DIGESTION. 



289 



secretions and excretions mingled with it in the stomach and intestines; 

 3. It is probable that a certain mutual interchange occurs between the 

 gases contained in the alimentary canal, and those present in the blood 

 of these gastric and intestinal blood-vessels; but the conditions of the 

 exchange are^not known, and it is very doubtful whether anything like a 

 true and definite secretion of gas from the blflod into the intestines or 

 stomach ever takes place. There can be no doubt, however, that the in- 

 testines may be the proper excretory organs for many odorous and other 

 substances, either absorbed from the air taken into the lungs in inspira- 

 tion, or absorbed in the upper part of the alimentary canal, again to be 

 excreted at a portion of the same tract lower down in either case as- 

 suming rapidly a gaseous form after their excretion, and in this way, 

 perhaps, obtaining a more ready egress from the body. It is probable- 

 that, under ordinary circumstances, the gases of the stomach and intes- 

 tines are derived chiefly from the second of the sources which have been 

 enumerated (Brinton). 



COMPOSITION OF GASES CONTAINED IN THE ALIMENTARY CANAL. 

 (TABULATED FROM VARIOUS AUTHORITIES BY BRINTON.) 



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 ftitestinal 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 intestinal 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, are com- 

 VOL. I. 19. 



