Main segments
Anatomically, the large intestine is traditionally divided into three consecutive segments:
- The cecum — caecum (the initial expanded segment).
- The colon — colon (the longest part).
- The rectum — rectum (the terminal fragment ending in the anal opening).
External distinguishing features
Macroscopically, the large intestine has a number of unique features that easily distinguish it from the small intestine. First of all, it has a larger diameter. In addition, specific structures form on its surface:
- Taeniae coli (taeniae coli) — longitudinal muscle bands.
- Haustra (haustrae coli) — characteristic sac-like outpouchings (segmental expansions) of the wall.
- Semilunar folds (plicae semilunares coli) projecting into the lumen of the organ.
- Omental appendices (appendices epiploicae) — small fatty tags on the outer surface.
Characteristics of the taeniae coli
The longitudinal muscle layer of the large intestine is distributed unevenly and concentrated in the form of three taeniae (taeniae coli):
- Free taenia — taenia libera.
- Omental taenia — taenia omentalis.
- Mesocolic taenia — taenia mesocolica.
Structure of the large intestine wall
The wall of the organ has a layered structure classic for the digestive tube. From inside to outside, they are located:
- Mucous membrane (tunica mucosa).
- Submucosa (tela submucosa).
- Muscular layer (tunica muscularis).
- Serous or adventitial layer (outer covering).
Features of the mucous membrane
The mucous membrane of the large intestine is adapted to its specific tasks. Its main feature is the absence of intestinal villi, which abundantly cover the mucosa of the small intestine.
Instead, the epithelial layer forms deep, numerous crypts — glandulae intestinales. Also, a prominent immune apparatus is located in the mucosa, represented by solitary and aggregated lymphoid nodules — noduli lymphoidei solitarii et aggregati.
Briefly: answers to topical questions
What parts does the colon include?
The colon forms a frame surrounding the loops of the small intestine and is subdivided into four parts:
- Ascending colon (colon ascendens).
- Transverse colon (colon transversum).
- Descending colon (colon descendens).
- Sigmoid colon (colon sigmoideum).
Where do all three taeniae coli anatomically converge?
All three taeniae coli anatomically converge on the cecum at the base of the vermiform appendix. At this site, the longitudinal muscle bundles transition into a continuous muscle layer.
This anatomical feature is of great clinical significance: the convergence point of the taeniae serves as a permanent landmark for surgeons when locating the base of the appendix during surgical interventions. The taeniae (taeniae coli) themselves represent the aggregated external longitudinal layer of the muscular tunic, which is absent on the vermiform appendix and rectum.
Which arteries supply blood to the large intestine?
The blood supply to the parts of the large intestine in the cited sources is described via branches of the superior mesenteric artery and their anastomoses:
- Ileocolic artery (a. ileocolica) and its branches — supply the cecum and appendix.
- Right colic artery (a. colica dextra) and anastomoses — supply the ascending colon.
- Middle colic artery (a. colica media) — enters the transverse mesocolon and divides into right and left branches.
- The right branch of a. colica media anastomoses with a. colica dextra and is involved in supplying the right half of the transverse colon.
- The left branch of a. colica media goes to the left half of the transverse colon and anastomoses with a. colica sinistra — this is a key anastomosis between the superior and inferior mesenteric arteries, part of Drummond's marginal artery.
How are the various parts of the large intestine covered by the peritoneum?
The various parts of the large intestine have different relations to the peritoneum.
| Large intestine segment | Peritoneal relations | Presence of Mesentery |
|---|---|---|
| Cecum | In 97% of cases covered on all sides, intraperitoneally | The source indicates that the cecum is intraperitoneal without a mesentery; pathological mobility may be due to a common mesentery with the small intestine |
| Vermiform appendix | Intraperitoneally | Has a mesoappendix |
| Ascending colon | Mesoperitoneally in 70.8% of cases; covered anteriorly and laterally, devoid of peritoneal covering posteriorly | In 1/3 of cases has a mesentery along its entire length |
| Transverse colon | Intraperitoneally | Has a well-defined mesentery — mesocolon transversum |
| Descending colon | Mesoperitoneally in 55% of cases | A pronounced mesentery or a mesentery present only over a certain length is possible |
| Sigmoid colon | Intraperitoneally | Has a well-defined mesentery |
What are the main functions of the large intestine?
The large intestine performs four key functional tasks ensuring the completion of digestive processes and the formation of feces. The main functions include:
- Absorption — intensive reabsorption of water from chyme.
- Fecal formation — binding of undigested food residues through active mucus production, facilitating their passage through the bowel.
- Excretory function — excretion of heavy metal salts, phosphates, as well as calcium and magnesium ions from the blood into the intestinal lumen.
- Bacterial activity — resident microflora ensures the synthesis of vitamins (groups K and B12) and partial breakdown of fiber (cellulose).
The external self-regulation link also ensures the evacuation of the rectum (the act of defecation).
To easily recall the names of the three muscle ribbons (taeniae coli), use the association: «The free (libera) omental (omentalis) ribbon hangs on the mesocolic (mesocolica) border».
Class and exam questions
How to distinguish the large intestine from the small intestine on a specimen?
The large intestine has a larger diameter, prominent longitudinal muscle ribbons, sacculated haustra, and fatty omental appendices.
Are there villi in the large intestine?
No, the mucous membrane of the large intestine completely lacks intestinal villi, but contains numerous deep crypts.
Where does the large intestine begin and where does it end?
It begins at the ileocecal junction and ends at the anal orifice of the rectum.
What else to study on this topic
- Topography and anatomy of the cecum (caecum)
- Structure of the colon (colon) and its parts
- Anatomical features of the rectum (rectum)
- Structure of the ileocecal junction
- Localization and function of lymphoid follicles in the intestinal wall
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