26 A MANUAL OF PHYSIOLOGY 



separated from them by a thin layer of lymph. And to 

 describe in a word the circulation of the food substances, we 

 may say that the blood feeds the lymph, and the lymph feeds the cell. 



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 blood- 

 vessels 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 trans- 

 parent 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 ; the colourless 

 elements are the white blood-corpuscles or leucocytes ; the 

 liquid in which they float is the plasma (' Practical Exercises, 1 

 p. 168). 



The Red Blood-corpuscles 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 surfaces, or biconcave, 

 and possess no nucleus. But the red corpuscles of the 

 llama and the camel, 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 //..* In the 

 frog the long diameter is about 22 AI, while in Proteus it is as 

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

 can be seen with the naked eye, nearly 80 p (Plate I., i). 



As regards the structure of the red corpuscles the most 

 probable view is that they are solid bodies, with a spongy 

 and elastic structureless framework, denser at the surface of 



* A micro-millimetre, represented by symbol j, is roVv millimetre. 



