STRUCTURE OF LEUKOCYTES. 97 



White blood-corpuscles (Fig. 41) or leukocytes, are most 

 appropriately, perhaps, called colorless blood-corpuscles. The 

 leukocytes of man are of several varieties, differing in some 

 particulars but similar in their general characteristics. 



In shape when at rest they are spherical, but during their 

 amoeboid movements they are irregular and changing in 

 form. 



In size the different varieties range from about 7 to 15 /u in 

 diameter, averaging about 10 fj. ; they are thus somewhat 

 larger than the erythrocytes. They are colorless, refractile, 

 and granular in appearance. 



Their surface is somewhat adhesive (unlike the red corpus- 

 cles), so that they adhere to the glass in microscopical prepa- 

 rations, and in the blood-current they roll slowly along at 

 the edge of the stream on the wall of the bloodvessel. 



Thev are of firmer consistency than the red corpuscles, and 

 their form is not so easily affected by mechanical influences 

 or changes in their environment. If the density of the fluid 

 in which they occur be much increased, as by the addition of 

 salts, they shrivel somewhat; while if the density be dimin- 

 ished by dilution with water, they assume the resting spheri- 

 cal form, become swollen, and within them appear coarse 

 protoplasmic granules which often exhibit the Brownian 

 movement. 



The number of leukocytes under normal conditions is about 

 7500 or 8000 in each cubic millimetre of blood ; the number is 

 greater in young children and during pregnancy ; and during 

 digestion, as three or four hours after a proteid meal, their 

 number is normally increased about one-third (the "digestion 

 leukocytosis "). The leukocytes are therefore far less numer- 

 ous than the red corpuscles, the ratio being normally about 1 

 white to 600 red. In pathological conditions the number of 

 leukocytes may vary widely. Leukocytes do not occur in 

 circulating blood alone, but are also met with as lymph-cor- 

 puscles in the lymphatic system ; as wandering cells in the 

 connective tissues ; in enormous accumulations in pus as pus- 

 corpuscles, etc. 



The structure of leukocytes is that of typical actively vital 

 cells. They have a cell-body of active protoplasm, well- 

 marked nuclei, and are said to be provided with centrosomes, 



7 Hist, 



