The only valid exceptions to this rule may be found in those cases in which the ileum and cmcum are filled simultaneously. Such a process of distention necessarily occurs in all cases of mechanical obstruction of the diges tive canal below this valve; as a result of the downward flow of the contents of the small intestines. And since it obviously distends the aperture by the application of a counter force from the side of the ileum, its mecha nical action is so simple as to require no further explanation. Its effect may indeed be seen in the cmcum, as usually inflated and dried.* The function of this valve therefore offers a complete contrast to that by which the stomach opens into the small intestine. For while it affords little or no obstacle to an onward transit of the contents of the canal, it resolutely bars the way to all regurgitation: —an action which me have already seen is exactly reversed by the pylorus. And even in the absence of information respecting the details of its active contraction, its structure entitles us to conjecture, that the greater part of its efficiency depends upon a passive, and therefore permanent mechanism ; and not, as is the case with the pylorus, on an in termittent (and vital) shortening of its mus cular fibres.
The use of the cmcum is evidently that of forming a receptacle, in which the contents of the small intestine may sojourn for a certain time, before passing onwards into the colon.
For not only are its shape, size, and direc tion such as admirably adapt it to this pur pose, but its development in different species and individuals closely corresponds to the degree in which such a delay is advantageous to digestion. Thus the large cwcum of the Herbivora is contrasted, in the Carnivora, by one of but inconsiderable size and development. While there are grounds for conjecturing, that the habitual use of a vegetable diet is capable of increasing its size in the human subject. In all of these respects, however, its develop ment does but parallel that of the remainder of the large intestine. We may therefore defer considering the nature of its secretion, and the changes undergone by its contents, until these segments of the bowel have also been noticed.
The vermiform appendix (g,fig. 275.) which is so named from its resemblance in shape and size to a worm, is a small, smooth, cylindrical tube ; that opens into the coecum below (and rather posterior to) the aperture of the small intestine. Its length varies from one to four or five inches : its diameter from about a fourth to a third of an inch. Its attached end of course shares the situation of the contiguous part of the cmcum. Its distal extremity is usually free ; and may hence be found in almost any situation Ay hich its length, and that of the short mesentery that binds it down, together allow it to take. Its opening into the coecum
is often partially occluded by a kind of trans verse fold or valve.
As regards its structure, the vermiform ap pendix exhibits all three of the ordinary coats. Its peritoneum is derived from that of the cwcum, and often forms a short fold or me sentery which is prolonged up a part of its length. Its muscular stratum is of uniform and considerable thickness, and is continuous with the three longitudinal bands which give the crecum its sacculated shape. Its mucous meinbrane is occupied by tubes and follicles, like those of the colon. And the latter structures are often present in such extra ordinary numbers, as to constitute almost a continuous layer of these rninute closed sacs. The calibre of the tube, which is in general disproportionately small, is occupied by a sparing quantity of glairy mucus ; and occasionally, by small fragments of the ordi nary intestinal contents.
The use of the vermiform appendix is un known. It has been suggested to be a mere relic of the umbilical duct of the fcetus :— an erroneous view, to which allusion will hereafter be made in speaking of the development of the intestinal canal. It is almost peculiar to Nlan ; in whom its situation often causes it to receive small solids in their transit through the cwcum, with the result of their becoming impacted in its narrow cavity. This accident is some times followed by inflammation and perfora tion of the tube, causing fatal peritonitis.
The colon* (formerly Great gut, Eng.; Grinznidarm, Germ.), which forms by far the greater part of the large intestine, extends from the ilio-cmcal valve to the rectum. Starting from the right iliac fossa, it passes vertically (a,.fig. 274.) up the posterior wall of the belly, and on the right side of the spine, until it reaches the under surface of the liver. A sudden turn at a right angle marks the end of this ascending portion, and the beginning of its transverse part. The latter segment, though tolerably horizontal, forms an arch (t,fig. 274.) with the convexity forwards, so as to pass around the projecting spine and aorta. Below the spleen it merges, by another rectan gular bend, into the descending colon (d, fig. 274.) This takes much the same course on the left side of the abdomen as the ascending colon does on the right ; and opposite to the crest of the ileum, it ends by becoming continuous with the sigmoid flexure (s, fig. 274.), The latter portion is attached by a short mesen tery to the left iliac fossa ; and it terminates in the rectum, at a point corresponding to the left sacro-iliac symphysis.