348 DIGESTION. 



different opinions have been expressed in regard to the 

 subject. Dr. Brinton supposes twelve hours to be occupied 

 by the journey of an ordinary meal through the small intes- 

 tine, and twenty-four to thirty-six hours by the passage 

 through the large bowel. 



On the Gases contained in the Stomach and Intestines. 



It need scarcely be remarked that, under ordinary cir- 

 cumstances, the alimentary canal contains a considerable 

 quantity of gaseous matter. Any one who has had occa- 

 sion, in a post-mortem examination, either to lay open the 

 intestines, or to let out the gas which they contain, must 

 have been struck by the small space afterwards occupied 

 by the bowels, and by the large degree, therefore, in which 

 the gas, which naturally distends them, contributes to fill 

 the cavity of the abdomen. Indeed, the presence of air in 

 the intestines is so constant, and, within certain limits, the 

 amount in health so uniform, that there can be no doubt 

 that its existence here is not a mere accident, but intended 

 to serve a definite and important purpose, although, pro- 

 bably, a mechanical one. 



The sources of the gas contained in the stomach and 

 bowels may be thus enumerated 



1. Air introduced in the act of swallowing either food or 

 saliva. 



2. Gases developed by the decomposition of alimentary 

 matter, or of the secretions and excretions mingled with it 

 in the stomach and intestines. 



3. It is probable that a certain mutual interchange 

 occurs between the gases contained in the alimentary 

 canal, and those present in the blood of the gastric and in- 

 testinal blood-vessels ; but the conditions of the exchange 

 are not known, and it is very doubtful whether anything 

 like a true and definite secretion of gas from the blood 

 into the intestines or stomach ever takes place. There can 



