Large Intestine - Anatomy & Physiology
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Introduction
The large intestine extends from the ileum of the small intestine to the anus. It absorbs water from the ingesta and stores faeces prior to defeacation. Every species has a large microbial population living in the large intestine, which is of particular importance to the hindgut fermenters. For this reason, hindgut fermenters have a more complex large intestine with highly specialised regions for fermentation.
The large intestine can be divided into:
Structure
Function
Vasculature
- The cranial mesenteric artery supplies the caecum, ascending and part of the transverse colon.
- The caudal mersenteric artery supplies the rest of the transverse colon, the descending colon and the rectum.
Innervation
- Like the small intestine, the large intestine recieves sympathetic and parasympathetic innervation.
- Neurones interact with the myenteric plexus to affect contractility, and with the submucosal plexus to affect secretions.
- The sympathetic have coeliac, cranial mesenteric and caudal mesenteric ganglia.
- As the sympathetic fibres leave the ganglia, they surround their respective artery.
- Parasympathetic innervation stimulates peristalsis.
Lymphatics
Histology
- The muscosa of the large intestine is smooth; there are no villi.
- The number of goblet cells in the mucosa increases compared to the small intestine.
- Mucus is very important for lubrication of the ingesta as it passes through the intestine, particularly as more water is absorbed from the lumen making it drier.
- There are numerous scattered lymph nodules.
- The number of lymph nodules increases compared to the small intestine.
- Taenia may be present.
- These are concentrations of the longitudinal muscle layer into long bands.
- When the taenia contract, they cause shortening of the large intestine, which produces saccualtions, or haustra.
- Many glands are present in the mucosa and skin of the anal region.
Species Differences
Carnivore
- The dog and cat posses two anal sacs. In the dog, these are the size of a hazlenut.
- They are located ventrolaterally between the internal and external anal sphincters.
- The fundus of the sac secretes a potent smelling fluid that drains through a single duct to an opening near the anocutaneous juncntion.
- The anal sacs get compressed during defecation, which causes the fluid to be expressed. The scent of the fluid is thought to act as a territorial marker.
Ruminant
Horse
Pig
- Taenia present