68 



BLOOD 



familiar red color of the blood is due to reflection of light from 

 the surface of its innumerable colored corpuscles. The suspen- 

 sion in the blood plasma of a multitude of transparent disks 

 which act in part as biconcave center of the corpuscle and in 

 part as biconvex circular lenses corpuscular rim accounts in 

 great measure for the opacity of thick layers of blood. 



The average diameter of a red blood cell is 7.5 /A (-j^W inch) ; 

 its thickness varies from 2 to 4 /*. These cells vary but little in 

 size. Seventy-five per cent of the red cells in human blood will 

 measure 7.5 /A, the remainder are either a little smaller or a little 

 larger than the average, the normal extremes being about 6 /* and 

 8 fji respectively. Those of extreme size are called megalocytes, 

 while the smallest are microcytes. 



FIG. 73. FROM A FRESHLY PREPARED, UNSTAINED SPECIMEN OF HUMAN BLOOD. 

 Three leucocytes, an eosinophile, a polynuclear, and a lymphocyte, are represented. 

 Many red blood cells, some " on the flat," some in rouloux and in profile, are also shown, 

 x 1200, but reduced somewhat in reproduction. (After Schafer.) 



The number of red corpuscles in the blood is subject to con- 

 stant variation between wide limits. Many physiological condi- 

 tions influence their total number, as well as the relative proportion 

 of red cells to the white. The average number of red blood cells 

 in the adult male is about 5,000,000 per cubic millimeter. In 



