DIGESTION OP FOOD. 353 



fold aspect. They contain undigested and indigestible rem- 

 nants, the ferments and certain decomposition products of the 

 digestive fluids, and true excretory matters. 



In carnivorous and omnivorous animals, including man, 

 the undigested materials are those that have escaped the action 

 of the secretions — such as starch and fats— together with those 

 substances that the digestive juices are powerless to attack, 

 as homy matter, hairs, elastic tissue, etc. 



In vegetable feeders a larger proportion of chlorophyl, cel- 

 lulose, and starch will, of course, be found. 



These, naturally, are variable with the individual, the spe- 

 cies, and the vigor of the digestive organs at the time. 



Besides the above, certain products are to be detected in the 

 faeces plainly traceable to the digestive fluids, and showing 

 that they have undergone chemical decomposition in the ali- 

 mentary tract, such as cholalic acid, altered coloring-matters 

 lilse urobilin, derivable probably from bilirubin; also choles- 

 terin, fatty acids, insoluble soaps (calcium, magnesium), to- 

 gether with ferments, having the properties of pepsin and 

 amylopsin. Mucus is also abundant in the fseces. 



We know little of the excretory products proper, as they 

 probably normally exist in small quantity, and it is not impos- 

 sible that some of the products of the decomposition of the 

 digestive juices may be reabsorbed and worked over or excreted 

 by the kidneys, etc. 



There is, however, a recognized non-nitrogenous crystalline 

 body known as excretin', which contains sulphur, salts, and 

 pigments, and that may rank perhaps as a true excretion of 

 the intestine. 



It is well known that bacteria abound in the ahmentary 

 tract, though their number is dependent on a variety of circum- 

 stances, including the kind of food and the condition in which 

 it is eaten. These minute organisms feed, of course, and to get 

 their food produce chemical decompositions. Skatol and indol 

 are possibly thus produced, and give the faecal odor to the con- 

 tents of the intestine. But as yet our ignorance of these 

 matters is greater than our knowledge — a remark which ap- 

 plies to the excretory functions of the alimentary tract gen- 

 erally. 



Pathological. — The facts revealed by clinical and pathologi- 

 cal studyjeave no doubt in the mind that the intestine at all 

 events may, when other glands, like the kidney, are at fault, 

 23 



