THE BLOOD. 



Fig. 76. 



HUMAN BLOOD-GLOBULES. a, Red glob- 

 ules, seen flatwise, b. Red globules, seen edge- 

 wise, c. White globule. 



globules , and, according to the observations of Schmidt, over 90 per 

 cent, of those contained in a single specimen have the same dimensions. 

 The smallest size observed is 4.50 mmm. (Harting), and the largest 9.3 



mmm.; while their average di- 

 ameter, as found in different 

 individuals, varies from 6.70 to 

 8,20 mmm. 



The form of the red globule 

 is that of a spheroid, very much 

 flattened on its opposite sur- 

 faces, somewhat like a thick 

 piece of money with rounded 

 edges. The globule accordingly, 

 if seen flatwise, presents a com- 

 paratively broad surface and a 

 circular outline (Fig. 76, o); 

 but if it be made to roil over, 

 it will present itself edgewise 

 during its rotation, and assume 

 the flattened form indicated 

 at b. The thickness of the 

 globule, seen in this position, 

 is about one-fifth of its transverse diameter. When the globules are 

 examined lying upon their broad surfaces, it can be seen that these 



surfaces are not exactly flat, 

 but that there is on each side a 

 slight central depression, so that 

 the rounded edges of the blood- 

 globule are evidently thicker 

 than its middle portion. This 

 inequality produces a remarka- 

 ble optical effect. The substance 

 of which the blood-globule is 

 composed refracts light more 

 strongly than the fluid plasma. 

 Therefore, when examined with 

 the microscope by transmitted 

 light, the thick edges of the 

 globules act as double convex 

 lenses, and concentrate the light 

 above the level of the fluid,, 

 Consequently, if the object-glass 

 be carried upward by the ad- 

 justing screw of the microscope, and lifted away from the stage, so that 

 the blood-globules fall beyond its focus, their edges will appear brighter. 

 But the central portion of each globule, being excavated on both sides, 

 acts as a double concave lens, and disperses the light from a point be- 



Fig. 77. 



RED GLOBULES OF THE BLOOD, seen a little 

 beyond the focus of the microscope. 



