98 ELEMENTARY PHYSIOLOGY less. 



reddish-brown substance containing iron, called hae- 

 matin, and a colourless protein substance. 



Each corpuscle therefore is not to be considered as a 

 bag or sack with a definite skin or envelope containing 

 fluid, but rather as a sort of sj)ongy semi-solid or semi- 

 fluid mass, like a disc of soft jelly ; and as such is capable 

 of imbibing water and swelling up, or giving out water 

 and shrinking according to the density of the fluid in 

 which it may be placed. Thus, if the plasma of blood be 

 made denser by dissolving saline substances, or sugar, in 

 it, water is drawn from the substance of the corpuscle to 

 the dense plasma, and the corpuscle becomes still more 

 flattened and very often much wrinkled. On tlie other 

 hand, if the plasma be diluted with water, the latter foi'ces 

 itself into and dilutes the substance of the corpuscle, 

 causing the latter to swell out, and even become spherical ; 

 and, by adding dense and weak solutions alternately, the 

 corpuscles may be made to become successively spheroidal 

 and discoidal. 



The stroma or framework constitutes but a very small 

 part, 10 per cent., of the solid matter of which the red 

 corpuscles are composed, the remaining 90 per cent, con- 

 sisting of the colouring matter or haemoglobin. The 

 corpuscles may therefore be regarded as simply so many 

 tiny masses of haemoglobin. Now haemoglobin, we may 

 say at once, possesses the remarkable property of uniting 

 in a peculiar way with considerable quantities of oxygen, 

 and thus confers on the red corpuscles their one great 

 characteristic of acting as the carriers of oxygen from the 

 lungs to the tissues of all parts of the body. We have 

 already pointed out (p. 25) the general importance of this 

 transference of oxygen to the tissues ; the details con- 

 nected with the relationshi[) of hjiemoglobin to this trans- 

 ference may be more appropriately dealt with when we 

 study respiration in the next Lesson. 



The colouring matter of the corpuscles is further 



