Changes

==Introduction==

==Introduction==

The omasum is the third chamber in the ruminant stomach. It lies within the intrathoracic part of the abdomen so cannot be palpated manually. Instead it is examined by ausculation.

The omasum is the third chamber in the ruminant stomach. It lies within the intrathoracic part of the abdomen so cannot be palpated manually. Instead it is examined by ausculation. The omasum has biphasic contractions which squeeze fluid out of the food before allowing the ingesta to continue into the [[Abomasum - Anatomy & Physiology|abomasum]]. Absorption of [[Volatile Fatty Acids|volatile fatty acids]] continues in the omasum.

 

The omasum has biphasic contractions which squeeze fluid out of the food before allowing the ingesta to continue into the [[Abomasum - Anatomy & Physiology|abomasum]].

 

Absorption of [[Volatile Fatty Acids|volatile fatty acids]] continues in the omasum.

==Structure==

==Structure==

[[Image:Omasum Anatomy Sheep.jpg|thumb|right|150px|Omasum Anatomy (Sheep) - Copyright RVC 2008]]

[[Image:Omasum Anatomy Sheep.jpg|thumb|right|250px|Omasum Anatomy (Sheep) - Copyright RVC 2008]]

*Right of the midline

The omasum is right of the midline. The [[Rumen - Anatomy & Physiology|Rumen]] and [[Reticulum - Anatomy & Physiology|reticulum]] are located to the left and the liver and body wall to the right. It is covered by lesser omentum and is bilaterally flattened. It is located at ribs 8-11. The lower pole has an extensive connection to the fundic region of the [[Abomasum - Anatomy & Physiology|abomasum]]. An '''omasal canal''' is present known as the '''omasal groove'''. The floor is smooth except for low ridges and projections around the upper opening. The opening to the [[Reticulum - Anatomy & Physiology|reticulum]] is at the cranial end of the omasal canal. The opening to the [[Abomasum - Anatomy & Physiology|abomasum]] is at the caudal end of the omasal canal. It is a large oval opening, partly covered by overhanging abomasal folds.

 

*[[Rumen - Anatomy & Physiology|Rumen]] and [[Reticulum - Anatomy & Physiology|reticulum]] to the left, liver and body wall to the right

 

*Covered by lesser omentum

 

*Bilaterally flattened

 

*Ribs 8-11

 

*Lower pole has an extensive connection to fundic region of the [[Abomasum - Anatomy & Physiology|abomasum]]

 

*Omasal canal present

**Known as the omasal groove

**Floor smooth except for low ridges and projections around the upper opening

 

*Opening to the [[Reticulum - Anatomy & Physiology|reticulum]] at the cranial end of the omasal canal

 

*Opening to the [[Abomasum - Anatomy & Physiology|abomasum]] at the caudal end of the omasal canal. It is a large oval opening, partly covered by overhanging abomasal folds.

 

==Function==

==Function==

A function of the omasum is absorption. Food enters omasum at second biphasic reticular contraction. There are biphasic contractions. The first contraction expels fluid by squeezing the ingesta from the omasal canal between the lamellae. The second contraction expels solids by mass contraction of the omasum. Contractions are slower than the rumenoreticular contractions (see [[Rumination - Anatomy & Physiology|rumination]]). Food is squeezed between lamellae.

 

*Food enters omasum at second biphasic reticular contraction

 

*Biphasic contractions

 

*First contraction expels fluid by squeezing the ingesta from the omasal canal between the lamellae

 

*Second contraction expels solids by mass contraction of the omasum

 

*Contractions are slower than the rumenoreticular contractions (see [[Rumination - Anatomy & Physiology|rumination]])

 

*Food squeezed between lamellae

 

==Vasculature==

==Vasculature==

*Cranial mesenteric artery

The blood supply to the omasum includes the '''cranial mesenteric artery''', the '''caudal mesenteric artery''' and the '''left gastric''' and '''left gastroepiploic arteries'''.

 

*Caudal mesenteric artery

 

*Left gastric and left gastroepiploic arteries

==Innervation==

==Innervation==

*Dorsal vagus nerve ([[Cranial Nerves - Anatomy & Physiology|CN X]]) (most important)

The omasum is innervated by the '''dorsal vagus nerve''' ([[Cranial Nerves - Anatomy & Physiology|CN X]]) (most important) and the '''ventral vagus nerve''' ([[Cranial Nerves - Anatomy & Physiology|CN X]]).

 

*Ventral vagus nerve ([[Cranial Nerves - Anatomy & Physiology|CN X]])

==Lymphatics==

==Lymphatics==

*Numerous small lymph nodes are scattered in the omasal curvatures

There are numerous small lymph nodes that are scattered in the omasal curvatures. The lymph drains to larger atrial nodes between the cardia and the omasum, then to the cistera chyli.

 

*The lymph drains to larger atrial nodes between the cardia and the omasum, then to the cistera chyli

 

 

==Species Differences==

==Species Differences==

===Small Ruminant===

===Small Ruminant===

*Smaller omasum

The small ruminants have a smaller omasum. The omasum is bean shaped.

 

*Omasum is bean shaped

===Bovine===

===Bovine===

*Lower pole of omasum contacts the the abdominal floor below the costal arch

The lower pole of the omasum contacts the abdominal floor below the costal arch.

==Test yourself with the stomachs of the ruminant flashcards==

==Links==

[[The Stomachs of the Ruminant - Anatomy & Physiology - Flashcards#The Omasum|The Omasum Flashcards]]

'''Test yourself with the [[The Stomachs of the Ruminant - Anatomy & Physiology - Flashcards#The Omasum|The Omasum Flashcards]]'''

[[Rumen - Anatomy & Physiology|The Rumen - Anatomy & Physiology]]

'''Click here for information on [[Rumen - Anatomy & Physiology|rumen - Anatomy & Physiology]]'''

[[Reticulum - Anatomy & Physiology|The Reticulum - Anatomy & Physiology]]

'''Click here for information on [[Reticulum - Anatomy & Physiology|reticulum - Anatomy & Physiology]]'''

[[Abomasum - Anatomy & Physiology|The Abomasum- Anatomy & Physiology]]

'''Click here for information on [[Abomasum - Anatomy & Physiology|abomasum- Anatomy & Physiology]]'''

'''Video'''

'''Video links:'''

[http://stream2.rvc.ac.uk/Anatomy/bovine/Pot0052.mp4 Pot 52 Lateral view of the Abdomen of a young Ruminant]  

[http://stream2.rvc.ac.uk/Anatomy/bovine/Pot0052.mp4 Pot 52 Lateral view of the Abdomen of a young Ruminant]  

[[Category:Stomach - Anatomy & Physiology]]

[[Category:Stomach - Anatomy & Physiology]]

[[Category:To Do - A&P]]

[[Category:To Do - AimeeHicks]]