The Oesophagus
The oesophagus is a musculomembranous tube that extends from
the pharynx to the stomach. In terms of topography, it begins immediately
caudal to the pharynx. Initially it begins dorsal to the trachea, however, at
the 3rd and 4th cervical vertebra it moves laterally to
the left side where it remains until approximately the level of the 1st
thoracic vertebra.
It lies lateral to the trachea and medial to the left
carotid artery, vagus, sympathetic and recurrent laryngeal nerves. When it
reaches the thoracic inlet, it drops to be ventral to the trachea.
Here the trachea is on the medial side of the oesophagus.
Lateral to the oesophagus is the first rib, the roots of the brachial plexus,
and the left cervicothoracic ganglion. The oesophagus then rises sharply so
that at the tracheal bifurcation, it sits above the aortic arch. It continues
caudally within the mediastinum and runs ventral to the longus colli muscle.
Here it has the aorta on its left and the right azygous vein and right vagus
nerve on the right. It continues caudally and veers to the left to be
accompanied by the dorsal and ventral oesophageal artery.
The oesophagus then reaches the diaphragm and passes through
the oesophageal hiatus and ends at the cardia of the stomach. The left and
right dorsal vagal branches run along the dorsal surface of the oesophagus and
unite about 2 cm cranial to the diaphragm and pass through the oesophageal
hiatus. The left and right ventral vagal branches run along the ventral aspect
of the oesophagus and unite immediately behind the oesophageal hiatus and then
pass through.
Basic External Form
The stomach in the dog, pig and horse can be described as a
single chambered musculoglandular sac. It lies caudal to the diaphragm and is
covered by the liver. The oesophagus enters the stomach dorsally and on the
left at a region called the cardia. The exit of the stomach leads to the
duodenum and is called the pylorus, it is on the left. The pylorus is always
more ventral than the cardia. Above the cardia is a blind dome called the
fundus, the body extends from the cardia to the ventral border, while the
pylorus is the distal tubular portion.
The stomach has two main surfaces: the parietal, which is
adjacent to the liver and diaphragm; and the visceral, which is adjacent to the
intestines and makes contact with the pancreas. The stomach lies to the left of
the midline in a caudal concavity of the liver known as the gastric depression.
It also has a greater and lesser
curvature to which is attached the greater and lesser omenta, respectively.
The stomach is supplied with blood from the coeliac artery,
while it is drained by the portal veins. It’s lymph is drained to the gastric,
hepatic and splenic lymph nodes. It receives an autonomic nerve supply only and
this comes from the vagus and coeliac plexuses.
When the stomach is full, it makes contact with the ventral
wall of the abdomen. When it is empty it stays some distance from the ventral
wall.
Ligamentous Support and Connections
The stomach has several ligamentous structures which provide
support, hold the stomach in position, allow it to expand and contract, and
provide a framework for blood and lymph vessels. Some of these are named
ligaments but in reality are quite diffuse compared to the ligaments and
tendons found in the musculoskeletal system.
The gastrophrenic ligament extends from the greater
curvature on the left cardiac region to the diaphragm. The lesser omentum
connects the lesser curvature of the stomach and duodenum to the visceral
surface of the liver. The greater omentum arises from the greater curvature of
the stomach and proximal duodenum. It then runs caudally and reflects back on
itself to insert on the dorsal surface of the stomach. The gastrosplenic
ligament connects the left greater curvature of the stomach to the hilus of the
spleen.
Luminal Structures
The oesophageal mucosa, which doesn’t have any glands,
passes into the cardia. Immediatley surrounding the cardia is a collar of
cardiac glands which secrete mucins. Most of the stomach is made up of the fundic
gland zone. This area is a yellow-brown colour and secretes enzymes, HCl, and
pepsin. The pyloric gland zone includes the caudal third of the stomach and
secretes mucins.
Rumen - External Aspects
The stomach of sheep and cattle is known as a complex
stomach and contains four compartments: rumen, reticulum, omasum and abomasum.
The first three chambers are known as forestomachs and are non-glandular. The
fourth compartment is the true glandular stomach. The stomach complex fills
most of the left side as well as a large amount of the right side of the
abdominal cavity.
The rumen makes up about 80% of the volume of the stomach
complex in cattle and about 70% in sheep. It lies on the left, its cranial
portion is adjacent to the diaphragm and it displaces the liver to the right.
It extends caudally to a line drawn between the tuber coxae and the femur. Dorsally,
the rumen is in contact with the psoas major and minor, and quadratus lumborum
muscles, as well as the aorta and caudal vena cava. Its ventral aspect lies
above the rectus abdominus muscle and the reticulum cranially. Its caudal
aspect is in contact with the intestines.
The rumen has two surfaces: the parietal and visceral. The
parietal surface is on the left hand side and is adjacent to the diaphragm,
left abdominal wall and spleen. The visceral surface is on the right and is in
contact with the omasum, abomasum, intestine, liver, pancreas, kidney, aorta,
caudal vena cava and the left adrenal gland.
Grooves
The rumen has a cranial
groove which divides the cranial region of the rumen into an atrium (which
is continuous with the dorsal sac) and the cranial portion of the ventral sac.
The left longitudinal groove starts
at the cranial groove and ends at the caudal groove. It runs dorsally, gives
off an accessory groove and then runs ventrally. It separates the dorsal and
ventral sacs of the rumen. The right
longitudinal groove does the same except that it is on the right aspect of
the rumen. The dorsal and ventral
coronary grooves come from the left longitudinal groove at the caudal
region of the rumen and separate the dorsal and ventral blind sacs. The ruminoreticular groove is very distinct
ventrally and less so dorsally
Strong ridges, called pillars, are present within the rumen
and these correspond to the grooves on the external surface. Thus, cranial,
caudal and coronary pillars exist. The coronary pillars arise off the caudal
pillar (which is the most developed) and form the blind sacs.
At the cranial end of the rumen is the ruminoreticular
orifice which is bound by the ruminoreticular fold. On the external surface,
this appears as the ruminoreticular groove.
The reticulum is the most cranial and smallest compartment
of the complex stomach. It lies on the left below the rumen. It has a
diaphragmatic and visceral surface. The diaphragmatic surface is in contact
with the liver and diaphragm while the visceral surface is flattened against
the rumen. It communicates with the rumen through the ruminoreticular orifice.
The mucous membrane of the reticulum is folded into
hexagonal “cells”. Within the cells are subdivisions of papillae.
The reticular groove is also present. It runs from the
cardia (where the oesophagus enters) to the reticulo-omasal orifice. It is
formed by right and left muscular lips and the floor of the reticulum. In the
calf, the lips are able to close and this provides a passageway for food to
enter the omasum, thus bypassing the rumen and reticulum.
This is a chamber on the right hand side of the ruminal
complex. Cranially it lies mostly against the liver but also is in contact with
the diaphragm. Ventrally it may contact the abdominal wall, but not in sheep.
The rest of the omasum is in contact with the rumen, reticulum and abomasum. An
omaso-abomasal orifice connects the omasum and abomasum.
The omasal groove is a continuation of the reticular groove
and allows ingesta to pass directly from the rumen and reticulum to the
abomasum. Folds, which have large papillae, are on either side of the groove.
The omasum has lamina which are mucosal sheets. They have a
large surface area, are muscular and have papillae. Their function is unknown.
This is the true digestive compartment of the complex
stomach. It is an elongated sac which lies on the right on the ventral
abdominal wall. It is suspended by mesentery and may be moved around by other
organs. It has three regions which are similar to the simple stomach:
- Fundus: a cranial blind area adjacent to the reticulum, right atrium and ventral sac.
- Body: this is further caudal and lies between the omasum and ventral sac, mainly on the left.
- Pylorus: this is on the right, caudal to the omasum and communicates directly with the duodenum, which is oriented dorsally.
The abomasum has two external surfaces: parietal and
visceral. The parietal is in contact with the ventral abdominal wall. The visceral
is in contact with the rumen and omasum. Greater and lesser curvatures are also
present and these give rise to the greater and lesser omenta, respectively.
That should be what you need to know about the upper portion of the alimentary tract. Let me know if you have any questions :)
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