BLOOD 



483 



IV. Cells of Blood. 

 1. Leucocytes— White Cells. 



These are much less numerous than the red cells, and 

 their number varies enormously in normal conditions. On 

 an average there are about 7500 per cubic millimetre. 

 {The method of counting must he studied practically.) 



They are soft, extensile, elastic, and sticky, and each 

 contains a nucleus and a well-developed double centrosonie. 

 In size they vary considerably, most being larger than the 

 red cells, some slightly smaller. The character of the 



'^mi's>^ 



a 



Fig. 197.— Cells of the Blood, a, erythrocytes ; h, large, and c, small lym- 

 phocyte ; d, polymorpho-nuclear leucocyte ; e, eosinophil leucocyte. 



nucleus varies greatly, and from this and from variations in 

 the protoplasm, they may be divided into three classes. 



(1) Lymphocytes. — Cells with a clear protoplasm and a 

 more or less circular nucleus. Some are very small, while 

 others are larger. They constitute about 20 to 25 per cent, 

 of the leucocytes (fig. 197, 6 and c). 



(2) Polymorpho-nuclear leucocytes have a much-distorted 

 and lobated irregular nucleus and a finely granular proto- 

 plasm whose granules stain with acid and neutral stains. 

 These constitute about 70 to 7 5 per cent, of the leucocytes 

 (fig. ] 97, d). 



(3) Eosinophil or oxyphil leucocytes have a lobated nucleus 



