The colon commences, as we have said, in the right iliac region, by a sudden and considerable expansion. Hence it ascends through the right hypochondriac, and right lumbar region, from which it crosses the fore part of the belly immediately below the stomach, to the left hypochondriac, and left lumbar region, from which it de scends through the left iliac region, where it makes two remarkable turns, one towards the fore part of the belly, and another towards the sacrum, forming what is called, from its resemblance to the letter S, the sigmoid flexure of the colon; and here it imperceptibly terminates in the rectum. Hence this intestine has been divided in to four portions, (not including the cxcum), denominated the right lumbar colon, the transverse colon, or the arch of the colon, the left lumbar colon, and the left iliac co lon, or sigmoid flexure. The direction and general ap pearance of these last three portions are well expressed in Fig. 11, of Pl. XIX.1- The diameter of the colon is greatest at its commencement in the curcum, where it forms a large bag nearly cubical, into which the ilium projects in such a manner as to form a sort of valye, which in general prevents any return of the contents of the colon into the small intestines. The situation and structure of this valve \VIII be better understood when we shall have described the int.( real membrane of the alimentary canal. The lower part of this commence ment of the colon extends dowl1W3I'dti, or s/Irrilel, into a. large close cavity, whence the name of exclun, while in the opposite direction appears the cavity of the right lumbar colon. Towards the right and ha( Is part or the cercum, on its cxternal surfac•, there is a retnarl,able ap pendage, forming a hollow tube of small diameter, call ed, from its resemblance to a worm, the vcrinifiJrni ap is composed of coats, that are very thick in proportion to the diameter of its cavity ; is smooth and whitish on its exterior surface, while within it is lined by a mucous membrane, from which exudis a considerable quantity of a glutinous fluid, nhictt is poured through an opening into the upper part of the c‘rcum near the iliac valve. From the to the transverse arch of the colon, the diameter of this intes tine is smaller than in any other part, being very little superior to that of the small intestines; but the trans verse arch is of very considerable size. Along the whole peripheral surface of the colon, we observe there longitudinal bands, which have by some been called li gamentous, though there can be no doubt that they are composed of longitudinal muscular fibres. One or these is distinctly seen at Y, Y, V, Fig. 11, Pl. XIX. By these bands the colon is divided longitudinally into three parts, and by the contraction of the muscular fibres, it is puckered up into numerous divisions, which are com monly called the cells of the colon.
The colon is firmly fixed in the different regions, and connected to the neighbouring parts by membranes, that will be presently noticed, through its whole extent, ex cept at the sigmoid flexure, where it floats loosely in the cavity of the belly, like the small intestines. Two particular parts of the peritoneum, by which it is fasten ed on the right and left sides, are called the right and left /iganunts of the colon.
The colon terminates at the end of the sigmoid flexure, in a portion that runs almost perpendicularly downwards, along the concave surface of the sacrum, and which, from this direction, has Lc en called illtestinum rectum, or the straight gut. See P1. XIX. Fig. 11. This bowel, at its commencement, inclines obliquely to the right, but almost immediately takes its course in the direction of the menial line. It forms a more regular cylindrical ca vity than the colon, not being, like that portion, divided into cells by the longitudinal muscular bands. It is close
ly connected as it passes downwards, and is supported partly by the concave surface of the sacrum, and partly by the cocyx. Where it terminates in the anus, it is surrounded by numerous strong muscular fibres, form ing two layers, and called, from their office, the .spinc ter muscles of the anus. At this part too there are two remarkable bundles of longitudinal fibres, forming a muscle on each side, called tetztator uni. The rectum gradually grows larger as it descends, and is capable of considerable dilatation and contraction.
Having thus, in conformity with the general custom of anatomists, considered the intestinal canal as divided into different portions, we must now return to the point from which we set out, and examine more minutely the structure of the whole canal, and some of the membranes by width. it is connected to the neiy,11lionring pa: Is. The into stit,a1 canal, like the stoma( II, lets It ( IA (I a, t Anpos..d of lour i oats, or layers ; ai, ontor, .h• pt o a Mar coal, a net cots, and a c it Ions coat. By most ntodern anatomit.ts the li.;rd of the.— is lc ly prop, rly omitted, as it consist; ihercly of (Altd.i: h ing between musLulor and t illons sneer There are thcrrfore three layers of the rah.' WIII(11 wi• have to whole intestinal canal is not inyt loped in the sane oleo.) -layer. What a e have called the ',mall intestim 11,c v. ',ole or the colon ; and thc whole: of the re( tum, cred yuith a coat Iron: the seron iu•ol,rme, t ailed the /1( that lines the sac ; but the ducuicnum, it ing as it cc ere in hind lir rwocr UM, receives only partial coverings mum membrany, and is surrounded chiefly with prodm from motl“.r. inenibrane, that connects the colon to di( neighbouring. o•tuoi. The peritom al coat of the inte . tines Forms the prinripal connecting- hill. tote arils the ster• nal and lateral parts of the belly ; and from the smoothness and humidity of its peripheral surface, gives to the whop that polished appearance which we oh-,crt.e 011 OpCliihe the belly. We cannot at present attempt to (lest ribs tLt manner in which this outer coat passes successively oy the several parts of the intestinal canal ; but as its rour.,e is important, and not generally understood, w e trail no tice it particularly, after having examined the other r on tents of the belly. Within the outer membrane of tie intestines lies the muscular coat, which differs, in respects, in the several parts of the canal. In the d..mum it consists chiefly of circular fibres, resembling. both in their disposition and colour, the transverse lihres of the stomach. In the small intestines there are a rev longitudinal fibres towards the convex part of the bow• els, or that side which looks outwards. In the colon, the longitudinal muscular fibres arc, as we have seen, n markable for being disposed in three hands, and they differ from the longitudinal fibres of any other part oi die alimentary canal, by being considerably shorter than the length of the tube to which they belong, thus facili Latins- the division of the large intestines into cells. by rendering unnecessary so great a muscular contraction as takes place in most other muscular parts of the body In the rectum, the disposition of the muscular fibres is very similar to what we have seen in the gulIct. They are here more numerous, and stronger than in the rest the intestinal canal, forming, as in the gullet, a periphe ral layer disposed longitudinally, and a central circul.o. layer. In the part of the rectum next the colon. the muscular fibres resemble those of the other intestines in their White colour ; hut towards the sacral extremity of the rectum, they become red, and are more like time external muscles of the body.