1 88 THE ENGINES OF THE HUMAN BODY 



stomach is placed in the medulla, and the nerve messages 

 which set it in operation are received from the mouth 

 (nerves of taste), from the nose (nerves of smell), and 

 from the eye (nerves of sight). It is strange that Nature 

 should have adopted such means to set in operation the 

 chemical factory which produces our gastric juice. She 

 has, however, taken good care to protect her contrivance 

 from abuse. For the controlling centre is fully excited 

 only when there is need of food — when there is hunger 

 and appetite. Hunger is the result of hard physical work, 

 and therefore a price has to be paid in the form of labour 

 before the stomach has sauce ready for another meal. 

 There is, however, a further means which is employed 

 for keeping up a flow of gastric juice — one recently dis- 

 covered by Professor Edkins. When digestion of food 

 has begun in the stomach a substance — we may call it a 

 drug, — which is formed in minute amount within the 

 lining membrane of the pyloric part of the stomach, is 

 liberated and passes into the blood. This substance — 

 gastrin — although it circulates all over the body, influ- 

 ences only one part of it, and that is the glandular inner 

 coat of the stomach. It stimulates the minute glands or 

 retorts to pour out the solvent needed for the digestion 

 of food. Thus the stomach, so long as digestion and 

 absorption is proceeding, forms its own stimulant. 



One very large group of foods — the carbohydrate 

 group : sugar, starch, and fats — are scarcely influenced 

 by gastric juice. They are passed on to be turned into 

 tissue-fuel in factories further along the alimentary tract. 

 Its digestive force is spent on nitrogen-carrying foods 

 — meats of all kinds : flesh, fish, fowl, egg, and milk. 

 Those substances are dissolved so that they may be taken 

 into the wall of the stomach, and thus passed on into the 

 blood stream. They are not only made soluble, but they 

 are also changed- — split up into simpler chemical com- 

 pounds, the first step in the preparation of, not tissue- 

 fuel, but tissue-food. Carbohydrate foods provide tissues 

 with the chief elements needed for the production of heat 

 and work, but muscle-cylinders, bone-builders, and other 



