The ruminant stomach consists of four compartments: the rumen, reticulum, omasum, and abomasum. The rumen is the largest compartment and has contractions that move food and send it back to the mouth. It contains microbes that aid in breaking down food particles via fermentation. The reticulum forms a honeycomb structure that traps large food particles. The omasum has many parallel folds, and the abomasum is glandular like a simple stomach. A gastric groove connects the compartments and allows milk to bypass the rumen in young ruminants.

1. Anatomy of the
Ruminant Stomach
By
Prof. Dr. Abdelmohaimen Mostafa Saleh
Professor of Anatomy & Embryology
Assiut University

2. The Ruminant stomach consists of 4 comparatments which are:-
-Rumen, reticulum and omasum, represented the forestomach
(proventriculus) , lined with nonglandular mucosa.
-The fourth comparatments; abomasum, lined with glandular mucosa
(Ture Stomach).

3. The rumen
The rumen is the first compartment of the ruminant stomach. It is the largest
chamber and has regular contractions to move food around for digestion,
eliminate gases and send food particles back to the mouth for remastication.
The rumen breaks down food particles through mechanical digestion and fermentation
with the help of associated microbes.Volatile fatty acids are the main product of
ruminant digestion.

4. Anatomical description:
It has two surfaces and two curvatures:
 Parietal (left) surface: related to
diaphragm, spleen and left bdominal
wall. Direct contact of rumen with left
flank facilitates auscultation, palpation
,trocherization and surgical operation.
 Visceral (right) surface: related to
intestine, liver, omasum and
abomasum.
 Both parietal & visceral surfaces
characterized by ruminal grooves.
 Dorsal curvature: lies against
diaphragm and roof of abdominal
cavity.
 Ventral curvature: lies against
abdominal floor.

5. Ruminal grooves
 Rumen is divided externally into
several parts by a number of ruminal
grooves which are arranged as follow:
 Ruminoreticular groove : between
the rumen and reticulum.
 right and left longitudinal grooves: on
the visceral and parietal surfaces
respectively.
 Right accessory groove: extends from
right long. groove ,describes a curve
dorsally and rejoins right long. groove
enclosing insula ruminis.
 left accessory groove: is small extends
dorsally from left long. groove .

6.  Cranial and caudal transverse grooves which
unite with right & left long. grooves.
 Right & left dorsal and ventral coronary
grooves: extend from caudal end of right
and left longitudanal grooves and demarcate
the caudoventral & caudodorsal blind sacs.
Ventral coronary grooves reach ventral
curvature of rumen, while dorsal grooves
don't reach its dorsal curvature.
 Ruminoreticular groove: separates rumen
(ruminal atrium) from reticulum ventrally.

7.  Parts of rumen :
1-Dorsal and ventral ruminal sacs: result
from union of longitudinal and
transverse grooves.
2-Ruminal atrium and ruminal recess:
represent cranial parts of the dorsal
and ventral ruminal sacs respectively
3-Caudodorsal & caudoventral blind
sacs: represent caudal parts of the
dorsal & ventral ruminal sacs
respectively.
= Esophagus enters stomach at cardia
which lies at the junction of rumen and
reticulum.

8. Interior of rumen:
 Ruminal pillars:
-With exception of
ruminoreticular groove, all
grooves are represented by
muscular pillars which
project into the cavity of
rumen.
-Cranial pillar projects
between ruminal atrium and
recess, but caudal pillar
projects between the two
blind sacs.

9. Ruminal papillae:
-The mucosa forms large conical or tongue shaped ruminal papillae.
They are well developed in ventral sac, blind sacs, and in ruminal
atrium, but decrease in size toward pillars on which they are absent.
Most roof lacks papillae.
-The papillae increase surface area of mucosa through which fatty acids
and sodium are absorbed.

10. The reticulum
Position:
It is the most cranial chamber,
spherical in shape. It lies in
intrathoracic part of abdominal
cavity between diaphragm and
rumen.
Dorsally, it is continued with
ruminal atrium. But ventrally it is
separated from rumen by
ruminoreticular groove.
It is involved in traumatic reticulitis
which may lead to
reticulopericarditis.

11. • Anatomical description:
 It has two surfaces;
• A- diaphragmatic S.: convex, related to
diaphragm B- visceral S.: related to rumen.
 It is related:
-on right side: to liver, omasum and
abomasum.
-on left side: to costal part of diaphragm.
-ventrally: to sternal part of diaphragm
and xiphoid
cartilage.
-cranially: to diaphragm.
-caudally: to rumen.

12. Interior of reticulum:-
- It has well developed muscular
wall.
- The mucosa forms permanent
crests of about 1 cm high,
which intersect to form
honeycomb like cells. Each cell
is subdivided by lower
secondary crests.The crests
and the cells are studded with
small papillae.

13. The omasum
Position:
- It lies ventrally in intrathoracic part of
abdominal cavity to the right of median
plane, between rumen and right abdominal
wall.
Anatomical description:
- It is spherical in shape and larger than
reticulum in ox.
- It has two curvatures; two surfaces and short
neck.
• Greater curvature: faces dorsocaudally and to
right,
• lesser curvature: faces in opposite direction.
• Visceral (left) surface: related to rumen.

14. • Parietal (right) surface: related to diaphragm and liver, below the latter a
small area lies against right abdominal wall.
• Ventrally: it related to reticulum and abomasum.
• Caudally: to the jejunum.
• Neck: is very short narrow part connects omasum and reticulum.
- Omasum of small rum. is oval and smaller than reticulum. It does not in
contact with right abdominal wall.

15. Interior of omasum:
- It communicates with reticulum through
reticuloomasal opening and with abomasum
through omasoabomasal opening.
- The inner wall forms different sized parallel
folds termed omasal laminae.Their surfaces
covered with horny papillae.They are
separated by interlaminar recesses.The
laminae are arranged as; 1,4.3,4,2,4.3,4,1 etc.
(1= highest; 4= lowest fold).

18. The abomasum
Position:
- The abomasum or true stomach is the most distal compartment in
form of an elongated sac. It lies chiefly on abdominal floor.The
fundus lies on abdominal floor in left side, the pyloric part on right
side and the body crosses median plane from left to right.
- The abomasum is closely related to the ruminal recess.

19. Anatomical description:
- The abomasum like simple stomach is divided
into fundus, body and pyloric parts.
- The abomasum has:
= two curvatures: greater faces ventrally and
attaches with greater omentum; and lesser faces
dorsally and attaches with lesser omentum.
= two surfaces: parietal ( related mainly to
abdominal floor) and visceral ( related to rumen
and omasum).

20. Interior of abomasum:
- It has glandular mucosa which presents
cardiac, fundic, and pyloric regions.
1-The cardiac region: is narrow area
surrounding omasoabomasal opening.
2-The fundic region: includes most of
fundus and body, it contains permanent
spiral folds which absent at lesser
curvature
3-The pyloric region: includes pyloric
part.
- The pyloric sphincter is not well
developed and presents large torus
pyloricus which is round protuberance
inside lesser curvature.

21. The gastric groove
- The gastric groove of ruminant stomach is well developed and of
considerable physiological importance.
- It extends from cardia through reticulum, omasum, and abomasum to
pylorus. it is divided into three segments; reticular, omasal, and abomasal
grooves.
Function:-
- Through gastric groove swallowed milk in suckling animals is conducted
through reticular and omasal grooves directly into abomasum.

22. Parts of gastric groove:
1-Reticular groove: extends from
cardia to reticuloomasal opening.
It is formed of two muscular spiral
ridges.
2-Omasal groove: extends from
reticuloomasal opening to
omasoabomasal opening. It is
boumded by two mucosal ridges.
3-Abomasal groove: extends from
omasoabomasal opening to
pylorus. It is represented by band
like area extends along lesser
curvature of abomasum which is
free from spiral folds.

23. References: -
- Nickel , Schummer (1995): The Viscera of Domestic Mammals (The Anatomy of the Domestic Animals) .
- Dyce, Sack, andWensing's (2015):Textbook of Veterinary Anatomy
- Klaus Dieter Budras (2011): Bovine Anatomy.
- https://www.google.com.eg/imghp?hl=ar&tab=wi&ogbl