312 



BIOLOGY 



separating two liquids, i. e., the capillary walls separating the 

 blood and the lymph. Under these conditions osmosis will 

 take place, and thus the same general force which was con- 

 cerned in the passage of the materials from the intestine into 

 the blood, will cause the passage of the same materials from 

 the blood vessels into the lymph in the tissues. This lymph 

 lies in direct contact with the living cells, and these living cells 

 can now take from the lymph the food material that they 

 need. This latter function, by which the living cells take the 

 material that they need, is not explained by any known force, 

 so we speak of it as due to what we still call vital force. 



Respiration. The absorption of oxygen by the blood in the 

 lungs of a frog or the gills (branchiae) of a fish, and the elimi- 

 nation of the carbon dioxid, are also processes which are ex- 

 plainable by simple chemical laws. The blood contains certain 

 substances which have a chemical affinity for oxygen, and 

 others which have a chemical affinity for carbon dioxid. The 

 red coloring matter, haemoglobin, has a chemical affinity for 



oxygen, and will absorb the gas 

 whenever it is in contact with it, 

 provided the pressure of the oxy- 

 gen is sufficient. But this union 

 is a peculiar one. If the atmos- 

 phere contains oxygen under 

 high pressure, the haemoglobin 

 will unite with the oxygen, but if 

 the oxygen pressure is low the 

 haemoglobin will let go of the oxy- 

 gen. As a result, whenever blood 

 passes through the lungs, where 

 there is a large quantity of air and 

 where oxygen is under high pres- 

 sure, the haemoglobin combines 



with oxygen; Fig. 138. The blood is then carried around the 

 body, and when it reaches the active tissues, like the muscles, 



vein 



1 artery 



FIG. 138. AN AIR SAC OF 

 THE LUNGS 



_ Showing the blood vessels distributed 

 in the wall in position to absorb oxygen 

 from the cavity of the sac and excrete 

 carbon dioxid into it. 



