FOOD AND DIGESTION. 403 



into acetic acid also. Carbonic acid gas may be formed at the same time 

 and cause flatulence. Cellulose and other insoluble carbohydrates are 

 decomposed with the formation of marsh gas and hydrogen, which 

 escape by the rectum. 



In putrefaction the process is nearly the same as in tryptic digestion, 

 the proteids being broken down into peptones, leucin, tyrosin, and a 

 long row of other substances which have strong odors and belong to the 

 aromatic group. It also results in the production of various gases, such 

 as carbon dioxide, sulphuretted hydrogen, ammonia, hydrogen and 

 methane (marsh gas), and of a high percentage of the volatile fatty acids, 

 valeriauic and butyric. Of the aromatic substances the most important 

 are indol and skatol, though their toxicity has been greatly over- 

 estimated. Some undergo oxidation, indol and skatol forming indoxyl 

 and skatoxyl ; they are usually carried off in the faeces, fcut when the 

 bowel is obstructed they are absorbed and eventually appear in the urine, 

 indoxyl and skatoxyl forming respectively indoxyl- and skatoxyl-sulphuric 

 acids and their salts. Tyrosin is further broken down into para-oxy- 

 phenol-proprionic acid, paracresol and phenol; para-oxy-phenol-acetic 

 acid is also. formed. 



MOVEMENTS OF THE INTESTINES. 



It remains only to consider the manner in which the food and the 

 several secretions mingled with it are moved through the intestinal 

 canal, so as to be slowly subjected to the influence of fresh portions of 

 intestinal secretion, and as slowly exposed to the absorbent power of all 

 the villi and blood-vessels of the mucous membrane. The movement of 

 the intestines is peristaltic or vermicular, and is effected 'by the alternate 

 contractions and dilatations of successive portions of the muscular coats. 

 The contractions, which may commence at any point of the intestine, 

 extend in a wave-like manner along the tube. In any given portion, 

 the longitudinal muscular fibres contract first, or more than the circular; 

 they draw a portion of the intestine upward, or, as it were, backward, 

 over the substance to be propelled, and then the circular fibres of the 

 same portion contracting in succession from above downward, or, as it 

 were, from behind forward, press on the substance into the portion 

 next below, in which at once the same succession of action next ensues. 

 These movements take place slowly, and, in health, commonly give rise 

 to no sensation; but they are perceptible when they are accelerated 

 under the influence of any irritant. The movements of the intestines 

 are sometimes retrograde; and there is no hindrance to the backward 

 movement of the contents of the small intestine. But almost complete 

 security is afforded against the passage of the contents of the large into 

 the small intestine by the ileo-caecal valve. Besides, the orifice of 



