Physiological Pharmaceutics

Colon and rectal drug delivery 145Figure 7.1 Anatomy and perfusion of the colonextends downwards from the splenic flexure to the pelvic brim and is approximately 30 cmlong. The colon then turns towards the midline to form the coiled sigmoid colon, which isabout 40 cm in length. This in turn joins the rectum in front of the third part of the sacrumand travels for approximately 12 cm before joining the anal canal. This is 3 cm long andits diameter is narrower than that of the rectum to which it connects. It is identified by thepresence of the anal sphincters which replace the muscular coats of the rectum. The slingof the puborectales muscle supports the ano-rectal junction.In humans the rectum is formed by the last 15 to 20 cm of the large intestine. Theinternal cavity of the rectum is divided into three or four chambers; each chamber is partlysegmented from the others by permanent transverse folds (valves of Houston) thatapparently help to support the rectal contents. A sheath of longitudinal muscle surroundsthe outside wall of the rectum, making it possible for it to shorten in length.Interspecies differences in structureThe variation in relative dimension of the large intestine is largely correlated with diet. Inherbivores, such as horses and rabbits, which depend largely on microbial fermentation fornutrition, the large intestine is very large and complex. Omnivores like pigs and humanshave a substantial but smaller large intestine. Carnivores such as dogs and cats have a smalland simple large intestine. The structure and function of the caecum varies in many animals.Vertebrates such as rabbits and horses, which live on a diet composed only of plant life, havea larger caecum that is an important organ of absorption, since it contains bacteria that helpdigest cellulose. Animals that eat only meat have a reduced or absent caecum. In cats anddogs, muscle contractions of the caecum are much more vigorous and are reversible.Materials already passed to the next region of the large intestine can be brought back to thecaecum for mixing with new food substances.