Digestion. 149 



them are not readily recognised. In the third colon the 

 material becomes firmer, but only slightly so, and bubbles 

 of gas are being constantly given off from its surface. In 

 the fourth colon the entire ingesta are like thick soup, the 

 surface covered with gas bubbles, and the material com- 

 posing them is in a finely comminuted condition ; for the first 

 foot or so of the single colon this condition is maintained, 

 when quite suddenly the contents are found solid and formed 

 into balls. The remarkable suddenness of this change is 

 invariable in a state of health, and indicates the most 

 active absorption, perhaps the most active absorption of 

 fluid in the intestinal canal. 



The entire contents of the colon are yellow in colour or 

 yellowish green, rapidly becoming brown or olive-green on 

 exposure to light, or, what is more probable, to the oxygen 

 of the air. The contents of the colon are normally alkaline 

 throughout. I once, however, found them acid. 



The muscular movements of the large intestine are much 

 slower than those of the small bowels, for the food has to 

 remain a longer time in contact with the absorbing surface ; 

 at least forty-eight hours. 



In the colon the food undergoes a further elaboration. 

 Thanhoffer has claimed that starch may be converted into 

 sugar, and proteids into peptones, and I see no reason why 

 the latter, at any rate, should not take place, for it is posi- 

 tively certain that in the colon much material may be 

 found which requires further acting upon ; this we may see 

 in almost any feeding experiment, particularly with grain. 

 Cellulose, no doubt, is here also acted upon ; perhaps the 

 chief action of the colon is directed against the cellulose. 



In my opinion it is impossible that this enormous intes- 

 tine can exist simply as a reservoir for ingesta, as has been 

 suggested. Such a view is incompatible with its structure 

 or the appearance of its contents. 



The marked fermentative changes occurring in the fourth 

 portion of the colon are probably especially associated with 

 cellulose digestion, as we know that the intestinal gases 

 consist amongst others of CH 4 . 



