BIGGEST AND BUSIEST FACTORY 199 



meated by a close-meshed capillary field. The blood 

 which flushes the network of capillaries not only supplies 

 the tubes with the materials needed for the production of 

 the digestive juice, but is also conveniently placed for 

 absorbing the finished products of digestion. Lymph 

 spaces and vessels which are drained by the lacteals also 

 abound in the lining membrane (fig. 42). 



The small bowel thus consists of millions of minute 

 test-tube factories spread out on the interior of a soft- 

 walled pipe, extending to a length of 20 feet, or more, 

 in a fully-grown man. It is manifest, however, that the 

 process of preparation would be assisted if a free supply 

 of digestive juice could be added to the chyme as soon 

 as it has entered the intestine. For this reason an out- 

 growth or annex has been formed from the duodenum — 

 one composed solely of the units which produce the 

 essential ingredients contained in the digestive juice. In 

 this way a special factory — the pancreas or sweet-bread 

 (fig. 41) — has been established. In the course of a day 

 it produces considerably more than a pint of secretion. 

 The pancreas is set into action by a simple mechanism 

 which was discovered some sixteen years ago by Professors 

 Bayliss and Starling. Other observers had noticed that 

 the flow of pancreatic juice began as soon as the acid 

 chyme from the stomach came into contact with the 

 lining of the duodenum. It was known, too, that no 

 " touch-button " mechanism was involved, for the flow 

 could still be started even if all the nerves to and from 

 the pancreas were divided. Professors Bayliss and Starling 

 guessed that a new kind of machinery was set in motion 

 by the acid chyme coming in contact with the lining 

 membrane — a chemical machinery. They found that the 

 contact of acid caused the lining membrane of the 

 duodenum to form a minute quantity of a substance they 

 named secretin. Secretin acts as a missive or letter ; it 

 is posted in the blood and carried round the body until 

 it reaches its proper address, namely, the pancreas. 

 There it sets the enzyme-producing factories at work 

 and at once produces a flow of pancreatic juice which is 



