The omasum, also known as the green,The Chambers Dictionary, Ninth Edition, Chambers Harrap Publishers, 2003
/ref> the fardel, the manyplies and the psalterium, is the third compartment of the

stomach The stomach is a muscular, hollow organ in the upper gastrointestinal tract of Human, humans and many other animals, including several invertebrates. The Ancient Greek name for the stomach is ''gaster'' which is used as ''gastric'' in medical t ...

ruminants Ruminants are herbivorous grazing or browsing artiodactyls belonging to the suborder Ruminantia that are able to acquire nutrients from plant-based food by Enteric fermentation, fermenting it in a specialized stomach prior to digestion, principa ...

. The omasum comes after the

rumen The rumen, also known as a paunch, is the largest stomach compartment in ruminants. The rumen and the reticulum make up the reticulorumen in ruminant animals. The diverse microbial communities in the rumen allows it to serve as the primary si ...

and

reticulum Reticulum is a small, faint constellation in the southern sky. Its name is Latin for a small net, or reticle—a net of crosshairs at the focus of a telescope eyepiece that is used to measure star positions. The constellation is best viewed b ...

and before the

abomasum The abomasum, also known as the maw,The Cham ...

. Different ruminants have different omasum structures and function based on the food that they eat and how they developed through evolution.

Anatomy

The omasum can be found on the right side of the cranial portion of the rumen. It receives food from the

through the reticulo-omasal orifice and provides food to the abomasum through the omaso-abomasal orifice. The omasum is spherical to crescent shape and has multiple leaflets similar to that of a book called omasal laminae. The omasal laminae are made of thin muscular layers covered with a nonglandular mucous membrane. The omasal laminae come from the sides of the large curvature and project towards the inside of the omasum, extending from the reticulo-omasal orifice to the omaso-abomasal orifice. They greatly increase the surface area of the omasum. The laminae are covered in omasal papillae that are claw-like in some ruminants or blunt cones in others. These papillae further increase the surface area but they also provide increased friction against the food particles.

Function

The function of the omasum is not completely understood. During the second contraction phase of the reticulum, the reticule-omasal sphincter opens for a few seconds allowing a small volume of finely dispersed and well-fermented ingesta to enter the omasum. The omasum has two physiological compartments: omasal canal that transfers food from the reticulum to the omasum, and the inter-laminate recesses between the mucosal laminae which provide the area for absorption. The omasum is where food particles that are small enough get transferred into the abomasum for enzymatic digestion. In ruminants with a more sophisticated omasum, the large surface area allows it to play a key role in the absorption of water,

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

s, volatile fatty acids, minerals, and the fermentation of food. Young ruminants that are still drinking milk have an esophageal groove that allows milk to bypass the rumen and go straight from the esophagus to the omasum.

Species differences

An early version of the omasum is seen in early ruminants like

duiker A duiker is a small to medium-sized brown antelope native to sub-Saharan Africa, found in heavily wooded areas. The 22 extant species, including three sometimes considered to be subspecies of the other species, form the subfamily Cephalophinae ...

s and

muntjac Muntjacs ( ), also known as the barking deer or rib-faced deer, (URL is Google Books) are small deer of the genus ''Muntiacus'' native to South Asia and Southeast Asia. Muntjacs are thought to have begun appearing 15–35 million years ago, ...

s, where it is a little more than a strainer sieve which prevents un-chewed foods from entering the

abomasum The abomasum, also known as the maw,The Cham ...

. The smallest omasum belongs to ruminants that consume high quality diets like the

moose The moose (: 'moose'; used in North America) or elk (: 'elk' or 'elks'; used in Eurasia) (''Alces alces'') is the world's tallest, largest and heaviest extant species of deer and the only species in the genus ''Alces''. It is also the tal ...

and roe deer, while the largest belongs to those who are un-selective grass and roughage eaters like cattle and sheep. The omasum is not only bigger in grass and roughage eaters but there is greater differentiation in the book-like structure; seen as an increase in the number of laminae.

Culinary uses

References

{{Authority control Mammal anatomy Digestive system Ruminants

Anatomy

Function

Species differences

Culinary uses

See also

References