350 DIGESTION 



family as well as for himself, and proceeded with his investigations, 

 A second time St. Martin went back to his native country, and a second 

 time the zealous investigator of the gastric juice, at heavy expense, 

 secured his return. And although his experiments were necessarily less 

 exact than would be permissible in a modern research, the modest book- 

 in which he published his results is still counted among the classics of 

 physiology. The production of artificial fistulae in animals, a method 

 that has since proved so fruitful, was first suggested by his work. 



Gastric juice when obtained pure, as it can be from an acci- 

 dental fistula in man, or, better, by giving a dog with an oesophageal 

 as well as a gastric fistula a ' sham-meal ' (p. 402), is a clear, thin, 

 colourless liquid of low specific gravity (in the dog 1003 to 1006) 

 and distinctly acid reaction. The total solids average about 

 5 parts per thousand, of which the ash (chiefly sodium and potas- 

 sium chloride, with small quantities of calcium and magnesium 

 phosphate) represents about a fourth, and heat-coagulable sub- 

 stances (proteins, nucleoprotein) about a third. None of these has 

 any special importance in digestion. Of quite a different significance 

 are the three ferments present: pepsin, which changes proteins 

 into peptones; rennin, which curdles milk; and a fat-splitting fer- 

 ment or lipase which, under certain conditions at least, splits 

 up emulsified neutral fats e.g., the fat of milk into the 

 alcohol (glycerin) and the fatty acids linked with it, but has so 

 little action upon non-emulsified fat, that when this is taken into 

 the stomach, it eventually passes into the duodenum practically 

 unchanged. The acidity is due to free hydrochloric acid, the other 

 important constituent of the juice. In the dog the proportion of 

 this acid varies from 0-46 to 0-58 per cent. In such analyses as 

 have been made of approximately pure human gastric juice a smaller 

 percentage of hydrochloric acid has usually been obtained (at most 

 0-35 to 0-4 per cent.). But there is some reason to believe that if 

 the human juice could be collected in a faultless manner, and 

 especially free from any admixture with saliva or with a pathological 

 secretion of mucus, it would show as high a percentage of acid as 

 the dog's juice. 



In cases of cancer, whether the growth is situated in the stomach 

 or not, the free hydrochloric acid of the gastric juice is usually 

 much reduced, and often absent. Under such conditions some 

 lactic acid may be present in the stomach, being produced from the 

 carbo-hydrates by the action of bacteria (Bacillus acidi lactici), 

 which are normally held in check by the hydrochloric acid, although 

 not rendered incapable of growth when they have passed on into 

 the intestine. Even in the strength of 0-07 to 0-08 per cent, hydro- 

 chloric acid prevents the formation of lactic acid from dextrose. 

 Indeed, when all the hydrochloric acid of the gastric juice is com- 

 bined with proteins, the protein-acid compound still inhibits the 

 growth of bacteria in the stomach, although not so efficiently as the 



