Difference between revisions of "Anus - Anatomy & Physiology"
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[[Image:Anal Glands.jpg|thumb|right|150px|Anal Glands- Copywright RVC 2008]] | [[Image:Anal Glands.jpg|thumb|right|150px|Anal Glands- Copywright RVC 2008]] | ||
*At the anus, the columnar intestinal epithelium is replaced by the stratified squamous keratinised epithelium of the skin. | *At the anus, the columnar intestinal epithelium is replaced by the stratified squamous keratinised epithelium of the skin. | ||
| + | *The '''anal canal''' joins the bowel to the exterior and is the last 2-3cm of the alimentary tract. | ||
| + | **This is a short passage derived from the proctodeum (formed by invagination of the surface ectoderm). | ||
| + | *Sebaceous and apocrine sweat glands both occur in this region in association with the anal sphincters. | ||
| + | **Before joining the anal canal, the rectum becomes dilated to form the ''rectal ampulla''. | ||
| + | *At the '''rectoanal''' junction, the lumen is constricted by longitudinal folds in the mucosa. | ||
| + | **These are normally pressed together to occlude the lumen. | ||
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*As the muscosa changes from columnar to cutaneous, three zones are created: | *As the muscosa changes from columnar to cutaneous, three zones are created: | ||
**'''Columnar''' | **'''Columnar''' | ||
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***Surrounds the anus. | ***Surrounds the anus. | ||
***Excretory ducts of the '''anal sacs''' open into this region. | ***Excretory ducts of the '''anal sacs''' open into this region. | ||
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==Species Differences== | ==Species Differences== | ||
Revision as of 11:48, 15 July 2008
Introduction
Structure
- There are two anal sphincters:
- Internal anal sphincter, formed by thickening of the circular smooth muscle of the gut and under autonomic control.
- External anal sphincter, formed from striated skeletal muscle and under voluntary control.
Function
Vasculature
Innervation
Lymphatics
Histology
- At the anus, the columnar intestinal epithelium is replaced by the stratified squamous keratinised epithelium of the skin.
- The anal canal joins the bowel to the exterior and is the last 2-3cm of the alimentary tract.
- This is a short passage derived from the proctodeum (formed by invagination of the surface ectoderm).
- Sebaceous and apocrine sweat glands both occur in this region in association with the anal sphincters.
- Before joining the anal canal, the rectum becomes dilated to form the rectal ampulla.
- At the rectoanal junction, the lumen is constricted by longitudinal folds in the mucosa.
- These are normally pressed together to occlude the lumen.
- As the muscosa changes from columnar to cutaneous, three zones are created:
- Columnar
- Has many longitudinal folds.
- Divided from the rectum by the anorectal line.
- This is a line where the mucosa is stratified squamous epithelium containing lots of lymphoid tissue.
- Intermediate
- Divided from the cutaneous zone by the anocutaneous line.
- Cutaneous
- Skin.
- Stratified squamous keratinised epithelium.
- Surrounds the anus.
- Excretory ducts of the anal sacs open into this region.
- Columnar
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.
- Large, coiled apocrine tubules.
- Have many glands in their walls.
- These glands are tubuloalveolar and produce a fatty secretion.
- Anal sacs are clinically important as they are commonly diseased in dogs - frequently, they become enlarged due to accumulated secretion.