388 A TEXTBOOK OF PHYSIOLOGY 



In man, but not in the dog, there is probably an invertase present 

 which converts cane-sugar into dextrose and levulose. 



Why does the Stomach not Digest itself ? It may be asked: " Why 

 does the stomach not digest itself ?" There are probably several con- 

 ditions contributing to this; among the chief reasons assigned are 

 the following: 



1. The large amount of mucin secreted is protective against the 

 secreted juice. 



2. The circulation of a large amount of alkaline blood keeps the 

 cells of the wall alkaline in reaction. 



The cells of the wall are living, and living substance can, by its 

 reactions, resist attack, neutralize acid, secrete antiferment, etc. 

 Any part of the stomach wall from which the blood-supply is cut 

 off e.g., by thrombosis is attacked. Gastric ulcers result from 

 lessened powers of resistance. The injection of gastric mucous mem- 

 brane of guinea-pig into rabbit may cause a specific cytolysin to form 

 in the rabbit's serum. Injection of this serum into the guinea-pig 

 may cause gastric ulcer. It has been shown that if the hind-leg of 

 a living frog is introduced into the stomach it is digested by the gastric 

 juice. Probably, acid first kills the tissue and then digestion takes 

 place. On the other hand, the alkali of the pancreatic juice is not 

 sufficiently strong to alter the structure, and so no digestion takes place. 



Absorption in the Stomach. The absorption of water from the 

 stomach is very small in amount. This has been shown to be the 

 case by injecting a measured quantity through the pyloric orifice, 

 keeping this orifice closed, and drawing the water off again after a 

 given time. There is a small but definite absorption of salts and sugar, 

 while the absorption of protein in the form of peptides (abiuretic bodies) 

 amounts to 20 to 30 per cent. The absorption of alcohol is rapid, and 

 bodies soluble in it are therefore well absorbed, such as strychnine 

 and other drugs. 



The Examination of Gastric Contents. Much importance is attached 

 by some clinicians to the examination of the gastric contents after a 

 test-meal, which is given in the morning after fasting twelve hours. 

 The meal usually consists of 50 grammes bread and a little tea without 

 milk ; it is important that it should be withdrawn by the stomach-tube 

 at a definite time (three-quarters of an hour) after the meal. It 

 should be noted that the contents consist of 



1 . The remnants of the test-meal. 



2. The mixed secretions of the stomach. 

 3*. The swallowed saliva. 



It must be remembered that the amount of secretion may vary, as 

 may also the rate of emptying of the stomach and the rate of absorption. 

 For this reason it is difficult from a test-meal alone to say definitely 

 what is (1 ) the secretory power, (2) the motor power, (3) the absorptive 

 power of the stomach. The chief points to ascertain about gastric 

 contents are the total amount of the acidity of its contents ; its nature, 



