PREPARATION OF TISSUE-FUEL 189 



living units need also a nitrogen-carrying element for 

 their maintenance. It is the preparation of this nitro- 

 genous or proteid element which is the speciality of the 

 factory we are now considering. The gastric juice works 

 at and dissolves the outer stratum of the food mass in 

 the stomach — the layer in contact with the lining mem- 

 brane. The juice also penetrates the food contents 

 of the stomach and prevents them from fermenting or 

 going bad. 



Why does not the gastric juice digest the lining mem- 

 brane in which it is formed ? John Hunter saw that it 

 did attack and digest the wall of the dead stomach ; also 

 that it could not attack certain worms which live in the 

 stomach until they were dead. When we say the lining 

 of the stomach escapes being digested because it is living, 

 or that it has developed an immunity to gastric juice, we 

 do not explain the mystery of the fact ; Nature has 

 developed a protective mechanism of some kind in the 

 gastric lining, the nature of which we do not yet know. 



We now come to one of the marvels of the animal 

 body. We have been speaking of digestion, but have 

 said nothing of absorption — the passage of soluble 

 contents from the cavity of the stomach to the rich 

 capillary blood-field of its lining membrane. We know 

 that certain substances pass through the gastric wall very 

 quickly ; we are conscious of the influence of wine 

 within a few minutes of drinking it, so quickly is alcohol 

 absorbed. Salts, certain poisons, and a percentage of 

 some sugars also enter the blood through the gastric 

 membrane, but starches, fats, and most sugars pass along 

 untouched. Only a small proportion of the meat we eat 

 — perhaps as much as a third — is dissolved and absorbed 

 in the stomach ; the remainder has to be dealt with 

 further along the alimentary tract. The lining membrane 

 into which these substances are absorbed is paved with 

 microscopic living blocks — living units — which are con- 

 stantly forming and throwing out mucus — a glairy lubri- 

 cating substance which keeps the surface clean and 

 moist. Everywhere the pavement is pierced by the 



