On the whole, therefore, the greater solidity of the contents of the large intestine implies a greater resistance to the contraction of its muscular coat. And the structure of this coat shows no proportionate increase, but rather a decrease of strength. Hence we might almost conjecture, that the general movement of the small intestine is here ex changed for one which is slower, feebler, and hence less effective : and which, besides being much weaker, is possibly interrupted by longer periods of rest.1 But it is probable that this comparatively feeble character of the general movement of the large intestine is capable of being more than compensated by that heightened efficacy which the peculiar arrangement of its mus cular coat can concede. The thin muscular stratum that forms the wall of its numerous pouches, is evidently capable of assuming a much smaller surface, in obedience to the thick and powerful bands of its transverse and longitudinal coats. Indeed, the sirnul taneous contraction of these two sets of fibres would convert them into what would form, for the time, a very thick and powerful expanse of unstriped muscle, capable of ob literating the entire calibre of' the intestine.
In this way the same muscular apparatus which generally concedes to the contents of the large intestine a long delay, and a slight movement over a large absorptive surface, would still be quite capable of effecting their rapid and vigorous expulsion, when required to do so. Nay, more, since much of this ex pulsive act seems, as it were, removed from the sacculi themselves, and concentrated in the bands around them, we may conjecture that the mechanism of these latter contrac tile masses is also specially concerned in the nervous part of the process. At least, it is not impossible, that the irritation or stretching of these two sets of muscles (which can be effected only by the general distention of the calibre of the tube, and not by the local dis tention of its cells) may constitute the im mediate stimulus of the evacuation of the whole large intestine, or of any particular segment.
The mucous membrane of the colon may be described as only differing from that of the small intestine in the fact that it is somewhat thicker, and quite devoid of Its more numerous tubes are about one-third longer than those of the small intestine ; and their diameter seems to be, in general, still more increased. Its solitary follteles are also more numerous, and of larger size. And the depressions which mark the site of each on the internal surface of the bowel are here represented by a distinct pit ; which leads by vertical, or even somewhat divergent sides, to the follicle that occupies its base. But the
secretions of these structures, and the cell growth which lines them, do not present any noticeable difference from their analogues in the small intestine.
The rectum (r, fi . 274. ; r 1, r 2, r 3, fig. 276.) (formerly straight gut, Eng. ; Mastdarm, Germ.), which intervenes between the sig mold flexure of the colon (s) and the anus (n), forms the terminal segment of the large intestine. It has received the above name from the comparatively direct course which it takes. Its length is about seven to eight inches. Its width is at first only equal to that of the small extremity of the colon, with which it is continuous : but gradually increases, so as to form a kind of dilatation or pouch in the immediate neighbourhood of its lower aperture. Here it suddenly contracts to the narrow orifice of the onus ; which is situated in the perineal space, that closes the pelvis, and forms the lower extremity of the trunk.
Strictly speaking, however, the rectum is by no means straight. Beginning at the left sa cro-iliac symphysis (e, fig. 276.), it first passes obliquely downwards and to the right side, towards the median line of the trunk; which it reaches at a point that nearly corresponds to the body of the third sacral vertebra (at r 1, fig. 276.). And though, from hence to its termination, it continues to occupy the middle line, still as it rests upon the concave anterior surface of the remainder of the spinal column, it assumes a curve similar to that of the sacrum. It thus acquires a second and more remarkable bend (at r 2,fig. 276.), which is convex backwards. Finally, at the tip of the coccyx, it quits the antero-posterior direction given it by this bone ; and, bending suddenly downwards at a right angle, becomes vertical for a very short distance (at r 3, fig. 276.) prior to ending in the anus.
The above successive changes in its di rection are associated with others, which af fect its relations to the peritoneum and to the surrounding structures. In accordance with these, it is usual to describe the rectum as subdivided into three portions.
The first or oblique segment (between s and r 1, fig. 276.) is about 31 inches long. It is covered on every side by peritoneum ; which, at its commencement, attaches it to the pelvis by a short meso-rectum, very simi lar to the meso-colon of the sigmoid flexure.* It lies on the sacrum, the upper border of the pyriformis muscle, and the sacral plexus. In front of it is the bladder, from which it is generally separated by some convolu tions of the small intestine. On either side it is related to the ureter, and to the branches of the internal iliac vessels.