THE BLOOD-CORPUSCLES 15 



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 com- 

 paratively small, round, colourless cells. The oval bodies are the 

 red or coloured corpuscles, or erythrocytes ; the colourless elements 

 are the white blood-corpuscles, or leucocytes; the liquid in which 

 they float is the plasma (Practical Exercises, p. 193). 



The Red Blood-Corpuscles, or Erythrocytes, differ in shape and 

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

 ians, such as the frog and the newt, they are flattened ellipsoids 

 containing a nucleus, and the same 

 is true of nearly all the other ver- 



tebrates, except mammals. In 



i- 

 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- 



i , j 11- -j i i Fig. i. Diagram showing Relative 



nucleated, are ellipsoidal in shape, | ize of Re corpuscles of Various 



like those of the lower vertebrates. Animals. 



As to size, the average diameter 



in man is between 7 and 8 fj,.* In the frog the long diameter is 



about 22 /j,, while in Proteus it is as much as 60 /j,, and in Amphiuma, 



the corpuscles of which can be seen with the naked eye, nearly 



80 ft (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 physiological envelope 

 which permits the passage of certain substances into or out of the 

 corpuscles, and hinders the passage of others. In the large oval 

 corpuscles of Necturus (see Frontispiece) the envelope can be clearly 

 demonstrated as a detachable membrane comparable to the mem- 

 brane surrounding the nucleus. 



Envelope and spongework are sometimes spoken of as the stroma 

 of the corpuscle, in contradistinction to its most important con- 

 stituent, a highly complex pigment, the haemoglobin. This pigment 

 is not in solution as such, for its solubility is not nearly great 

 enough to permit this, but either in solution as a compound with 

 * A micro-millimetre, represented bv symbol /*, is ^-^ millimetre. 



