vi INTESTINE AS AN OBGAN OF EXCEETION 349 



II. The excreta which accumulate in the lower part of the 

 intestine accordingly comprise two kinds of substances : 



(&) Those derived from the food, i.e. indigestible or undigested 

 alimentary residues of ingesta, or non-absorbed and non-absorbable 

 decomposition products. 



(&) Chemical compounds discharged from the wall of the 

 alimentary canal as secretory products of the adjoining glands, 

 which are not or cannot be reabsorbed by the lymph and blood ; 

 and detritus shed off in the epithelial regeneration of the mucous 

 coat. 



The exact determination of the first group of substances is of 

 great importance in denning the digestibility, or better the more 

 or less perfect utility -value, of the individual food-stuffs. It is 

 clear that the less the amount and the simpler the chemical 

 composition of these products the more complete will be the utility 

 of any given diet. 



The determination, on the other hand, of the total waste pro- 

 ducts in the second group, is more valuable in determining the 

 importance of the intestine as an organ of excretion ; also in decid- 

 ing whether the intestine expels specific products of metabolism, 

 and to what point it is able to assist or replace the excretory 

 function of the kidneys. 



No exact distinction between the two kinds of substances has, 

 however, yet been possible, either because the analytical methods 

 at our disposal are inadequate for the quantitative determination 

 under different circumstances of the individual components of the 

 faeces (which, moreover, vary greatly even within physiological 

 limits), or because a considerable proportion of these can arise both 

 from decomposition of the food, and from katabolic processes in 

 the glands adjoining the intestinal canal, and in other tissues of 

 the body. 



A simpler problem, capable of experimental solution, is to 

 decide which of the two groups forms the largest and most 

 important part of these excreta, the alimentary residues, or the 

 residues of the intestinal secretions and epithelial detritus. 



From the fact that in ordinary, normal faeces the residues of 

 the digestive secretions, especially bile and the epithelial detritus, 

 are present in small quantities only, it was formerly supposed that 

 the faecal mass consisted mainly of undigested alimentary residues. 

 C. Yoit and his school (1860, 1884, 1892) first demonstrated the 

 fallacy of this theory. 



Voit investigated the meconium, which is formed and collects 

 during intra-uterine life in the intestine. He also found that a 

 blackish, pitchy, faecal mass, similar to meconium, is formed in the 

 dog's intestine during a protracted fast. On a flesh diet, moreover, 

 similar faeces are formed in the dog, with the same characters as in 

 fasting ; the amount is scanty, sometimes increasing if more meat be 



