ft is the extreme freedom of movement which such a mode of attachment concedes to the small intestine, that gives rise to the convoluted appearance so characteristic of this part of the tube. The exact figure of these convolutions is probably never quite alike at any two different times in the same indi viduak—being the conjoined result of the muscular movements of the canal, the nature and amount of its contents, the size of the neighbouring viscera, and the state of the abdominal parietes. The effect of dilatation resembles that seen in some other parts of the alimentary canal : — namely, distention of the tube always causes it to split up the loosely connected laminx of peritoneum, and extend backwards between them, so as to shorten the length of its tether of mesentery.
The terms jejunum and ileum refer to a division of the small intestine which, though to some extent an arbitrary one, is not only too convenient to be altogether dispensed with, but is also connected with certain pecu liarities in the structure of the mucous mem brane, that will be hereafter alluded to. The jejunum includes the upper two-fifths, and the ileum the lower three-fifths, of the small intestine.* Muscular The muscular coat of the small intestine consists of the fibre-cells pre viously described, the bundles of which are arranged in two layers,— an outer or longitu dinal, and an inner or circular. The first con stitutes a very delicate lamina, which is often scarcely visible at the mesenteric border of the tube, but is thickened at the opposite margin, where it is firmly united to the perito neum. The circularfibres form a much stronger and more perfect stratum ; and many of their bundles, like those of the same layer in the stomach, seem to take a slightly oblique di rection ; so as to join with others above and below them. Both layers (and especially the transverse) are somewhat stronger at their commencement in the duodenum. But from the middle of the jejunum their thickness remains unaltered throughout the rest of the small intestine.
Movements of the The muscular actions of the intestine have long been re duced to two :— a normal peristalsis, which urges the contents forwards towards the anus ; and an abnormal antiperistalsis, by which they are propelled backvvards towards the stomach. But each of these movements has rather been maintained as a doctrine, than verified as a fact.
From the mere tenuity of the muscular coat of the small intestine, we might infer that its movements are much less vigorous than those of the stomach and cesophagus, in which this tunic has a thickness from two to six times as great. Indeed, an active and
continuous peristalsis, like that which may be seen in these segments of the canal, would scarce allow the time necessary for the diges tive act. Even a slow progressive contrac tion of two inches per minute would traverse the whole length of the intestine in from two to three hours :— a speed which we have every reason to believe very unusual in the healthy subject.
To obtain direct evidence respecting these movements, various methods have been re sorted to.
In the healthy living intestine, it is but very rarely that any definite muscular action can be seen or felt through the wall of the belly. In some of the Polyps, however, the alimentary canal appears to exhibit a peri staltic, but intermittent, movement. And in Alan, the borborygmi which sometimes occur in conditions but little removed from those of health, constitute sufficient evidence of a valid intestinal movement. While in cases in which abnormal obstruction of the intestine has been followed by an accumulation of fluid in the segment above the occluded part, the wall of the belly often becomes so extremely distended and thinned, as to allow us to re cognise a progressive rolling contraction of the dilated bowel. Such observations at least prove that its muscular coat is capable of very vigorous contraction, while in this state of undue distention.
When the intestines of a healthy living animal are exposed by vivisections, surgical operations, or accidental injuries, they are found at rest. Hence, could we implicitly trust these appearances, we should assign but a very slight mechanical value to the intestinal contractions. But such an estimate would obviously be at variance with that propulsion of their contents which we know them* to effect. And, apart from this implied contradic tion, it is evident that such observations can never be regarded as affording us trustworthy evidence of what really obtains in the healthy uninjured animal. For not only is it possible that the slow and feeble contractions of the intestine are much interfered with by the pain and disturbance which such operations or accidents presuppose, but I would add, that there are considerable grounds for suspecting that irritation of the peritoneal tunic of the bowel can produce relaxation of its subjacent muscular coat.