THE BLOOD. 



213 



FIG. 43. 



HUMAN BLOOD-GLOBULES. a. Red globules, seen 

 flatwise, b. Red globules, seen edgewise, c. White 

 globule. 



their average diameter, in different individuals, varies from 6.70 to 

 8.20 mmm. 



The form of the red blood-globule is that of a spheroid, much 

 flattened on its opposite sur- 

 faces, somewhat like a thick 

 piece of money with rounded 

 edges. If seen flatwise it shows 

 a broad surface and a circular 

 outline (Fig. 43, a) ; but if 

 made to roll over, it presents, 

 during its rotation, the flattened 

 form indicated at b. Its thick- 

 ness is about one-fifth of its 

 transverse diameter. When 

 lying on their broad surfaces, 

 it can be seen that the globules 

 are not exactly flat, but that 

 there is on each side a central 

 depression, the rounded edges 

 being thicker than the middle 

 portion. This produces a differ- 

 ent appearance of the globules 

 when examined within and without the exact focus of the microscope. 

 The substance of which they are composed is more refractive than the 

 plasma in which they are immersed. When viewed, therefore, by trans- 

 mitted light, their thick edges act as double convex lenses, and con- 

 centrate the light above the level of the fluid. Consequently, if the 

 object-glass of the microscope be slightly raised, so that the globules 

 fall beyond its focus, their edges will appear brighter. But their cen- 

 tral portions act as double concave lenses, and disperse the light from 

 a point below the level of the fluid. They thus become brighter when 

 the object-glass is carried downward and the globules fall within its 

 focus. An alternating appearance of the globules may, therefore, be 

 produced by viewing them first beyond and then within the focus of the 

 instrument. When beyond the focus, they are seen with a bright rim 

 and a dark centre. Within it, they have a dark rim and a bright centre. 



When placed under the microscope, the blood-globules, after a fluc- 

 tuating movement of short duration, often arrange themselves in 

 slightly curved rows, adhering to each other by their flat surfaces, 

 and presenting an appearance like that of rolls of coin. This is prob- 

 ably due to the coagulation of the blood, which takes place very rapidly 

 when in thin layers and in contact with glass surfaces ; thus forcing 

 the globules into a position to occupy the least space. 



The color of the blood-globules, viewed by transmitted light and in 

 thin layers, is a light amber or pale yellow. By reflected light, or 

 in thick layers, it is deep red. Their consistency is nearly fluid. They 

 are very flexible, and easily elongated, bent, or distorted in passing 



