The Colon constitutes the principal tract of the large intestines, and exceeds them all in diameter: as accumulations in its cavity frequently produce various ill effects from their pressure, it becomes essential for the practitioner to know its direction and bearings. It commences in the cavity of the os ilium, on the right side; from thence, ascending by the kidney on the same side, it passes under the concave side of the liver, to which it is sometimes tied, as also to the gallbladder, which tinges it yellow in that place'; it then runs under the bottom of the stomach to the spleen, in the left side, to which it is also affixed; and thence, passing in the form of the Greek letter 2, it terminates in the upper part of the os sacrum in the rectum. It appears, therefore, to be contiguous to all the digestive organs, and may consequently produce much disturbance by its morbid distention. Its connexion with the duodenum is also a circumstance of much pathological importance: whatever motion takes place in the former intestine will be communicated, more or less, to the latter; and should it become unnaturally distended, it will press immediately upon the ascending part of the duodenum, and retard the progress of the alimentary matter, which has always to rise against gravity, when the body is in an erect position, or recumbent on the right side.

The colon has been divided into the ascending portion, which extends from the coecum to the right hypochondrium; into the transverse portion, or what is termed its great arch; and into the descending portion, including what has been called its sigmoid flexure. The coats of this intestine are much stronger than those of the others: its muscular layer has also a peculiar disposition; its longitudinal fibres form three straight bundles or bands, far separated from each other when the intestine is dilated; at the same time, its circular fibres form bands, equally separated from each other, but more numerous: from which arrangement it follows, that, in a great number of places, the intestine only consists of the peritonaeum and its mucous membrane; these places are generally formed into distinct cavities, which have been termed the cells of the colon: they serve to promote a gradual descent of the excrement; but, when the action of the canal is torpid, they give origin to much mischief, by unduly retaining its contents.

1 At least such is its appearance in the dead subject; whether a similar transudation takes place during life is very doubtful.

25. Several physiologists have supposed that the colon performs some other function than that of a mere recipient. Sir E. Home imagined that it formed fat; an hypothesis which would have received some slight support from the fact that the fattest animals have generally the largest colons, did we not know that persons have lived, and enjoyed good health, for many years, with an artificial anus formed by the coecal extremity of the small intestines, which sufficiently proves that the large intestines are not essential to perfect digestion, nor to the maintenance of life.

26. The rectum is the last portion of the intestinal canal; it begins at the upper part of the os sacrum, where the colon ends, and going straight down (whence its name), it is tied to the extremities of the coccyx by the peritonaeum behind, and to the neck of the bladder in men, but in women to the vagina uteri before; whence arises the sympathy between those parts. The coats of the rectum are more thick and fleshy than those of any other of the intestines: it has not in general any valves, but several rugae; had the former existed, the expulsion of the faeces would have suffered inconvenient delay. The figure of the rectum varies, as it is full or empty; when empty, it is regularly cylindrical, and contracts in transverse folds: it is capable of very great distention, and may even be extended to the size of a large bladder: the quantity of faeces that sometimes accumulates is prodigious, and cannot be removed except by mechanical means.