516 THE GASTRIC JUICE [CH. XXXIII. 



The question has been often raised why the stomach does not digest itself during 

 life. The mere fact that the tissues are alkaline and pepsin requires an acid 

 medium in which to act is not an explanation, but only opens up a fresh difficulty 

 as to why the pancreatic juice which is alkaline does not digest the intestinal wall. 

 To say that it is the vital properties of the tissues that enable them to resist 

 digestion only shelves the difficulty and gives no real explanation of the mechanism 

 of defence. Recent studies on the important question of immunity (see p. 474) 

 have furnished us with the key to the problem ; just as poisons introduced from 

 without stimulate the cells to produce antitoxins, so harmful substances produced 

 within the body are provided with anti-substances capable of neutralising their 

 effects ; for this" reason the blood does not normally clot within the blood-vessels, 

 and Weinland has shown that the gastric epithelium forms an antipepsin, the 

 intestinal epithelium an antitrypsin, and so on. The bodies of parasitic worms that 

 live in the intestine are particularly rich in these anti-bodies. 



Mett's Tubes. 



A method which is now generally employed for estimating the proteolytic 

 activity of a digestive juice is one originally introduced by MetL Pieces of 

 capillary glass-tubing of known length are filled with white of egg. This is set into 

 a solid by heating to 95 C. They are then placed in the digestive fluid at 36 C., 

 and the coagulated egg-white is digested. After a given time the tubes are 

 removed ; and if the digestive process has not gone too far, only a part of the little 

 column of coagulated protein will have disappeared ; the length of the remaining 

 column is easily measured, and the length that has been digested is a measure of 

 the digestive strength of the fluid. 



Hamburger has used the same method in investigating the digestive action of 

 juices on gelatin. The tubes are filled with warm gelatin solution, and this jellies 

 on cooling. They are placed as before in the digestive mixture, and the length of 

 the column that disappears can be easily measured. These experiments must, how- 

 ever, be performed at room temperature, for the usual temperature (36 40 C.) at 

 which artificial digestion is usually carried out would melt the gelatin. He has also 

 used the same method for estimating amylolytic activity, by filling the tubes with 

 thick starch paste. 



Colour Tests for Gastric Acids. 



Hydrochloric acid is absent in some diseases of the stomach, notably in 

 cancer ; many colour tests for hydrochloric acid have been introduced from time 

 to time, but by far the most characteristic and delicate is the following : 



Topfer's test. A drop of dimethyl-amido-azo-benzol is spread in a thin film 

 on a white plate. A drop of dilute hydrochloric acid (up to 1 in 10,000) strikes with 

 this in the cold a bright red colour. 



Lactic acid is soluble in ether, and is generally detected by making an ethereal 

 extract of the stomach contents, and evaporating the ether. If lactic acid is present 

 in the residue it may be identified by the following way : 



A solution of dilute ferric chloride and carbolic acid is made as follows : 



10 c.c. of a 4-per-cent solution of carbolic acid. 



20 c.c. of distilled water. 



1 drop of the liquor ferri perchloridi of the British Pharmacopoeia. 



On mixing a solution containing a mere trace (up to 1 part in 10,000) of lactic 

 acid with this violet solution, it is instantly turned yellow. Larger percentages of 

 other acids (for instance, more than 0'2 per cent of hydrochloric acid) are necessary 

 to decolorise the test solution, but they do not turn the solution yellow. 



Another colour test, that of Hopkins, is performed as follows : 5 c.c. of 

 sulphuric acid and 3 drops of a saturated solution of copper sulphate are added 

 to a few drops of lactic acid dissolved in alcohol. The mixture is placed in boiling 

 water for five minutes, and then cooled; 2 drops of 0'2 per cent, alcoholic solution 

 of thiophene are then added ; on replacing the tube in boiling water, a cherry-red 

 colour develops. 



