562 SPECIAL PHYSIOLOGY. 



of the acetic or lactic acid, during the use of an excess of vegetable 

 diet, the contents of the whole intestinal canal may be acid. In car- 

 nivora, the contents of the caecum, from the presence of ammonia, ex- 

 hibit an alkaline reaction, whilst in the herbivora, they are always 

 acid, from the presence of lactic acid. 



The chemical composition of the contents of the small intestine is 

 dependent on the nature of the food taken. It must also vary at 

 different parts of the canal, according to the composition and quantity 

 of the secretions mixed with it, and according to the relative quantity 

 and nature of the substance's which have been absorbed from it. 

 Thus, the contents of the first part of the duodenum consist of the 

 acid chyme, with bile and pancreatic juice, i. ., of a mixture of the 

 food taken, whether this be bread, milk, meat, or eggs, together with 

 saliva, gastric juice, bile, pancreatic juice, and mucus, minus a certain 

 amount of the water and dissolved substances* which have already 

 been absorbed. These substances, which are almost exclusively ab- 

 sorbed by the bloodvessel, consist of saline matters, unaltered starch, 

 sugar, whether pre-existing in the food, or produced by conversion of 

 starch, dissolved albuminoid and gelatinoid substances, in the shape of 

 albuminose and gelatin-peptone, salivin, pepsin, creatin and other ex- 

 tractive matters, and lastly, traces of alcoholic, ethereal, acid, and 

 various sapid, substances. The sugar found here, is said, by some, to 

 be grape sugar, the conversion of cane sugar into grape sugar being 

 chiefly accomplished, in this part of the alimentary canal, by the 

 agency of the intestinal juice. No fatty matter is yet absorbed, but 

 it all remains in the contents of the upper part of the duodenum. 

 Even after the admixture of the bile and pancreatic juice, all the sub- 

 stances just enumerated still continue to undergo solution and ab- 

 sorption, and the fatty matters, also now emulsified and rendered 

 absorbable, are gradually taken up, together with some of the fatty 

 acids of the bile. The contents of the small intestine are thus pro- 

 gressively robbed of all their dissolved or emulsified nutrient sub- 

 stances, in which they become by degrees poorer. Finally, passing 

 into the large intestine, they acquire a greater consistence and a 

 darker hue. 



The contents of the large intestine have been supposed to undergo 

 an imperfect secondary digestion in the caecum ; and there are reasons 

 for believing that such a process, due to the action of lactic or other 

 acids and of the intestinal juices, may, especially after heavy meals, 

 be continued along the rest of the intestine. This may explain the 

 digestion and absorption of the nutrient substances in enemas, by 

 means of which the system, as is well known, may be for a long time 

 supported. Whether starch is changed, or fat emulsified, is uncertain. 

 The final residue consists chiefly of the insoluble or undigested por- 

 tions of the food, broken down into small fragments. In it are found 

 particles of vegetable matter, such as unaltered starch-grains, woody 

 fibre, remains of vegetable epidermic and other cells, with portions of 

 spiral and annular ducts. Of animal substances there are present 

 portions of yellow elastic tissue, cartilage-cells, unchanged fat, epider- 

 moid and epithelial cells, unchanged fragments of fibrous tissue, such 



