THE MICROSCOPE IN MEDICINE AND SANITATION. 109 
of the white corpuscles will be treated somewhat more fully 
in the next section; but their presence in urine is of 
much significance since it is these cells which mainly 
make up the whitish pus discharged from inflamed 
surfaces. When the walls of the genito-urinary tract are 
seriously affected, cells detached from its lining epi- 
thelium will be noted in the urine. The appearance 
of some of these elements is indicated in Fig. 42. 
Finally, most renal diseases are characterized by the de- 
posit in the fine tubules of the kidney of an albuminous 
substance which coagulates to form minute twisted tubes 
(Fig. 43). These casts found in the urine may be clear 
and hyaline or may be rendered granular by the presence 
of disintegrated epithelial cells or may contain undecom- 
posed epithelial cells or pus cells or fat globules or bacteria 
according to the special pathologic condition from which 
they arise. So-called waxy casts are denser and more 
sharply marked than any other type and indicate an 
advanced stage of renal disease. 
4. Examination of Blood.—The blood is a colorless 
fluid or plasma containing cells of several distinct types 
whose number and relative proportions vary in many 
physiologic and pathologic states. Few clinical tests are 
of more value to the physician both in diagnosis and 
prognosis than the examination of this body fluid under 
the microscope. 
For the study of the various types of cells the blood 
must be dried and stained; and the preparation of a 
good blood smear for staining is a matter of some deli- 
cacy. The blood may be obtained from the lobe of 
