The Small Intestine between the duodenum and caput caecum is on an average about 22 feet long, and is divided into two portions-jejunum and ileum-the first being about 8 feet, and the latter 14 feet, in length. There is no definite point of junction between the two, the transition being gradual. The jejunum is about 1¼ inches in diameter, and the ileum 1 inch, but muscular contraction may render the bowel very narrow, while gaseous distension may render it very wide. The jejunum is about ¼ inch wider, thicker walled, heavier, and more vascular than the ileum. With regard to the arrangement of the bowel in the abdominal cavity, as a rule the coils on the left side of the spine and high up are jejunum, while these in the pelvis and right iliac fossa are ileum. In the region of the pubes may frequently be found that portion of bowel with the longest mesentery (about the jejuno-ileal junction), and also the lower portion of the ileum.
It is often of importance in abdominal operations to determine roughly to what portion of the intestinal tract a loop which has been picked up belongs. It is not easy to do this, but some guiding points may be mentioned. The valvules conniventes, or transverse folds of mucous membrane, are most marked in the upper portón of the jejunum ; are fewer beyond that point ; and absent, or almost so, beyond the middle of the ileum. These may be seen by transmitted light, if the bowel be fairly translucent. The Peyer's patches, which are oblong collections of lymphoid tissue, about 1 inch long and ½ inch broad, situated opposite the mesenteric attachment, and numbering between twenty and thirty in all, are larger and more numerous in the ileum than in the jejunum. Further, the vessels supplying the lower ileum form one or even two sets of arches in the mesentery, prior to running into the bowel, and the mesentery becomes progressively more fat-laden toward the lower end, sometimes so much so as to obscure the vascular arches.
Peyer's patches are favourite sites for both tubercle and typhoid bacilli to settle in and cause ulceration. The former rarely perforate the serous coat, but do not respect the anatomical margins of the patch, tending to extend circularly round the lumen of the gut, and hence, in healing, to cause stricture. The latter are prone to perforation, remain localized to the patch, and heal generally with a flat cicatrix, stricture being an uncommon sequela.
Lieberkilhn s glands occur throughout the entire intestinal tract, and are a frequent starting-point both for adenoma and carcinoma of the intestine. Where it is necessary from gangrene or malignant growth to remove a portion of bowel (enterectomy), care should be taken to allow healthy mesentery to project beyond the cut end of the bowel on either side, and, further, to cut the bowel obliquely, removing more of the side opposite the mesenteric attachment, the object of these precautions being to secure sufficient blood-supply. In suturing (enterorraphy) care is taken to apply serous surfaces to one another, as these unite readily. The position where delay in healing generally occurs is at the mesenteric attachment, where, owing to divergence of the two layers of the mesentery (5/16 inch), there is no serous coat. As a rule, if more than a third of the total length of the intestine be removed, the patient's nutrition suffers. It may be necessary to open a piece of bowel (enterotomy) to remove an impacted body, or to short circuit a piece involved in a tumour.
Wounds of the small intestine are generally more serious the nearer the stomach they occur. Where the wound is small, particularly if punctured, no harm will probably result. Thus the bowel has been frequently punctured for tympanites without untoward consequences, and, even when the wound is a little larger, the mucous membrane generally protrudes into and fills up the wound. This protrusion of mucous membrane, together with muscular contraction, may prevent extrusion of material even in small incised wounds. As a rule, longitudinal wounds gape more than do transverse, the circular layer being the stronger. Transverse wounds gape most when situated opposite to the mesenteric attachment, and jejunal wounds generally gape more than those of the ileum.