viii THE FUNCTIONS OF THE BODY 151 



Then there is the production of bile by the liver. 



' It seems to have a slight solvent, emulsifying, and saponifying 

 action on fats ; in some animals it is said to have a slight power 

 of converting starch into sugar ; by its alkalinity it helps the action 

 of the trypsin of the pancreas (which, unlike pepsin, acts in an alka- 

 line fluid) ; it affects cell-membranes, so that they allow the passage 

 of small drops of fat and oil ; and it is said to have various other 

 qualities." * 



We have referred to this in a little detail because the 

 contrast between the old pronouncement that the func- 

 tion of the liver is to secrete bile and what we know of 

 its multiple vital functions is an index of physiological 

 progress. 



6. Respiration. Life has been spoken of as a process 

 of slow combustion, and it is certain that oxidations are 

 continually occurring. To keep the fire of life burning, 

 oxygen is needed, and that is the plus side of respira- 

 tion. But the oxidations involve the formation of 

 carbon dioxide, and the getting rid of this poisonous gas 

 is the minus side of respiration. In general, the output 

 of carbon dioxide is a measure of the rate of metabolism. 

 In green plants in daylight there is a nutritive process 

 of utilising the carbon dioxide of the atmosphere in the 

 synthesis of sugar and the like, splitting it up, and return- 

 ing the oxygen to the air ; this disguises the respiratory 

 process, which must be studied at night. There are 

 some very puzzling cases among animals e.g. parasitic 

 threadworms living in a medium with no oxygen ; it is 

 probable that in these animals an oxygen-producing 

 ferment or oxidase forms oxygen at the expense of some 

 constituent of the body. 



In Protozoa and simple aquatic animals the oxygen 

 mixed in the water (familiarly seen as bubbles on the 

 glass when a tumbler of cold water is brought into a 

 warm room) diffuses into the living matter, which has a 

 great affinity for it, and carbon dioxide diffuses out. As 

 organisms became bigger and tougher, two improve- 

 ments were effected, the blood, or it might be some 



1 The Author's Outlines of Zoology, 6th edition, 1914, 



