Omasum - Anatomy & Physiology

ALIMENTARY SYSTEM

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

Omasum Anatomy (Sheep) - Copyright RVC 2008

• Right of the midline

• Rumen and 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

• 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

Omasum Histology (Sheep) - Copyright RVC 2008

• Keratinised stratified squamous epithelium

• Lamellae thrown into leaves
  • Different sizes
  • Divides the lumen into narrow and uniform recesses

• Firm

• No glands

• Papillae
  • Most small and lenticular
  • Some large and conical

• Cicrular tunica muscularis extends into papillae of long laminae

• Lamina muscualris extends into papillae encircling tunica muscularis

• 3 smooth muscle layers in papillae so they are very motile

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

The Omasum 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

Right sided topography of the Ovine Abdomen

Structure of the ruminant forestomachs