 

 of fat produces the characteristic honeycombed appearance of 

 the colostral bodies and such cells are only found in patho- 

 logical conditions and at the beginning and end of the lacta- 

 tion period. Some observers report that these large cells may 

 contain several nuclei, but Ernst never found more than one 

 and suggested that the apparent multiplication of nuclei was 

 due to mononuclear cells becoming superimposed. 



Blood Cells, (a) Red blood cells or erythrocytes appear as 

 biconcave discs or as thorn-apple shaped cells containing meta- 

 chromatic granules. 



(b) Leucocytes. These constitute a very considerable per- 

 centage of the total cells in normal physiological conditions 

 and may entirely predominate in pathological ones. All 

 varieties of leucocytes may be found but the usual frequency 

 of occurrence is in the following order: polymorphonuclears, 

 lymphocytes, large mononuclears, and transitionals. 



The polymorphonuclear leucocytes, of which the majority 

 are neutrophylic in their staining properties, are usually 7.5 to 

 10 M in diameter and stain characteristically with methylene 

 blue as a deeply stained lobed or polymorphonucleus sur- 

 rounded by faintly coloured protoplasm. The lymphocytes are 

 usually considerably smaller (5.7 /x) than the " polymorphs " 

 but vary very considerably in size. The nucleus is round and 

 occupies practically the whole of the cell. Mononuclear leu- 

 cocytes are much larger than the lymphocytes (usually 13-16 ju 

 but may be 25 M in diameter) and two to three times the size of 

 erythrocytes. The nucleus is large and oval and is eccentrically 

 situated in a relatively large amount of protoplasm. With 

 methylene blue the nucleus stains moderately well and the 

 cytoplasm contains fine amorphous particles which produce 

 the appearance of ground glass. With Leishmann's stain the 

 nucleus is ruby coloured and the cytoplasm blue but containing 

 a few ruby granules. The transitional cells are about the size of 



