NME2.19: GROSS ANATOMY – LOWER ALIMENTARY TRACT
18/02/08
LEARNING OUTCOMES
Describe the parts of the small and large intestines




The small intestine has three components:
o Duodenum – C-shaped structure 20-25cm long; retroperitoneal except for superior part
o Jejunum – 2.5m long; wider diameter and thicker wall than ileum
o Ileum – 4m long
The tube of the small intestine thins progressively from duodenum to ileum
The small intestine is attached to the posterior abdominal wall by mesentery of the small intestine
The large intestine is 1.5m long and extends from the caecum to the anus
o Taeniae coli are longitudinal muscle fibres that run the length of the colon
o Haustra are sacculations along the length of the colon
o Appendices epiploicae are fat accumulations
Describe what is meant by mesentery and by retroperitoneal structures


Mesentery are the peritoneal membranes connecting the intestines with the posterior abdominal wall
Retroperitoneal structures lie posterior to the peritoneum e.g. kidneys, most of duodenum, pancreas
Describe the blood supply of the intestines








The superior mesenteric artery arises below the celiac trunk from the aorta at L1
o Inferior pancreatoduodenal artery is its first branch supplying the duodenum
o Jejunal and ileal arteries follow in large numbers on the left
o These later anastomose forming arches – arterial arcades
o Vasa recta are straight arteries arise from the terminal arcades
The number of arterial arcades increases towards the distal end of the gut
The vasa recta are:
o Long and close together along the jejunum
o Short and far apart along the ileum
The superior mesenteric vein drains the small intestine and proximal 2/3 of the large intestine
The inferior mesenteric vein drains the distal 1/3 of the large intestine from the descending colon
The middle colic artery branches on the right from the superior mesenteric artery to form two
branches that anastomose with the left and right colic arteries
o The left colic artery arises from the inferior mesenteric artery
The right colic artery branches lower down on the right from the superior mesenteric artery
o At the colon it divides into two supplying the ascending colon
The ileocolic artery branches lower down on the right from the superior mesenteric artery
o At the right iliac fossa it divides into two, the superior anastomosing with the right colic while
the inferior divides further:
 Colic supplies lower ascending colon
 Caecal (anterior and posterior) supply the caecum
 Appendicular supplies the appendix

 Ileal supplies the distal ileum
Remember portosystemic anastomosis (see NME2.5)
Describe the structure of the rectum and anal canal


The rectum begins at S3 and passes through the pelvic floor to become the anal canal
o The rectum lacks the characteristic colonic structures outlined above
The anal canal is 3cm long with circular muscle fibres of the internal and external anal sphincters
o Puborectalis is a muscle with sphincteric action around the anorectal junction supporting
most of the weight of faecal mass
o Anal columns are characteristic longitudinal folds in the upper canal mucosa
o Anal valves are crescentic folds in the lower canal mucosa marking the pectinate line
o Anal sinuses are depressions superior to the anal valves