60 



THE BODY AT WORK 



gives the clue to the functions of the several constituents of 

 the blood. The red corpuscles carry oxygen in chemical com- 

 bination with their colouring matter. From them it passes 

 into solution in the plasma ; from the plasma through the 

 walls of the capillary vessels into lymph ; the tissues take it 

 from the lymph as they require it. As fast as it is removed 

 from lymph it is renewed from plasma. Carbonic acid excreted 

 by tissue-cells is dissolved in lymph. From lymph it is trans- 



FIG. 4. RED BLOOD-CORPUSCLES PRESENTING, SOME THE SURFACES, OTHERS THE EDGES, 



OP THEIR DISCS, TOGETHER VilTU SINGLE REPRESENTATIVES OF FOUR TYPES OF LEUCO- 

 CYTE. 



A, the most common type, highly amoeboid and phagocytic. Its protoplasm is finely granular, 

 its nucleus multipartite. B, a leucocyte closely similar to the last, but larger, and con- 

 taining an undivided nucleus. It is shown with a cluster of particles of soot in its body- 

 substance. C, a young leucocyte, or " lymphocyte." D, a coarsely granular leucocyte. 

 Its granules stain brightly with acid dyes e.g., eosin or acid fuchsin. 



f erred to plasma. The reception of carbonic acid by these 

 fluids is not quite so simple as the transference of oxygen from 

 blood to lymph. It is aided by the presence of alkaline carbo- 

 nates which are always ready to form " acid " salts : not acid 

 to litmus-paper the blood is always alkaline but containing 

 more than one unit of acid to one of base. Sodic carbonate has 

 the formula Na 2 C0 3 . With an additional molecule of carbonic 

 acid it becomes Na 2 C0 3 C0 2 (HO) bicarbonate. When in 

 solution it can hold still more carbonic acid. If carbonic acid 



