The large intestine is all about 1.5 m long and goes from the caecum in the right iliac fossa to the anus in the perineum.
Aside from the transverse colon and sigmoid colon, it is more fixed in position compared to the small intestine.
The functions of the large intestine are:
- Absorption of water from fluid contents in it to help create the feces.
- Storage, lubrication, and expulsion of feces.
- Synthesis of vitamin B complex by normal bacterial flora present its lumen.
- Protection from invasion by microorganisms by its mucoid secretion that is rich in IgA group of antibody.
For illustrative purposes, the large intestine is split into the following 4 parts:
- Caecum and appendix.
- Anal canal.
The colon is further split into 4 parts: ascending colon, transverse colon, descending colon, and sigmoid colon.
All the parts of the large intestine are retroperitoneal and mended with the exception of appendix, transverse colon, and sigmoid colon that are intraperitoneal and possess mesenteries. The mesenteries of these parts are called mesoappendix, transverse mesocolon, and sigmoid mesocolon, respectively.
The structure of the large intestine differs in various parts. For this reason, the subsequent report copes only with the structure of colon- the longest part, as a representative of the large intestine. Like the small intestine, the colon also includes 4 layers. From inside outward these are mucosa, submucosa, muscle layer, and serosa.
The mucosa will not present transverse circular folds (plicae circulares) and villi. Nonetheless, temporary folds affecting mucosa and submucosa exist in the undescended colon. The surface epithelium is absorptive in nature and is composed of columnar cells bearing microvilli.
Intestinal glands (crypts of Lieberkuhn) are long and tubular and possess a large number of goblet cells. The lamina propria of the mucous membrane includes ample diffuse lymphatic tissue but is devoid of Peyer’s patches.
It’s created from loose areolar tissue and includes blood vessels, lymph vessels, and nerve plexus (Meissner’s plexus).
It contains outer longitudinal and inner circular layers of smooth muscle. The outer longitudinal layer is not continuous, instead, it’s ordered into 3 thick longitudinal bands referred to as teniae coli. The inner circular layer of smooth muscle Teniae coli is thin when compared with the small intestine.
It covers the transverse colon and the sigmoid colon but the parts of ascending and descending colons are covered by tunica adventitia.
Cardinal (Distinguishing) Features
The 3 cardinal features of the large intestine are the presence of (a) teniae coli, (b) appendices epiploicae, and (c) sacculations (or haustrations).
These are 3 ribbon-like bands of the longitudinal muscle layer. These bands converge proximally at the base of the appendix and spread out distally to become constant with the longitudinal muscle layer of the rectum. Hence, teniae coli are present on all the parts of colon and caecum.
In the caecum, ascending colon, and descending colon, the places of teniae are anterior (Teniae libera), posteromedial (Teniae mesocolica), and posterolateral (Teniae omentalis), however in the transverse colon the corresponding places are inferior, posterior, and superior, respectively.
All these are small bags of visceral peritoneum filled up with fat connected to the teniae of large intestine. Consequently, they may be absent in the appendix, rectum, and anal canal. The appendices epiploicae are most numerous on the sides of sigmoid colon and posterior surface of the transverse colon.
Sacculation (or Haustrations)
All these are a series of pouches/dilatations in the wall of caecum and colon between the teniae. They may be generated because length of teniae fall short of the length of circular muscle coat. The sacculations are accountable for the characteristic puckered look of the large intestine.
Differences between the Small and Large Intestines
Medically, it’s essential for the pupils to understand that (a) the small intestine is a standard site for worm infestation, typhoid, and tubercular ulcers on the other hand the large intestine a standard site for amebiasis and carcinoma. (b) The infection and irritation of the small intestine result in diarrhea, on the other hand the infection and irritation of the large intestine cause dysentery.