The motions of the large intestines have nearly the same characters as those of the small intestines, like these last, they are not always in the same direction, but push the substances which are contained in their cavity, sometimes towards the ileum and sometimes towards the anus. But by means of this motion, these substances which have already the character of _feces_, can never re-enter the small intestines. The cause that prevents their return is different from that which prevents the return into the stomach of the substances contained in the duodenum. The obstacle in this case, we have said, is produced by the contraction of the contractile rings, which are found at the extremity of the two cavities; in the other, it is produced by a cause purely mechanical, by the arrangement of the ileo-cecal valve. Hence it follows, that if the mode of contraction of the different parts of the intestinal canal be perverted by any cause, it might happen that their contraction towards the pylorus would not take place when the duodenum was affected with its anti-peristaltic motion, and then the substances contained in it, pushed by the contraction of the annular fibres, would re-enter the stomach. At the coecum, on the contrary, as the obstacle is purely mechanical, so long as the ileo-cecal valve is not broken, it will present an insurmountable obstacle to the return of the _feces_ into the small intestines. The motions of the large intestines, sufficient to carry the feces into the rectum, would not, in a state of health, be powerful enough to expel them entirely, by overcoming the resistance which the sphincter constantly presents; in expelling the feces, the contraction of the intestine is assisted by the pressure which arises from the lowering of the diaphragm, and by the contraction of the abdominal muscles.