226 MANUAL OF PHYSIOLOGY. 



tical plates, which are slightly convex oil each side, and contain 

 a distinct oval nucleus in their centre. 



The size of the corpuscles varies greatly in different classes of 

 animals, but is strikingly constant in the same class. A glance 

 at the diagram, Fig. 99, in which the corpuscles are drawn to 

 scale, will give an idea of their relative sizes in examples of the 

 different classes of animals, and will make the following points 

 more rapidly obvious than any description. 



The size of the animal has no general relation to the size of the 

 corpuscles. The human red disks are of a fair average size when 

 compared with those of other mammals, and therefore man's blood 

 cannot be distinguished from that of the other mammalia. 



The mammalian corpuscles are, on the whole, small when com- 

 pared with those of the other vertebrates. The batrachians are 

 distinguished by the great size of the corpuscles. Those of the 

 Amphiuma Tridactylum are visible to the naked eye. 



The following measurements are given by Welcker for the hu- 

 man disks : 



Diameter, . . . 0.0,077 of a millimetre = ?sW n of an inch. 



Thickness, . . . 0.0,019 of a millimetre = T2 |j rn th of an inch. 



Volume, . . 0.000,000,077 of a cubic millimetre. 



Surface, . . . 0.000,128 of a square millimetre. 



The last measurement would give about 2816 square metres for 

 the entire blood of an adult. A surface of 11 square metres is 

 exposed every second in the lungs for the absorption of oxygen. 



When circulating in the vessels, or immediately after removal, 

 the red corpuscles are very soft and elastic, being bent and altered 

 in shape by the slightest pressure, and easily stretched to twice 

 their diameter. But the moment pressure or traction is removed, 

 they return to their normal biconcave, disk-shape if the medium 

 in which they lie continue of the normal density. (See Fig. 97, 

 p. 220.) 



Changes take place in the blood shortly after it is removed 

 from the body, which seem to be associated with the loss of func- 

 tion (death) of the red disks, as shown by their rapid destruction 

 if reiutroduced into the circulation. 



