vi INTESTINE AS AN OEGAN OF EXCRETION 347 



striated border being still visible ; more often, however, they are 

 imperfect, or reduced to the bare nuclei. 



This completes our definite knowledge of the composition of 

 human faeces, under normal conditions. 



The development of gases in the digestive canal, which mix 

 with the air swallowed with the food and the saliva, is largely 

 associated with the complex process of formation of the faeces. 

 This development of gases arises from the fermentative and putre- 

 factive processes of the intestinal bacteria. It is therefore entirely 

 absent during iutra-uterine life, when the contents of the intestine 

 are destitute of microbes. Since the development of gases is due 

 to decomposition of the various food -stuffs, it follows that the 

 gaseous mixture must vary in composition according to the nature 

 of the diet. 



The oxygen of the swallowed air is entirely or almost entirely 

 absent in the intestinal canal, no doubt because it is rapidly 

 absorbed by the blood, through the mucous membrane which func- 

 tions as the respiratory surface. This absorption of oxygen with 

 simultaneous excretion of carbonic acid takes place almost exclu- 

 sively in the gastric cavity, where the presence of the hydrochloric 

 acid of the gastric juice normally checks any fermentation of the 

 chyme, with ebullition of gases. The gaseous mixture in the 

 stomach, therefore, consists of the air swallowed with the food and 

 the saliva, and to a less extent of the duodenal gases, which may 

 penetrate the pyloric orifice, or diffuse through it. Planer found 

 in the gases of the dog's stomach 66-68 per cent nitrogen, 23-33 

 per cent carbonic acid, and only - S-6 - l per cent oxygen. These 

 data show that a respiratory exchange takes place in the stomach, 

 the oxygen of the air swallowed being absorbed by the blood 

 circulating in the capillaries of the mucous membrane, while the 

 carbonic acid of the blood passes into the air of the stomach, and 

 partially mixes with that which comes from the duodenal gases. 



The gases of the intestinal contents were analysed by Planer in 

 the dog. They vary in composition in the small and large intes- 

 tine with a flesh and a vegetable diet, as shown by the following 

 table of volumetric percentages : 



