These antiperistaltic waves follow one after another like the peristaltic waves of the stomach (see Figs. 5, 6, and 10). They begin either on the more advanced portion of the food in the colon (when only a small amount is present), or at the nearest tonic constriction, which is usually at the turn between the transverse and descending colon (Figs. 7 and 8.) The waves rarely run continuously for a long time. When the colon is full, it is usually quiet. The first sign of activity is an irregular undulation of the walls, then very faint constrictions passing along the gut towards the cæcum. These constrictions may first appear only on the ascending colon. As they continue coursing over the intestine they become deeper and deeper, until there is a marked bulging between successive constrictions. When the waves have thus become more prominent, they are seen to start near the end of the transverse colon and pass without interruption to the end of the cæcum. After these deepest waves have been running for a few minutes the indentations grow gradually less marked, until at last they are so faint as to be hardly discernible. The final waves are sometimes to be observed only at the end of the transverse colon. Such a period of antiperistalsis lasts from two to eight minutes, with an average duration of four or five minutes. The periods recur at varying lengths of time; in one instance a period began at 1.38 P.M. and was repeated at 2.06, 2.34, 2.55, 3.15, and at 3.36, when the observation ceased; in another instance a period began at 2.43 P.M., and was repeated at 2.57 and at intervals of from ten to fifteen minutes thereafter while the animal was being watched. The waves have nearly the same rate of recurrence as those in the stomach; about five and a half waves pass a given point in a minute, _i. e._, eleven waves in two minutes. This rate has proved fairly constant in different cats and at different stages in the process of digestion; in one case, however, the waves passed at the rate of nine in two minutes. The stimulating effect of rectal injections on the movements of the small intestine has already been noted. Enemata have also pronounced stimulating action on the antiperistalsis of the colon. Usually the almost immediate result of a rectal injection of warm water is the appearance of deep antiperistaltic waves, which often continue running for a long period. In one case, after an injection of 50 c.c. of warm water, the waves followed one another with monotonous regularity during an observation lasting an hour and twenty minutes. The manner in which this antiperistaltic mechanism affects nutrient enemata introduced into the bowel will be discussed in the section devoted to the question of antiperistalsis.