Omasum:
It is ellipsoidal in form (in small ruminants is smaller than reticulum), the log axis is nearly vertical, it lies chiefly to the right of the median plane opposite to the 7th to the 11th rib. On right surface of the rumen and caudal to the reticulum. It has:
A-parietal surface (right): related to the diaphragm and liver.
B-visceral surface (left): contact with the rumen, reticulum and abomasums.
C-greater curvature: directed posteriorly and to the right.
D-lesser curvature: connected with the reticulum by a very short narrow neck of the omasum.
Internal surface: It has:
1-omasal lamina: about (100) are present in the Ox and about (80) in Sheep which carried small papillae and differ in size or length separated by inter lamina recesses.
2-omasal groove: the omasal groove portion of the gastric groove, it passes from reticulum to abomasums.
It has two openings:
1-reticulo-omasum orifice. 2-omasal-abomasums orifice.
Relations of omasum:
dorsally: rumen ,ventrally: abdominal wall, cranially: reticulum &-Caudally: intestine
Abomasum:
1-it is an elongated sac which lies chiefly on the abdominal floor on right side.
2-it has:
A-cranial blind end: the fundus is relation with the reticulum and ventral sac of the rumen.
B-body: extend caudally between the ventral sac of rumen and the omasum. Lying more on the left than on the right of the median plane.
C-pyloric part: turns to the right caudal to the greater curvature of omasum and joins the duodenum at the pylorus.
3-externally it has two surfaces and two curvatures:
A-parietal surface: contact with the abdominal floor.
B-visceral surface: related to the rumen and omasum.
C-greater curvature: attached to the greater omentum found ventrally to the left.
D-lesser curvature: attached to the lesser omentum directed dorsally to the right and contact with the omasum.
4-the abomasums is lined by glandular mucous membrane.
There is a narrow, light colored cardiac gland zone at the omasoabomasal orifice.
The fundus and body contain gastric glands (proper gastric gland region), the mucous membrane has a reddish color and is marked by large prominent fold, and the folds are not spiraling, but oblique (12-14) extending from sides of the abomasal groove to curvature.
Abomasal groove is the third segment of the gastric groove, the pars pylorica (pyloric region) lined by yellowish, wrinkled mucous membrane with out folds, It contains the pyloric glands.
The torus pyloricus: is a half rounded protuberance in the pylorus at the end of the lesser curvature and instead of pyloric sphincter in the horse.
Relation of abomasum:
dorsally: intestine, ventrally: abdominal wall , left side: rumen &right side: liver
Gastric groove (ventricular groove):
1-extend from the cardia through the lesser curvature of the reticulum, base of omasum and into the abomasums. It is very clear and useful in the young animal to directed milk into abomasums directly. It is divided into 3 segments corresponding to the compartment of the stomach complex, through which it passes (reticular groove, omasal groove and abomasal groove).
It reflex in young animals during sucking to form a tubular canal to transport milk directly to the abomasums. In adult the gastric groove does not close except in response to pharmacologic levels of certain salts. It can be closed by oral administration of (0.4) molar copper sulfate solution (sheep) or 10%NAHCO3 solution (cattle), closure usually lasts less than 10 seconds.
Comparative:
1-in the sheep and goats, the reticulum is relatively larger than in the ox.
2-the omasum of sheep is much smaller than the reticulum. It is oval in shape.
3-the abomasum in the sheep and goats, is relatively larger and longer than in the ox.