Omasum - Anatomy & Physiology
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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 has biphasic contractions which squeeze fluid out of the food before allowing the ingesta to continue into the abomasum.
Absorption of volatile fatty acids continues in the omasum.
Structure
- Right of the midline
- Covered by lesser omentum
- Bilaterally flattened
- Ribs 8-11
- Lower pole has an extensive connection to fundic region of the 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 at the cranial end of the omasal canal
- Opening to the abomasum at the caudal end of the omasal canal. It is a large oval opening, partly covered by overhanging abomasal folds.
Function
- Absorption
- 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)
- Food squeezed between lamellae
Vasculature
- Cranial mesenteric artery
- Caudal mesenteric artery
- Left gastric and left gastroepiploic arteries
Innervation
- Dorsal vagus nerve (CN X) (most important)
- Ventral vagus nerve (CN X)
Lymphatics
- Numerous small lymph nodes are scattered in the omasal curvatures
- The lymph drains to larger atrial nodes between the cardia and the omasum, then to the cistera chyli
Histology
Species Differences
Small Ruminant
- Smaller omasum
- Omasum is bean shaped
Bovine
- Lower pole of omasum contacts the the abdominal floor below the costal arch
Test yourself with the stomachs of the ruminant flashcards
Links
The Rumen - Anatomy & Physiology
The Reticulum - Anatomy & Physiology
The Abomasum- Anatomy & Physiology
Video
Pot 52 Lateral view of the Abdomen of a young Ruminant
Pot 175 Sections of the Ruminant Stomach
Pot 47 Ovine Omasum and Abomasum
Left sided topography of the Ovine Abdomen and Thorax