264 



THE BLOOD AND THE LYMPH 



FIG. 137 



no nucleus can be made out in the completed corpuscle it is usually 

 supposed that none exists. Fehrsen always found nucleated red cor- 

 puscles up to the third hour after birth. It is barely possible, however, 

 that the erythrocyte has a nucleus in a finely divided, scattered, granular 

 condition, such as Gruber has called attention to in the unicellular 

 rhizopod Pelomyxa. The abundance of nuclein present in the corpuscle 

 and the closeness of its union with the hemoglobin (see page 256), 

 tend to make this supposition at least possible and it has at present 

 some support from observers. On general biological principles it is 



more or less probable, but inasmuch as 

 reproduction of the erythrocytes them- 

 selves or any suggestion of it has seldom 

 or never been observed, if perfect cells 

 structurally they have become highly 

 enough differentiated to lose one of the 

 most basal of cellular functions. The 

 so-called Poggis' corpuscles may prove 

 to have some explanatory value in this 

 respect. 



In shape the erythrocytes are biconcave 

 disks with a diameter four times the great- 

 est thickness. This is their shape when 

 lying flat or free from restraint, but they 

 are very flexible and elastic and are readily 

 bent nearly double and distorted in various 

 ways by the pressure of the circulation in 

 the capillaries, etc. This biconcave-disk 

 shape is perfectly adapted to best serve 

 their function, for it makes them at once 

 very flexible and very resilient, and these 

 are important properties, as may be seen 

 in a moment's observation of the circu- 

 lation in a frog's foot or mesentery. It 

 also places at its maximum the surface- 

 area which they expose to the tissue -cells 

 and to the alveoli of the lungs, thus making 

 more rapid the diffusion and absorption of oxygen and perhaps of carbon 

 dioxide, which are their sole known functions. Their combined surface- 

 area in an adult of average size is not far from thirty-two hundred square 

 meters, an almost astounding figure explained, despite the extreme 

 smallness of each corpuscle, by the fact that the five liters of a man's 

 blood contains about twenty-five million million (25,000,000,000,000) of 

 these tiny masses of oxygen-bearing hemoglobin. Their shape, more- 

 over, is that which exposes a maximum of surface in using a minimum 

 of hemoglobin. Various physical agents, for example, heat and elec- 

 tricity, cause the erythrocytes to become distorted into many curious 

 shapes, the most common of which is the crenated form, seen also as the 



i 



An erythrocyte in cross-section. 

 The triangle a, b, c, suggests how the 

 surface-area of the corpuscle is in- 

 creased by the concavity with a less 

 amount of material. 



