DIGESTION IN THE SMALL INTESTINE. 419 



absorbed, while the nitrogen remains unaltered. Therefore, the oxygen 

 will be in smaller relative proportion, as contrasted with the nitrogen, 

 than is found in the atmosphere; the carbon dioxide, which is also 

 present, will, on the other hand, be in relative excess, since a certain 

 quantity of this gas diffuses from the venous blood into the interior of 

 the alimentary canal. Hydrogen, ammonia, and carburetted and sulphu- 

 retted hydrogen are also found as the results of various decompositions. 



Y. INTESTINAL DIGESTION IN DIFFERENT ANIMALS. The contents 

 of the stomach do not pass suddenly, but gradually, into the small 

 intestine, their entrance into this portion of the alimentary canal 

 not being due to the forcible contraction of the stomach, but to a 

 series of periodic relaxations of the pylorus, as a result of special 

 stimuli ; during the intervals, the pylorus is tightly closed. Transition 

 into the intestine rarely occurs before digestion has. made considerable 

 advances; the necessary stimuli, therefore, cannot be of a purely 

 mechanical nature, especially as it has been found- that mechanical stimuli 

 lead to more marked contraction of the p}'loric ring rather than to its 

 relaxation. The stimulus which leads to a relaxation of the pyloric 

 sphincter is probabty of a chemical nature, but its exact mode of opera- 

 tion is entirely unknown. It would seem, however, that fat is almost 

 incapable of causing an opening of the pylorus, for, no matter how much 

 fat be given in the food, nearly the same amount will be found in the 

 intestine after four, five, or twent3'-one hours, showing that the pylorus 

 allows no more to pass than may be taken up by the villi ; therefore, 

 there is never an accumulation of fat in the intestine. 



If, during the latter stage of gastric digestion, a transverse section 

 is made through the duodenum, on the mucous surface will be found a 

 white, pasty emulsion ; then comes a yellowish, cheesy precipitate, pro- 

 duced by the bile, and in the centre will be found a thin, j^ellowish-brown 

 liquid, containing particles of undigested food. If such a section be 

 made still lower down in the intestine, further from the stomach, the 

 fatty layer will be found to have decreased in amount, while the central 

 fluid will be relatively more abundant, and becomes darker and darker 

 in color, until at the lower portion of the small intestine it is of a deep- 

 green color. Some of the fluid constituents contain many gas-bubbles. 

 The central fluid has alwaj r s a more strongly acid or less alkaline reac- 

 tion than the portion of the intestinal contents in contact with the 

 intestinal walls. 



As we have seen, the chyme, as it issues from the stomach, is sub- 

 jected to the action of the intestinal secretions before being ready for 

 absorption. The character of the chyme, as indicating the-character of 

 the digestive processes, varies in different animals, and has been closely 

 studied by Colin, whose description is here mainly followed. In 



