The contents of the small intestine are propelled forwards by a combination of pendular movements producing rhythmic compression and relaxation, and true peristalsis at the rate of approximately 1 inch per minute, but in addition are thoroughly mixed by a process called "segmentation," described accurately by Cannon and more recently as seen by Hertz under the X-rays. "The shadow of a short length of small intestine, at first uniform in thickness, becomes constricted in its cavity, the constriction increases until the single shadow is more or less divided into two. Then each half undergoes a similar division, but the two central segments of the four thus produced join together; the new central segment then divides again, the segmentation cont'nuing in one place at the rate of ten divisions in one minute and a half."

By this means the food is thoroughly mixed with the bile, pancreatic and intestinal juices, and is brought into frequent contact with the absorbing mucous membrane - a fact of the first importance.

After a mixed meal the average time when food begins to appear in the caecum is four and a half hours.

Judging from the sounds heard by the stethoscope, the contents of the ileum are projected into the caecum in jets, and on a mixed diet fats reach the caecum before proteins, proteins earlier than bread, and more than half of a quantity of milk ingested reaches the caecum in two hours at the outside. Most of the chyme is absorbed before the caecum is reached, only 9-10 per cent. of the proteins of a meal appearing there, and a further 2-3 per cent. being absorbed in this portion of the colon, whilst only 2 per cent. of carbohydrates and 5 per cent. of fats reach the caecum, and of these quantities a further absorption of 1/2 per cent. of the former, and 1-2 per cent. of the latter takes place. There are no movements in the large intestine until a sufficient amount of material has been aceumulated in the caeeum and ascending colon to produce a certain amount of distention, and then peristalsis at a lower rate than in the small intestine gradually propels the food along to the pelvic portion of the colon. In addition to pure peristalsis, other movements take place in various parts of the colon; antiperistalsis, oscillatory movement, and a general contraction occurring in the ascending colon, while shortening of the longitudinal fibres and a downward movement take place in the ascending colon.

A little fat, sugar, and coagulable protein in different stages of digestion are included in the 350 grams of fluid material passing through the ileo-caecal valve. This quantity is reduced to about 110 grams, which is the daily amount of the faeces, and hence a certain amount of absorption is effected by the colon.

To Hertz must be credited the most accurate observations on the length of time required for the contents to pass along the alimentary canal. He says: "The average times taken are four and a half hours to the caecum, six and a half hours to the hepatic flexure, nine hours to the splenic flexure, and eleven hours to the commencement of the iliac colon." Some of this material may be evacuated along with the faeces, but the greater portion remains in the pelvic colon for another twenty-four hours, whence it should be expelled at a subsequent defaecation.

The whole of the large intestine below the splenic flexure should be emptied during a normal defaecation.