A POSTAL SYSTEM 223 



by the living units lining the upper part of the 

 duodenum. The arrival in the duodenum of the 

 acid contents of the stomach caused the dispatch of secre- 

 tin, the substance or hormone which acts as a missive 

 for stimulating the pancreas to action. Secretin is 

 posted in the nearest letter-boxes or capillaries in the 

 duodenal wall and is carried away in the general blood 

 circulation, which serves for all kinds of postal traffic. 

 In a postal system where there are no sorters and which 

 must be conducted by an automatic mechanism, letters or 

 missives cannot be addressed in the usual way. Their 

 destination is indicated not by their inscription but by 

 their shape. The molecules of secretin may be regarded 

 as ultramicroscopic Yale keys sent out to search for the 

 locks of letter-boxes which they can fit and enter. They 

 fit and can enter only the letter-boxes of the pancreatic 

 molecules, and hence they must circulate round the body 

 until they automatically find their destination. What is 

 still more wonderful in this system is that the letter- 

 boxes, or we may call them locks, have a positive attrac- 

 tion for the key-missives which are destined for them. 



To such key-missives as are posted in and delivered 

 by the general circulation, Professor Starling gave the 

 name of hormones. The muscular engines of the body 

 use the carbon dioxide which they cast into the circulation 

 as a hormone or missive to inform the respiratory centre 

 in the medulla of their needs and to stimulate it to 

 action. The harder the muscles work the more is their 

 need for oxygen and the greater is the quantity of carbon 

 dioxide which has to be got rid of ; the respiratory 

 bellows must meet their requirements by ventilating the 

 lungs more rapidly. The more the carbon dioxide 

 accumulates in the circulation, the more acid becomes 

 the reaction of the blood ; and the more the blood becomes 

 acid in reaction, the greater becomes its power to stimu- 

 late the respiratory centre. In this instance the carbon 

 dioxide missives deliver their message while on their way 

 to a final destination — the lungs. As our knowledge 

 increases we see that a postal system is extensively used 



