Difference between revisions of "Anus - Anatomy & Physiology"

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<big><center>[[Alimentary - Anatomy & Physiology|'''BACK TO ALIMENTARY - ANATOMY & PHYSIOLOGY''']]</center></big>
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==Overview==
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<big><center>[[Large Intestine - Anatomy & Physiology|'''BACK TO LARGE INTESTINE - ANATOMY & PHYSIOLOGY''']]</center></big>
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==Introduction==
  
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The anus is the terminal portion of the '''alimentary tract''' which communicates with the external environment. Two '''sphincters''' control it's aperture. It allows faeces and gas to leave the body. '''Defeacation''' is the process where faeces are expelled from the [[Rectum - Anatomy & Physiology|rectum]] through the anus.
 
  
 
==Structure==
 
==Structure==
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[[Image:Sheep Anus.jpg|thumb|right|250px|Anus (Sheep) - © RVC 2008]]
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[[Image:Sheep Anus.jpg|thumb|right|150px|Anus (Sheep) - Copywright RVC 2008]]
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*There are ''two'' anal sphincters:
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There are ''two'' anal sphincters:
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**'''Internal anal sphincter''', formed by thickening of the circular smooth muscle of the gut and under autonomic control.
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The '''internal anal sphincter''', formed by the thickening of the circular smooth muscle of the gut and under autonomic control and the '''external anal sphincter''', formed from striated skeletal muscle and under voluntary control.
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**'''External anal sphincter''', formed from striated skeletal muscle and under voluntary control.
  
 
==Function==
 
==Function==
  
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The function of the anus is to transmit faeces from the '''rectum''' to the external environment. '''Anal sphincters''' give control over the regularity of defeaction, and this may be consciously controlled in some species. The '''anal canal''' lubricates the passage of faeces.
 
  
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==Species Differences==
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==Vasculature==
  
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===Carnivore===
 
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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. They are large, coiled apocrine tubules that have many '''tubuloalveolar glands''' in their walls. These '''tubuloalveolar glands''' produce a fatty secretion. The '''fundus''' of the sac secretes a potent smelling fluid that drains through a single duct to an opening near the '''anocutaneous junction'''.
 
  
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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.
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==Innervation==
  
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Anal sacs are clinically important as they are commonly [[Anal Sac Disease - General|diseased]] in dogs - frequently, they become enlarged due to accumulated secretion. The '''dog''' has '''perianal glands''' that lie in the skin surrounding the anus. They are modified sebaceous glands.
 
  
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===Equine===
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==Lymphatics==
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Click here for information on the [[Alimentary System  - Horse Anatomy|equine]] anus.
 
  
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===Porcine===
 
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Also have '''anal sacs'''.
 
  
 
==Histology==
 
==Histology==
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[[Image:Anal Glands.jpg|thumb|right|150px|Anal Glands- Copywright RVC 2008]]
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*At the anus, the columnar intestinal epithelium is replaced by the stratified squamous keratinised epithelium of the skin.
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*The '''anal canal''' joins the bowel to the exterior and is the last 2-3cm of the alimentary tract.
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**This is a short passage derived from the proctodeum (formed by invagination of the surface ectoderm).
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*Sebaceous and apocrine sweat glands both occur in this region in association with the anal sphincters.
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**Before joining the anal canal, the rectum becomes dilated to form the ''rectal ampulla''.
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*At the '''rectoanal''' junction, the lumen is constricted by longitudinal folds in the mucosa.
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**These are normally pressed together to occlude the lumen.
  
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[[Image:Anal Glands.jpg|thumb|right|250px|Anal Glands- © RVC 2008]]
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*As the muscosa changes from columnar to cutaneous, three zones are created:
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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 large 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:
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**'''Columnar'''
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***Has many longitudinal folds.
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***Divided from the rectum by the ''anorectal line''.
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****This is a line where the mucosa is stratified squamous epithelium containing lots of lymphoid tissue.  
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**'''Intermediate'''
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***Divided from the cutaneous zone by the ''anocutaneous line''.
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**'''Cutaneous'''
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***Skin.
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***Stratified squamous keratinised epithelium.
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***Surrounds the anus.
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***Excretory ducts of the '''anal sacs''' open into this region.
  
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'''1.Columnar'''
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==Species Differences==
  
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The columnar zone has many longitudinal folds. It can be divided from the rectum by the ''anorectal line''. This is a line where the mucosa is stratified squamous epithelium containing lots of lymphoid tissue.  
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===Carnivore===
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*The dog and cat posses two '''anal sacs'''. In the dog, these are the size of a hazlenut.
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'''2.Intermediate'''
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**They are located ventrolaterally between the internal and external anal sphincters.
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**The fundus of the sac secretes a potent smelling fluid that drains through a single duct to an opening near the anocutaneous juncntion.
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The intermediate zone can be divided from the cutaneous zone by the ''anocutaneous line''.
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**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.
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***Large, coiled apocrine tubules.
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'''3.Cutaneous'''
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***Have many glands in their walls.
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***These glands are tubuloalveolar and produce a fatty secretion.
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The cutaneous zone is the skin. It is stratified squamous keratinised epithelium that surrounds the anus. The excretory ducts of the '''anal sacs''' open into this region.
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***Anal sacs are clinically important as they are commonly diseased in dogs - frequently, they become enlarged due to accumulated secretion.
  
 
==Links==
 
==Links==
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'''Click here for information on the [[Intestines, Small and Large - Pathology|Pathology Of The Small and Large Intestines]].'''
 
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[[Category:Large Intestine - Anatomy & Physiology]]
 
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[[Category:A&P Done]]
 

Revision as of 11:48, 15 July 2008

BACK TO ALIMENTARY - ANATOMY & PHYSIOLOGY
BACK TO LARGE INTESTINE - ANATOMY & PHYSIOLOGY

Introduction

Structure

Anus (Sheep) - Copywright RVC 2008
  • 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

Anal Glands- Copywright RVC 2008
  • 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.

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.

Links

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