THE CIRCULATING LIQUIDS OF THE BODY 15 



Morphology of the Blood. 



The blood consists essentially of a liquid part, the plasma, 

 in which are suspended cellular elements, the corpuscles. When 

 the circulation in a frog's web or lung or in the tail of a tadpole 

 is examined under the microscope, the bloodvessels are seen to 

 be crowded with oval bodies of a yellowish tinge in a thin 

 layer, but in thick layers crimson which move with varying 

 velocity, now in single file, now jostling each other two or three 

 abreast, as they are borne along in the axis of an apparently 

 scanty stream of transparent liquid. Nearer the walls of the 

 vessels, sometimes clinging to them for a little and then being 

 washed away again, may be seen, especially as the blood-flow 

 slackens, a few comparatively small, round, colourless cells. 

 The oval bodies are the red or coloured corpuscles, or erythro- 

 cytes ; the colourless elements are the white blood-corpuscles, 

 or leucocytes ; the liquid in which they float is the plasma 

 (Practical Exercises, p. 177). 



The Red Blood-corpuscles, or Erythrocytes, differ in shape 

 and size and in other respects in different animal groups. In 

 amphibians, such as the frog and. the newt, they are flattened 

 ellipsoids containing a 

 nucleus, and the same is 

 true of nearly all the 

 other vertebrates, except 

 mammals. In mammals 

 they are discs, hollowed 

 out on both the flat sur- 

 faces, or biconcave, and 

 possess no nucleus. But 



the red Corpuscles of the FlG X ._ DIAGRAM SHOWING RELATIVE SIZE OF 

 llama and the camel, RED CORPUSCLES OF VARIOUS ANIMALS. 



although non-nucleated, 



are ellipsoidal in shape like those of the lower vertebrates. As 

 to size, the average diameter in man is between 7 and 8 /A.* 

 In the frog the long diameter is about 22 /x, while in Proteus it 

 is as much as 60 /x, and in Amphiuma, the corpuscles of which 

 can be seen with the naked eye, nearly 80 //, (Plate I. frontispiece). 

 As regards the structure of the red corpuscles, the most prob- 

 able view is that they are solid bodies, with a spongy and elastic 

 structureless framework, denser at the surface of the corpuscle 

 than in its centre, but continuous throughout its whole mass 

 (Rollett). The denser peripheral layer constitutes a physio- 

 logical envelope which permits the passage of certain substances 

 into or out of the corpuscles, and hinders the passage of others. 



* A micro-millimetre, represented by symbol /*, is T ^Vu millimetre. 



m 





