Anatomy : Colon


Dilated distal GI tract segment, ~1m long, in continuity with small intestine and rectum.

General Features


In common with GI Tract:

Serosa - outer longitudinal muscle - inner circular muscle - submucosa - mucosa.

Distinctive 

(1) Variability of peritoneal covering:  Asc and desc colon retroperitoneal; transverse and sigmoid colon has mesentery and is invested with peritoneum. 

(2) Appendices epiploicae:  fatty projections from wall.

(3) Thickenings of longitudinal muscle layer form 3 taeniae:  taenia libera (anterior), taenia mesocolica (posterior on transverse; mesocolon) & taenia omentalis (superiorly on transverse; attaches omentum);
the circular muscle is in-between; gathers colon into
haustra
.


Caecum (264)

Invested in peritoneum in majority, retroperitoneal in a few

-           (caecal folds form the retrocaecal recess);

Av size = 6 x 8 cm.  Ileum enters posteromedially 2cm above appendiceal lumen.

Ileocaecal valve has medial and lateral frenula which are transverse lips with a weak sphincter effect.

Blood supply derived from colic branch of ileocolic artery -> gives off ascending branch, then anterior and posterior branches (anterior branch may run in fold in front of ileocaecal junction).

Deep is the peritoneal floor, iliacus and psoas fascia, and genitofemoral and lat fem cutaneous nerves (250).

Lymph --> nodes along ileocolic artery.

Recesses: (264) retrocaecal recess, also caecal folds, which hide the ileocaecal recesses.


Appendix (264)

6-10cm blind pouch arising from the posteromedial wall of caecum (constant), 2cm inferior & posterior to I-C valve.
Accords with
McBurney’s point on surface anatomy, 1/3 distance between ASIS and umbilicus.

Structurally similar to large bowel with all wall layers, except many lymphoid masses so irregular lumen. 

-           the three tinea coli coalesce here to make up its entire longitudinal muscle layer;

In the infant the caecum is conical and the appendix hangs downwards, but the lateral wall grows faster than the medial wall, hence its adult position.

-
positions: retrocaecal , subcaecal, pelvic , paraileal.  (

Appendicular artery (from ileal branch of ileocolic artery)an end-artery that runs in a peritoneal fold connected behind ileum (called 'mesoappendix' but not a true mesentery).

Bloodless fold of Treves (peritoneal fold) runs anterior to appendix -> ileum; aids in identification of terminal ileum.

Lymph: as for caecum.


Ascending Colon (267)

15cm. Usually retroperitoneal.  Variations from fully mobile on mesentery (embryonic position, risk of volvulus) to fixed .

Relations: quadratus lumborum, psoas posteriorly in upper extent.  Upper part has lower pole R kidney and often duodenum posteriorly, liver & GB anteriorly.
Lies on
iliac fascia and anterior lumbar fascia; medial is the infracolic compartment; lateral is the paracolic gutter.  (292)
R hepatic flexure overlies anterior right kidney, is in contact with inf surface of liver.

On mobilization to embryonic position, duodenum, HOP, ureter and gonadals come into view; at risk.

Blood: Asc branch of ileocolic below, R colic above. 
R colic is the most variable of colic vessels: 30% comes off middle colic, 20% off SMA, absent in 50%.  (287)


Transverse Colon, Greater Omentum (267)

45cm.  Invested in peritoneum, sits on a mesentery.

Connected to stomach (lies in its curvature) by the gastrocolic omentum (part of greater omentum)

-           however, omentum has no direct developmental relation to colon; “bloodless plane” exists between it & TC / mesentery;

-           the rest of the greater omentum drapes down off its surface.

-           omentum vessels arise from an arcade between R gastric and R gastroepiploic arteries. 

-           R edge of omentum finishes at D1; there runs upward as free edge of lesser omentum.  L aspect of omentum is continuous with gastrosplenic ligament. (255) and shortes as splenic flexure.

When mobilizing the mesentery, the tissue must be freed from the pancreatic head.
- posterior leaf tends to fuse with fasia over pancreas; small pancreatic veins from the head drain into SMV and may tear.

Middle colic artery supplies TC. Arises from SMA just below pancreatic head.  Divides into R & L branches ~5-7 cm from colon (second L branch present in 40%).  (287)

Relations: hangs anterior to small bowel; mesentery origin runs from inferior pole of right kidney, across descending duodenum and pancreas, to inferior pole of left kidney (Pl 261).


Descending Colon (267)

<30cm. Retroperitoneal in most (occasionally on a mesentery).

Relations: TC may lie anterior to upper DC.  Splenic flexure overlies lower pole L kidney (in contact with spleen and pancreas above 291).
Posterior
= ureter, gonadals, QL and iliopsoas (lumbar & iliac fascias) 257.

-           phrenicocolic ligament attaches splenic flexure to diaphragm at 10-11 rib levels (257); the part of the greater omental attachments that 'misses the spleen'

Blood (Pl 287): IMA (arises from L anterolateral aspect of aorta 3cm above bifurcation, at L3).

-           IMA gives off L colic and sigmoid arteries then continues as superior rectal artery.

-          Whole colon further supported by marginal artery of Drummond; this is poorest at the L colic flexure (ie where hindgut and midgut arteries meet)

-           Supplemented by collateral Arc of Riolan but only in 10% (from MCA in base of mesentery that joins an early branch of the L colic).

-           L colic branch goes steeply upwards; divides into asc and desc branches (and transverse branch in 25%); is  absent in 10%;  accessory L colic from SMA (limits mobilisation of flexure).

In L hemi, remove L colic and splenic flexures and left transverse colon.  Take L colic at origin off IMA.


Sigmoid Colon (267)

<45cm (greatly variable).  Invested in peritoneum, hangs on a mesentery and often coiled anterior to rectum.

Tinea coalesce into a circumferential band over its course; appendices epiploicae are better developed.

Root of sigmoid mesentery = inverted “V”; apex overlies bifurcation of L common iliac artery and the ureter (~apex of mesentery, on top of the artery) (311)

-           medial limb runs slopes into sacral hollow, stops at S3, lateral limb runs down over pelvic brim along iliac vessels halfway to inguinal ligament (~5cm).

Blood: 2-4 sigmoid branches of inferior mesenteric.; in anterior resection / sigmoid colectomy, divide after has given off its left colic branch.
And divide IMV below pancreas if need to relieve further tension.


Lymphatics

Epicolic nodes (along colonic wall) --> paracolic (along marginal artery) --> intermediate nodes (named by branches of SMA & IMA with which they run) --> principal nodes (superior & inferior mesenteric nodes).

High ligation for R colonic resection not feasible as some lymphatics run directly back to SMA. 


Nerve Supply

Sympathetic (vasoconstrictor): T10-L2 via coeliac & superior mesenteric ganglia (midgut); lumbar part of sympathetic trunk (hindgut).

Parasympathetic:  Same watershed; vagus above, pelvic splanchnics below.

Pain: runs with sympathetic, except in the rectum some appear to pass with parasympathetics.


Summary of Colonic Blood Supply

SMA - middle colic to TC, right colic (variable) to ascending and hepatic flexure, ileocolic to TI, caecum and prox ascending.

IMA - left colic: descending and spelic flexure, sigmoid artery, and superior haemorrhoidal arteries.

Marginal artery (of Drummond) = in most, courses from caecum to rectum, absent in splenic in 5-10%

Arterial arcade (of Riolan) = in a minority collateralizing arcade connecting SMA and IMA branches distinct to marginal artery.

Microscopic collaterals in bowel wall = further collateralization network; not robust.

Relative watershed areas: right colon (long vasa recta, can spasm), splenic flexure, sigmoid colon: susceptible to low-flow states.

  Veins largely follow arterial supply

Ureter Relationship

From renal pelvis to bladder, behind peritoneum.
Lie on psoas in the abdomen, in front of transverse processes
Gonadals are medial initially, then cross over to lie laterally and superficially.
Enter pelvis in front of bifurcation of common liac.

On R, ureter is protected by duodenum
- no need to see ureter below this; if gonadal vessels remain down then the ureter will be safe even if not seen.
On L, ureter not protected and crossed by vessels; needs to be observed.
- lower course = easily traced and identified and should not be at risk.

In pelvis, lie against lateral walls initially in front of the internal iliac vessels, and turns medially across pelvic floor to enter bladder.
- hence tend to lie anterior to the field of dissection in rectal surgery.