How surgeons distinguish the large intestine from the small intestine
During operations, physicians use four main anatomical features to identify the intestine:
- Diameter and color: The large intestine is wider (though gradually narrowing distally) and has a grayish, ash tint. The small intestine is pinker and brighter.
- Muscular teniae (teniae coli): The longitudinal musculature is not continuous, but gathered into three bands 3–5 mm wide running along the organ. These include the free (tenia libera), mesenteric (tenia mesocolica), and omental (tenia omentalis) teniae. They are absent on the rectum.
- Haustra (haustrae coli): Saccular outpouchings of the wall between the teniae, separated by constrictions. They form because the muscular teniae are shorter than the tube itself. On radiographs, they appear as a characteristic alternation of narrowings and expansions.
- Omental appendices (appendices epiploicae): Duplications of visceral peritoneum 4–5 cm long, filled with adipose tissue. They are usually absent on the cecum, arranged in a single row on the transverse colon, and most commonly in two rows on other segments.
Sections and peritoneal relations
The large intestine is divided into the cecum, colon (ascending, transverse, descending, sigmoid), and rectum. Each segment has its own fixation features:
- Cecum (caecum): The segment below the level of the ileal opening. The length in adults is 3–10 cm.
- Ascending colon (colon ascendens): In 70.8% of cases, it lies mesoperitoneally (covered by peritoneum anteriorly and laterally). If it retains a mesentery, the intestine becomes excessively mobile, creating a risk of volvulus.
- Transverse colon (colon transversum): Intraperitoneal, with a well-developed mesentery (up to 12 cm high at the midline). Due to high mobility, it can sag and enter hernial sacs (umbilical, epigastric).
- Descending colon (colon descendens): Predominantly mesoperitoneal. Posteriorly, it lies against the muscles of the posterior abdominal wall and the left ureter.
- Sigmoid colon (colon sigmoideum): Intraperitoneal, with a prominent mesentery whose height reaches a maximum in the middle of the loop. The root of the mesentery crosses the left ureter at the level of the 5th lumbar vertebra.
Ileocecal angle and vermiform appendix
The transition site of the small intestine into the large intestine is located in the right iliac fossa. Here, the terminal part of the ileum invaginates into the medial wall of the cecum, forming the ileocecal valve (ileocecal valve / valve of Bauhin). It performs a barrier function, preventing the reflux of contents back into the small intestine.
Inferior and posterior to the valve arises the vermiform appendix (appendix vermiformis). It is covered by peritoneum on all sides and has its own mesentery with blood vessels. The position of the appendix is extremely variable: it can descend into the lesser pelvis, lie parallel to the ileum, ascend beneath the liver, or hide behind the cecum (retrocecal). If in a retrocecal position the appendix lies extraperitoneally (retroperitoneally), its inflammation threatens severe complications involving the retroperitoneal cellular tissue, and diagnostics become significantly more complicated.
Colic flexures
The colon forms two important angles at the junction of its parts:
- Right (hepatic) flexure (flexura coli dextra): The transition of the ascending part into the transverse part. Located in the right hypochondrium. Superiorly and anteriorly, it contacts the liver and gallbladder; posteriorly, the right kidney; and medially, the descending part of the duodenum.
- Left (splenic) flexure (flexura coli sinistra): The transition of the transverse part into the descending part. Located in the left hypochondrium. It forms an acute angle, lies approximately 4 cm higher and significantly more dorsally (deeper) than the right flexure, approaching the lower pole of the spleen. Due to this topography, this area is the most difficult to access during surgical mobilization.
Briefly: answers to topical questions
How do the various segments of the large intestine relate to the peritoneum?
The segments of the large intestine have varying peritoneal relations: intraperitoneal, mesoperitoneal, or extraperitoneal.
- Cecum (caecum) — covered by visceral peritoneum only anteriorly.
- Vermiform appendix (appendix vermiformis) — intraperitoneal, has a mesentery.
- Ascending and descending colons (colon ascendens et descendens) — mesoperitoneal.
- Transverse and sigmoid colons (colon transversum et sigmoideum) — intraperitoneal, have a mesentery.
- Rectum (rectum) — upper part intraperitoneal, middle part mesoperitoneal, lower part lacks a peritoneal covering.
Which arteries supply blood to the colon?
Blood supply to the colon is derived from the superior and inferior mesenteric artery systems.
- Superior mesenteric artery (a. mesenterica superior) — gives off the ileocolic (a. ileocolica), right colic (a. colica dextra), and middle colic (a. colica media) arteries.
- Inferior mesenteric artery (a. mesenterica inferior) — gives off the left colic (a. colica sinistra), sigmoid (aa. sigmoideae), and superior rectal (a. rectalis superior) arteries.
What variants of the position of the vermiform appendix relative to the cecum exist?
The position of the tip of the vermiform appendix is extremely variable. The following positional variants are distinguished:
- Pelvic (descending) — directed downward into the cavity of the lesser pelvis (about 50% of cases).
- Medial — located parallel to the ileum.
- Lateral — directed into the right paracolic gutter.
- Anterior — lies on the anterior surface of the cecum.
- Ascending (subhepatic) — with the apex directed upward, sometimes reaching the subhepatic region.
- Retrocecal (posterior) — located behind the cecum. It can lie intraperitoneally (adherent to the intestinal wall) or extraperitoneally (retroperitoneally).
Where does venous drainage from the large intestine segments occur?
Venous drainage from the large intestine is carried out via extraorgan veins that form the superior and inferior mesenteric veins, belonging to the portal vein system (v. portae hepatis).
- Superior mesenteric vein (v. mesenterica superior) receives the ileocolic (v. ileocolica), right colic (v. colica dextra), middle colic (v. colica media), and appendicular (v. appendicularis) veins.
- Inferior mesenteric vein (v. mesenterica inferior) empties into the splenic vein (v. splenica) or, less frequently, directly into the portal vein.
To which lymph nodes does lymph drainage from the cecum and appendix occur?
Lymph drainage from the cecum and vermiform appendix goes to the visceral lymph nodes of the abdominal cavity.
- First-stage nodes are located on the intestinal walls and along the mesenteric border; in the ileocecal region, the ileocolic lymph node (nodus lymphaticus ileocolicus) is described.
- Regional nodes are localized along the nourishing vessels: for the appendix — along the a. ileocolica; for the cecum — along the a. ileocolica and a. colica dextra.
- Lymph subsequently drains to the superior mesenteric lymph nodes (nodi lymphoidei mesenterici superiores).
To quickly recall the names of the three muscular teniae of the large intestine, focus on their location: free (runs along the free border), mesenteric (along the line of mesentery attachment), and omental (along the line of omental fixation).
Class and exam questions
How can a surgeon quickly locate the base of the appendix?
A reliable anatomical landmark is the convergence point of all three longitudinal muscular teniae on the wall of the cecum — it is precisely there that they transition into the continuous muscular layer of the appendix.
What are dolichocolon and coloptosis?
Dolichocolon is an abnormal increase in the length of the large intestine. Coloptosis is its prolapse, which is often associated with an excessive height of the mesentery.
Why can epiploic appendages cause symptoms of an «acute abdomen»?
If the appendix is large and has a thin mesoappendix, it can undergo torsion. This leads to impaired blood supply, necrosis of the adipose tissue inside the appendix, and inflammation.
Which parts of the colon are the most mobile?
Transverse and sigmoid colons. They are covered by the peritoneum on all sides ( intraperitoneally) and have a well-defined mesentery.
What else to study on this topic
- Syntopy of the ascending and descending colons with retroperitoneal adipose tissue and muscles
- Variations in the position of the vermiform appendix and their impact on the clinical presentation of appendicitis
- Topography of the root of the transverse mesocolon
- Blood supply to the ileocecal region and lymphatic drainage pathways
- Relationship of the sigmoid colon to the ureter and iliac vessels
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