When the food has been distributed through the intestine so as to present the appearance shown in Figure 1, a noticeable feature in most or all of the loops is the total absence of movement. If the animal remains quiet, however, only a few moments elapse before peculiar motions appear in one or another of the loops, or perhaps in several, and last for some time. These motions consist in a sudden division of one of the long, narrow masses of food into many little segments of nearly equal size; then these segments are again suddenly divided and the neighbouring halves unite to make new segments, and so on, in a manner to be more fully described. I have called this process the rhythmic segmentation of the intestinal contents. Further observation reveals peristalsis here and there, and under certain circumstances the typical swaying movements may be seen. All these phenomena are now to be considered in detail. [Illustration: FIGURE 1.--Appearance of food in the intestines 5¾ hours after eating. This and other radiographs reduced two-thirds.] [Illustration: FIGURE 2.--Diagram representing the process of rhythmic segmentation. Lines 1, 2, 3, 4 indicate the sequence of appearances in the loop. The dotted lines mark the regions of division. The arrows show the relation of the particles to the segments they subsequently form.] _Rhythmic segmentation of the intestinal contents._--This is by far the most common and the most interesting mechanical process to be seen in the small intestine. The nature of the process may best be understood by referring to the diagram in Figure 2. A string-like mass of food is seen lying quietly in one of the intestinal loops (line 1, Fig. 2). Suddenly an undefined activity appears in the mass, and a moment later constrictions at regular intervals along its length cut it into little ovoid pieces. The solid string is thus quickly transformed, by a simultaneous sectioning, into a series of uniform segments. A moment later each of these segments is divided into two particles, and immediately after the division neighbouring particles (as _a_ and _b_, line 2, Fig. 2) rush together, often with the rapidity of flying shuttles, and merge to form new segments (as _c_, line 3, Fig. 2). The next moment these new segments are divided, and neighbouring particles unite to make a third series, and so on. At the time of the second segmentation (line 3, Fig. 2) the end particles are left small. Observation shows that these small pieces are not redivided. The end piece at A simply varies in size with each division; at one moment it is left small, at the next moment it is full size from the addition of a part of the nearest segment, and a moment later is the small bit left after another division. The end piece at B (probably the rear of the mass) shoots away when the end mass is divided, and is swept back at each reunion to make the large end mass again, only to be shot away and swept onward with each recurrence of the constrictions.