356 



DIGESTION 



different form from that in which it exists in the juice namely, as 

 a zymogen or mother-substance. But while the zymogen of the 

 pancreatic lipase is activated by bile, this is not the case with the 

 mother-substance of the gastric lipase. It appears that in the 

 suckling the lipase of the gastric juice plays a more important part 

 than in later life. This is obviously in accordance with the fact that 

 the specific food of the suckling milk contains as an essential con- 

 stituent a large proportion of emulsified fat. The conditions for 

 the emulsification of fat do not exist in the gastric juice, and this is 

 the reason why the gastric lipase has so slight an effect upon un- 

 emulsified fat, which presents a surface of contact proportionally so 

 small. In any case the amount of fat hydrolysed in the stomach 

 under ordinary con litions is small in comparison with the amount 

 split in the intestine, although it has been shown that with a diet 

 rich in fat some of the intestinal contents, including pancreatic 

 lipase, may pass back into the stomach. 



As regards the carbo-hydrates, the swallowed saliva will continue 

 to act on starch in the stomach, so long as the acidity is not too great ; 

 while the hydrochloric acid of the gastric juice is able to invert cane- 

 sugar, changing it into a mixture of dextrose and levulose,* and 

 also, doubtless, to hydrolyse to dextrose a portion of the maltose 

 formed by the saliva. Altogether, there is no doubt that the pro- 

 portion of the carbo-hydrates of the food digested in the stomach is 

 far from insignificant. 



The Antiseptic Function of the Gastric Juice. The stomach, with 

 its acid contents, forms during the greater part of gastric digestion 

 a valve or trap to cut off the upper end of the intestine from the 

 bacteria-infested regions of the mouth and pharynx, and to destroy 

 or inhibit the micro-organisms swallowed with the food and saliva. 

 The occasional presence in vomited matter of sarcinae or regularly 

 arranged groups of micrococci, generally four to a group, shows that 

 under abnormal conditions the gastric contents are not perfectly 

 aseptic ; and even from a normal stomach active micro-organisms 

 of various kinds can be obtained. But upon the whole there is no 

 doubt that the acidity of the gastric juice is an important check on 

 bacterial activity during the first part of digestion, and in the upper 

 portion of the alimentary canal. Koch has shown that the acidity 

 of the gastric juice of a guinea-pig is sufficient to kill the comma 

 bacillus of cholera. Normal guinea-pigs fed with cholera bacilli 



* These are both reducing sugars, but, as their names indicate, they rotate 

 the plane of polarization in opposite directions. The specific rotatory power 

 of levulose is greater than that of dextrose, so that when cane-sugar is com- 

 pletely inverted, although equal quantities of dextrose and levulose are pro- 

 duced, the plane of polarization is rotated to the left. Cane-sugar itself rotates 

 it to the right. The term ' inversion ' has been extended to include the 

 similar hydrolysis of other sugars of the disaccharide group e.g., maltose to 

 dextrose, and lactose to a mixture of dextrose and galactose, even although 

 the products are not levo-rotatory. 



