The large Intestine consists of caecum, colon and rectum and the general structure of these parts are similar. The structure of large intestine resembles that of the small intestine except for the following differences.
Mucosa
- There are no villi. Intestinal glands or crypts of Lieberkuhn are present throughout and they are longer and straighter. The surface epithelium consists of tall columnar cells but the Goblet cells far exceed in number than the columnar cells. The crypts of Lieberkuhn also contain numerous goblet cells and towards the distal portions of the intestine the crypts appear to be lined entirely by goblet cells.
- The mucous membrane of the rectum is usually thrown into a number of longitudinal folds. At the anus, the simple columnar epithelium is replaced by stratified squamous epithelium, which becomes continuous with the epidermis beyond the anal orifice.
- The submucosa and muscularis do not present any special features. The serous coat is absent in the terminal portion of the rectum and is replaced by a fibrous coat.
Species difference
- The large intestine of man, solipeds and pigs, possess flat bands of longitudinal muscle known as taeniae. (For details about the number and extent of these bands in the horse, refer splanchnology).
- In the dog and pig at recto-anal junction, tubulo-alveolar anal glands are present. In pigs, it produces a mucous secretion and in dogs it produces a fatty secretion.
- In the dog, circumanal glands occur at the site where anal mucosa becomes continuous with the skin. These consists of a sebaceous portion, which opens through a patent duct into an adjacent hair-follicle and a non-sebaceous portion, which exhibits no evidence of any secretory activity.
- Lateral and ventral to the anus in carnivores are the anal sacs. The wall of anal sac is covered by stratified squamous epithelium. In the loose subepithelial layer are apocrine tubular glands in the dog and in the cat, sebaceous glands are present in addition. The excretory ducts of anal sacs also contain tubular and sebaceous glands.
