TRANSIT ALONG THE COLON
Studies to estimate transit time through the alimentary tract have been undertaken over the years. Initially they used injected solid markers such as glass beads or dye substances such as carmine, and estimated the total time taken for the head of the column of marker to appear at the anus. These studies showed that normal persons might retain 30-40g of the beads ingested for as long as 9 days. More recently transit studies employing markers such as radio chromium, which can be easily counted, have shown that on average three-quarters of the radioactive marker is excreted in 96 hours although there is a wide variation between subjects and in the same subject on different occasions. The movements of a radioactive source can also be followed by appropriate surface scanning. There is no doubt that transit through the colon accounts for the greater part of the total time of passage through the alimentary tract. Alvarez has likened transit through the colon to a railway siding in which are standing three trucks. 'Every day a new one arrives and bumps off' the end one so that three remain. Occasionally one arrives with such force that it bumps off all three and then three days have to lapse before the siding is full enough so that the truck arriving at one end can push one out at the other.' This analogy is useful but falls down to the extent in which it suggests that any given days ingesta remains in a tightly defined packet, as the use of radio-opaque markers has shown that considerable mixing occurs in the colon. Following excessive or unnecessary laxative taking, the colon may be emptied and the bowels will not or cannot act until filling occurs, which may take several days. Thus the administration of a laxative to normal subjects is both illogical, meddlesome and potentially harmful.

NORMAL BOWEL HABIT
The majority of persons are largely unaware of the frequency of their motions and normality probably implies unobtrusiveness of the bowel habit. Over concern with regularity by normal subjects is not desirable. In published studies on the frequency of bowel habit it has been shown that the majority of normal persons have a bowel action every day. Only about 1 % of the population have an action less frequently than three times a week or more frequently than three times a day (Connell et al, 1965b). More than three bowel actions per day should probably be regarded as abnormal until proved otherwise. It is less easy to define other aspects of defaecation such as the degree of straining necessary. For example, a very small stool passed only with distressing straining cannot be regarded as normal even if a bowel action occurs once a day. Also persons who have persistent urgency of defaecation cannot be regarded as normal although frequency may be regarded as normal. A complete definition of normal defaecation should, therefore, include criteria for volume and consistency of the stool as well as frequency of bowel habit. However, any consideration of normal values must take account the conditions under which they are determined. For it is well known that the nature of the diet has a marked effect on the consistency and volume of the stool and the frequency of defaecation. Thus in those countries where unrefined grain products containing their natural fibre are the main source of dietary carbohydrate the stools are larger softer and more frequent than they are with our own highly refined diets.