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Omasum - Anatomy & Physiology (view source)

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==Introduction==
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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.
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The omasum has biphasic contractions which squeeze fluid out of the food before allowing the ingesta to continue into the [[The Abomasum|abomasum]].
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Absorption of [[Volatile Fatty Acids - Anatomy & Physioogy|volatile fatty acids]] continues in the omasum.
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==Structure==
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[[Image:Omasum Anatomy Sheep.jpg|thumb|right|150px|Omasum Anatomy (Sheep) - Copyright RVC 2008]]
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*Right of the midline
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*[[The Rumen - Anatomy & Physiology|Rumen]] and [[The Reticulum - Anatomy & Physiology|reticulum]] to the left, liver and body wall to the right
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*Covered by lesser omentum
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*Bilaterally flattened
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*Ribs 8-11
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*Lower pole has an extensive connection to fundic region of the [[The Abomasum|abomasum]]
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*Omasal canal present
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**Known as the omasal groove
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**Floor smooth except for low ridges and projections around the upper opening
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*Opening to the [[The Reticulum - Anatomy & Physiology|reticulum]] at the cranial end of the omasal canal
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*Opening to the [[The Abomasum|abomasum]] at the caudal end of the omasal canal. It is a large oval opening, partly covered by overhanging abomasal folds.
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==Function==
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*Absorption
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*Food enters omasum at second biphasic reticular contraction
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*Biphasic contractions
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*First contraction expels fluid by squeezing the ingesta from the omasal canal between the lamellae
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*Second contraction expels solids by mass contraction of the omasum
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*Contractions are slower than the rumenoreticular contractions (see [[Rumination - Anatomy & Physiology|rumination]])
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*Food squeezed between lamellae
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==Vasculature==
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*Cranial mesenteric artery
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*Caudal mesenteric artery
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*Left gastric and left gastroepiploic arteries
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==Innervation==
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*Dorsal vagus nerve ([[Nervous System - CNS - Anatomy & Physiology#Innervation - Cranial Nerves|CN X]]) (most important)
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*Ventral vagus nerve ([[Nervous System - CNS - Anatomy & Physiology#Innervation - Cranial Nerves|CN X]])
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==Lymphatics==
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*Numerous small lymph nodes are scattered in the omasal curvatures
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*The lymph drains to larger atrial nodes between the cardia and the omasum, then to the cistera chyli
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==Histology==
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[[Image:Omasum Histology Sheep.jpg|thumb|right|150px|Omasum Histology (Sheep) - Copyright RVC 2008]]
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*Keratinised stratified squamous epithelium
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*Lamellae thrown into leaves
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**Different sizes
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**Divides the lumen into narrow and uniform recesses
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*Firm
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*No glands
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*Papillae
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**Most small and lenticular
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**Some large and [[Oral Cavity - Tongue - Anatomy & Physiology#Types of Papillae|conical]]
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*Cicrular tunica muscularis extends into papillae of long laminae
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*Lamina muscualris extends into papillae encircling tunica muscularis
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*3 smooth muscle layers in papillae so they are very motile
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==Species Differences==
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===Small Ruminant===
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*Smaller omasum
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*Omasum is bean shaped
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===Bovine===
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*Lower pole of omasum contacts the the abdominal floor below the costal arch
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==Links==
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[[The Rumen - Anatomy & Physiology|The Rumen - Anatomy & Physiology]]
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[[The Reticulum - Anatomy & Physiology|The Reticulum - Anatomy & Physiology]]
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[[The Abomasum|The Abomasum- Anatomy & Physiology]]
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[[The Stomachs of the Ruminant - Anatomy & Physiology - Flashcards#The Omasum|The Omasum Flashcards]]
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'''Video'''
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[http://stream2.rvc.ac.uk/Anatomy/bovine/Pot0052.mp4 Pot 52 Lateral view of the Abdomen of a young Ruminant]
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[http://stream2.rvc.ac.uk/Anatomy/bovine/pot0175.mp4 Pot 175 Sections of the Ruminant Stomach]
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[http://stream2.rvc.ac.uk/Anatomy/ovine/Pot0047.mp4 Pot 47 Ovine Omasum and Abomasum]
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[http://stream2.rvc.ac.uk/Frean/sheep/LeftSideTopography.mp4 Left sided topography of the Ovine Abdomen and Thorax]
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[http://stream2.rvc.ac.uk/Frean/sheep/RightSideTopography.mp4 Right sided topography of the Ovine Abdomen]
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[http://stream2.rvc.ac.uk/Frean/sheep/RuminantStomachStructure.mp4 Structure of the ruminant forestomachs]
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