298 DIGESTION OF FATS. [BOOK IT. 



at present not exactly known. Leucin and tyrosin have been 

 found in the intestinal contents, and may therefore be formed 

 during normal digestion, but whether a large quantity or a small 

 quantity of the proteid material of food is thus hurried into 

 a crystalline form cannot be definitely stated. The extent to 

 which the action is carried is probably different in different 

 animals, and varies also according to the nature of the meal and 

 the condition of the body. Possibly when a large and unnecessary 

 quantity of proteid material is taken at a meal together with other 

 substances, no inconsiderable amount of the proteids undergo this 

 profound change, and, as we shall see, rapidly leave the body as 

 urea, without having been used by the tissues, their contribution 

 to the energy of the body being limited to the heat given out 

 during their formation. To this apparently wasteful use of 

 proteids we shall return in speaking of what is called the 'luxus 

 consumption ' of food. 



Possibly also, in the intestines as in the laboratory, this 

 pancreatic digestion of proteids in excess is accompanied by a 

 considerable development of bacteria and other organized bodies, 

 which create trouble by inducing fermentative changes in the 

 accompanying saccharine constituents of the chyme. That fer- 

 mentative changes may occur in the small intestine is indicated 

 by the facts that the gas present there may contain free hydrogen, 

 and that chyme after removal from the intestine continues at 

 the temperature of the body to produce carbonic acid and hydrogen 

 in equal volumes. This suggests the possibility of the sugar of 

 the intestinal contents undergoing the butyric acid fermentation 

 (during which, as is well known, carbonic anhydride and hydrogen 

 are evolved) and thus, so to speak, put on its way to become fat ; 

 and we shall see hereafter that sugar is somewhere in the body 

 converted into fat. Moreover it is probable that by other fermen- 

 tative changes a considerable quantity of sugar is converted into 

 lactic acid, since this acid is found in increasing quantities as the 

 food descends the intestine. 



Thus during its transit through the small intestine, by the 

 action of the bile and pancreatic juice, assisted possibly to some 

 extent by the succus entericus, the proteids are largely dissolved 

 and converted into peptone and other products, the starch is 

 changed into sugar, the sugar possibly being in part further con- 

 verted into lactic acid, and the fats are largely emulsified, and 

 to some extent saponified. These products, as they are formed, 

 pass into either the lacteals or the portal blood-vessels, so that the 

 contents of the small intestine, by the time they reach the ileo- 

 csecal valve, are largely but by no means wholly deprived of their 

 nutritious constituents. As far as water is concerned, the secretion 

 into the small intestine is about equal to the absorption from it, so 

 that the intestinal contents at the end of the ileum, though much 

 more broken up, are about as fluid as in the duodenum. 



