Page 93 - Clinical Anatomy

ECA2 7/18/06 6:42 PM Page 78

78 The abdomen and pelvis

different subjects; the average is some 24 feet (6.5m). Resection of up to one-
third or even half of the small intestine is compatible with a perfectly
normal life, and survival has been reported with only 18in (45cm) of small
intestine preserved.
The mesentery of the small intestine has a 6in (15cm) origin from the
posterior abdominal wall, which commences at the duodenojejunal junc-
tion to the left of the 2nd lumbar vertebra, and passes obliquely down-
wards to the right sacro-iliac joint; it contains the superior mesenteric
vessels, the lymph nodes draining the small gut and autonomic nerve
fibres.
The upper half of the small intestine is termed the jejunum, the remain-
der is the ileum. There is no sharp distinction between the two and this
division is a conventional one only. The bowel does, however, change
its character from above downwards, the following points enabling the
surgeon to determine the level of a loop of small intestine at operation.
1◊◊The jejunum has a thicker wall as the circular folds of mucosa (valvulae
conniventes) are larger and thicker more proximally.
2◊◊The proximal small intestine is of greater diameter than the distal.
3◊◊The jejunum tends to lie at the umbilical region, the ileum in the supra-
pubic region and pelvis.
4◊◊The mesentery becomes thicker and more fat-laden from above
downwards.
5◊◊The mesenteric vessels form only one or two arcades to the jejunum,
with long and relatively infrequent terminal branches passing to the gut
wall. The ileum is supplied by shorter and more numerous terminal vessels
arising from complete series of three, four or even five arcades (Fig. 59).

Large intestine
The large intestine is subdivided, for descriptive purposes, into:
•◊◊caecum with the appendix vermiformis;
•◊◊ascending colon (5–8in (12–20cm));
•◊◊hepatic flexure;
•◊◊transverse colon (18in (45cm));

Fig. 59◊The simple
arterial arcades of the
jejunum (a) compared
with the complex arcades
of the ileum (b).