712 
EXPERIMENTS ON DIGESTION. 
considerable consistence, the fatty matter, the resin, the colouring 
principle, and the residue of the bile, in order to constitute the ex- 
crement, properly speaking. The quantity, the consistence, and 
the colour of the excrement, vary considerably, according to the 
nature of the food ; and the process of digestion finishes by the ex- 
pulsion of it, from time to time, from the rectum. 
Such, in few words, are the changes which the food undergoes 
in the large intestines. The nature of the contents of the rectum 
vary materially, according to the nature of the ingesta, and accord- 
ing to certain states and influences of the digestive canal. 
The contents of the large intestine are strongly acid in the 
dog fed on liquid albumen — less so when the food consisted prin- 
cipally of gelatine ; and in other dogs it was neutral. That from 
horses fed on oats reddened the tincture of turnsol. It was the 
same with a sheep that was fed on hay. In another that had been 
fed on hay and grass, the contents of the rectum were neutral, con- 
taining ammonia neutralized by carbonic acid in excess. 
There was no albumen when the dog had been fed on milk or 
bones ; but much albumen was found in the colon of a horse that 
had been fed on oats. It was found in a smaller quantity in a 
calf and sheep fed on oats ; but none was discoverable in the dung 
of a sheep that had been fed on hay and grass. The fixed salts 
were very little different from those in the caecum. 
An analysis of the intestinal fluids of a horse fed on hay has 
shewn us, that, after its escape from the stomach, the portion of 
organic matter dissolved in this fluid is continually diminishing, 
while the saline substances that have been described are regularly 
increasing. This increase in the quantity of the salts was intended 
probably to preserve the organic substances from putrefaction, pro- 
perly so called, during their long continuance in the intestinal canal. 
In order to explain this, we are compelled to admit either that the 
lymphatic vessels have the faculty of absorbing the organic sub- 
stances in the intestinal fluids in preference to the salts, and to 
leave the salts in a manner untouched — or that the intestinal fluid 
which is secreted towards the lower part of the canal is more 
highly charged with these salts, and less rich in organic matter, in 
consequence of which it adds more and more of particular salts to 
the portion of alimentary matter not yet absorbed. 
