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THE CANINE GASTROINTESTINAL TRACT:
Embryology and Anatomy
with certain aspects of physiology and
clinical application
At an early stage of development, the part of the gut (enteron) that becomes the
stomach and intestine passes through the body as a simple tube. The diaphragm
is just beginning to develop. You can easily identify the part of the gut that is
going to become the stomach because it is increased in diameter relative to the
intestine and is just caudal to the developing diaphragm. In the figure below, a
first indication of the developing diaphragm is labeled pleuroperitoneal fold. The
dilatation of the stomach is unmarked but can be seen as an expanded part of
the gut just caudal to the pleuroperitoneal fold.
Fig. 1
The relationship of stomach and intestine to the lining membrane of the body
cavity, the membrane that will become the peritoneum, is shown in Figure 2:
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Fig. 2
The peritoneum that extends from the body wall to the stomach and intestine is a
double layer. Between the two layers of peritoneum is loose connective tissue in
association with vessels and nerves that supply the gut wall and the ducts that
pass from its accessory glands, the liver and pancreas. The part of this
peritoneum that extends from the abdominal surface of the diaphragm and dorsal
body wall to the stomach is designated dorsal mesogastrium; it carries vessels
and nerves to the stomach. The part that extends from the dorsal body wall to the
intestine (enteron) is the mesentery. The mesentery conveys vessels, nerves,
and the bile and pancreatic ducts to the intestine.
The stomach is not perfectly cylindrical but is spindle-shaped and curved on its
long axis so that it has a greater curvature dorsally and a lesser curvature
ventrally. The dorsal mesogastrium passes from the diaphragm and dorsal body
wall to the greater curvature of the stomach. The ventral mesogastrium passes
from the lesser curvature of the stomach and neighboring part of the intestine to
the ventral diaphragm and ventral body wall. Its attachment to the ventral body
wall extends caudally to the level of the umbilicus. This part of the peritoneum is
designated the ventral mesogastrium. The hepato-pancreatic bud, which will form
the liver and part of the pancreas, proceeds as a tubular evagination from the
neighboring intestine into the ventral mesogastrium (Fig. 3).
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Fig. 3
The hepato-pancreatic bud, enters the ventral mesogastrium and begins to
branch extensively, forming the liver. An extension from the first part of the bud
branches to form the ventral pancreas. Ultimately the liver becomes so large as
to completely separate the ventral mesogastrium into a part passing from the
lesser curvature of the stomach to the liver---this becomes the lesser omentum--;
and a part passing from the liver to the diaphragm and ventral body wall. This
part of the ventral mesogastrium becomes the falciform ligament. The falciform
ligament conveys the umbilical vein from the umbilicus to the liver. In the dog,
both before and after birth, parts of the falciform ligament are lost. A small,
remnant, fold can often be detected ventral to the caudal vena cava as it leaves
the liver to penetrate the diaphragm. A larger part of the falciform ligament
remains at the level of the umbilicus. It becomes fat-filled and is encountered in
every mid-ventral abdominal incision (e.g., spay) that takes place close to the
umbilibus. In the dog and cat, the umbilical vein is entirely involuted (lost) after
birth. In the horse, the umbilical vein remains and is the round ligament of the
liver. It is patent (carries blood) in about 80% of horses throughout life. In the
remaining 20%, it remains only as a fibrous cord.
With growth of the stomach, its greater curvature, which is at first dorsal, turns to
the left, and its lesser curvature then faces right. Coincident with this turning of
the stomach, the intestine continues to grow in length. Growth of the intestine
caudal to the stomach fills the abdomen and is probably responsible for pushing
the stomach forward against the developing liver and diaphragm. The stomach
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then lies in an oblique plane (caudodorsal to cranioventral) against the abdominal
face of the diaphragm and the developing liver.
Fig. 4
With growth and turning of the stomach, the dorsal mesogastrium enlarges
greatly. It forms a large saclike outpouching to the left, the greater omentum. On
the right side, the opening into the omental sac is the omental (L., omentum) or
epiploic (Gr., epiploon) foramen. With continued growth, the liver pushes up on
the right side and the omental foramen becomes fairly small. On the left side,
caudal to the greater curvature of the stomach, the spleen develops in the
mesenchymatous connective tissue between the two peritoneal layers of the
greater omentum. The part of the greater omentum between the spleen and the
greater curvature of the stomach is designated the gastrosplenic ligament. See
Figure 5, below, which diagrammatically represents the development of the
greater omentum and the rotation of the gut around the axis of the cranial
mesenteric artery. Excepting only the inset, which shows how it develops within
the greater omentum, the spleen is not shown in these diagrams;
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Fig. 5. The upper 3 figures show the development of the gut, its rotation, and the
development of the greater omentum from the left side; the lower 3 figures show
this from the right side. In addition there is an inset showing how the spleen
develops within the greater omentum. This occurs in the greater omentum a little
caudal to the omentum’s attachment to the greater curvature of the stomach.
Within the abdomen, the greater omentum exists as a flattened sac. The
superficial wall of the sac lies upon the ventral body wall. The deep wall of the
sac is collapsed, resting upon the superficial wall ventrally. Dorsally, it is in
relation to the coils of the small intestine, chiefly, the jejunum.
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Fig. 6
The pancreas develops from two sources: (1) as the ventral pancreatic bud from
the proximal part of the hepato-pancreatic bud; and (2), as a separate tubular
outpouching, the dorsal pancreatic bud, branching from the duodenum caudal to
the origin of the hepato-pancreatic bud. The two parts of the pancreas join
together and their branching duct systems unite and communicate. The entire
pancreas is within the mesoduodenum and the contiguous part of the greater
omentum. The part within the mesoduodenum is the right lobe. The left lobe
extends leftward in the dorsal part of the greater omentum, caudal to the
stomach. It reaches the dorsal end of the spleen. The body of the pancreas is the
part at the angle of junction of right and left lobes. The portal vein (see below)
always passes in relation to the body of pancreas.
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Fig. 7. Cranial view of the stomach, showing the continuity of mesoduodenum
and greater omentum, which occurs at the omental foramen. The omental
foramen (upper large arrow) is relatively large here; with growth of the liver, it is
much smaller. See the R and L lobes and body of the pancreas. See the portal
vein. Note that the hepatic branches pass to the porta of the liver with the portal
vein. They are given off from the hepatic artery as it passes ventral to the portal
vein.
The intestine first appears as a simple tube extending from the pylorus of the
stomach to the caudal part of the embryo where the anus will form. With this
simple tube, the large intestine is distinguished from the small intestine after the
cecum begins to develop. The cecum forms as a blind diverticulum of the first
part of the ascending colon of the large intestine. When the cecum develops, the
part pyloricward is the small intestine; the part analward is large intestine, which
ends at the anus. Development of the cecum is indicated in the figures on page
5.
This string is around the cecum.
This string is around the ascending colon.
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Fig. 8. The left figure shows the origin of the cecum from the ascending colon. The
right figure shows the protrusion of the ileum into the ascending colon, forming the
ileal papilla.
The mesentery suspending the intestinal tube is short cranially, where it
suspends the duodenum, and short caudally where it extends to the colon. The
mesentery between the two short-mesentery parts is much longer. Here the
intestine forms the large loop that becomes jejunum and ileum. When intestine is
suspended by a short mesentery, it is too closely attached to the body wall to do
any significant amount of coiling or twisting. Thus duodenum and colon are
seldom involved in a twisting of the mesentery with a shutting off of blood supply
and “strangulation.” The long mesentery of jejunum-ileum does permit extensive
coiling, and this part of the gut can more easily twist and “strangulate.” Also, the
long mesentery permits jejunum and ileum to reach the ventral body wall. The
jejunum especially is the part of the gut most frequently involved in an umbilical
or scrotal hernia.
Fig. 9. The long loop of jejunum and
ileum is at the level of the developing
cranial mesenteric artery. With growth
in length and its confinement within a
limited space, the body cavity, the
intestine begins to rotate around the
axis of the cranial mesenteric artery.
(see figures on page 5)
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The direction of rotation is normally clockwise when viewed dorsally. In rare
cases, the rotation can be counter-clockwise. The rotation is gradual and the
developing cranial mesenteric artery is not twisted in the rotation-process.
Ultimately, the rotation is about 360 degrees (some texts say 270 degrees). At
the origin of the cranial mesenteric artery, fusion of the twisting mesentery
Fig. 10. Left view. Normal topography
of the gut near the root of the
mesentery.
Note that, in Fig. 10, from the caudal flexure of the duodenum, the ascending
duodenum passes forward and dorsally. Its mesoduodenum fuses with the
mesocolon of the desending colon, forming the duodenocolic fold. From the
cranial end of duodenocolic fusion, the duodenojejunal flexure is formed. The
ascending duodenum, passes to the left of the cranial mesenteric artery (Fig. 5
shows this). Note also that the transverse colon will lie immediately caudal to the
deep leaf of the greater omentum (which bears the left lobe of the pancreas).
There is here a fusion of a thin slip (velum omentale) of the deep leaf with the
transverse mesocolon.
After rotation of the intestine, the short-mesentery duodenum and colon are each
in the shape of a J. The long arm of the J of the duodenum starts on the right
side, at the pylorus of the stomach. The long arm of the J of the colon is on the
left side and extends cranially from the anus. The long loop of jejunum and ileum
extends between the two short-mesentery parts of the intestine.
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Fig. 11
The J-shaped duodenum (Figs. 11, 12) exhibits a short cranial part, a
descending duodenum, a caudal flexure (and some add to this a “transverse
part”), and an ascending duodenum. Parts of the intestine that convey the
ingesta caudally are described as “descending”; parts that convey the ingesta
cranially are described as “ascending.” “Transverse” parts extend from one side
of the body to the other.
Fig. 12
The descending duodenum is the right-most part of the gut. It extends caudally
from the cranial part to the caudal flexure. The ascending duodenum passes
from the caudal flexure craniodorsally to the left of the cranial mesenteric artery.
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Its suspending mesentery, which is designated mesoduodenum, fuses with the
mesocolon of the descending colon, the area of fusion being designated the
duodenocolic fold. At the cranial end of the duodenocolic fold, the duodenum
passes over into the jejunum at the duodenjejunal flexure. The flexure is easily
recognized because it is just cranial to the fused area of the duodenocolic fold;
the beginning longer mesentery of the jejunum (mesojejunum) appears at the
bend.
From the duodenojejunal flexure, the jejunum and ileum (the two parts of the
intestine are often abbreviated as jejunoileum) pass to the ileum’s joining the
ascending colon. The jejunum is much the longer part. It is thrown up into
numerous coils that for the most part are ventral, separated from the ventral body
wall only by the thin, collapsed sac of the greater omentum. The ileum is marked
by the ileocecal fold, which extends from ileum to cecum. The ileum is also
somewhat more heavily muscled than the duodenum and jejunum an adaptation
to its propelling the ingesta through the ileal sphincter into the ascending colon.
The J-shaped, short-mesentery colon is distinguished as ascending colon,
transverse colon, and descending colon. The descending colon is continuous
caudally with the rectum that joins the anus. The ileum joins the short arm of the
J, which is the ascending colon. The ascending colon passes cranially from the
ileocolic junction to the transverse colon. Its short mesocolon is fused in part with
the mesoduodenum of descending duodenum. The mesocolon of the ascending
colon is shorter than the mesoduodenum of the descending duodenum that
covers it. Therefore, to see the ileocolic junction, the ascending colon, and the
cecum, you have to lift up the mesoduodenum of the descending duodenum. The
transverse colon is also short, extending from right to left cranial to the cranial
mesenteric artery. On the left, it is continued as the long arm of the J, which is
the descending colon and rectum. Within the abdomen, the descending colon is
the leftmost part of the intestine. The mesoduodenum that suspends the
ascending duodenum is shorter than the mesocolon of the descending colon,
and fused with it to form the duodenocolic fold. Therefore to see the ascending
duodenum and its attachment to the mesocolon of descending colon, you have to
lift up the descending colon.
Fig. 13
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Fig. 13. The
path of the gut
(diagrammatic)
Is all of this an esoteric academic discussion of little utility in the real world of
veterinary medicine? Read these paragraphs and then decide. Get yourself in
the habit of knowing this relationship that results from (1) rotation of the intestine
around the cranial mesenteric artery and (2) fusion of the mesenteries to form the
root of the mesentery and the duodenocolic fold. The relationship to understand:
If, following your ventral median abdominal incision, you want to find an
obstruction in the descending colon, you run your hand down the left abdominal
wall to the place where it comes to a stop: that place is where the mesentery is
reflected from the dorsal body wall. You bring your hand up with the first part of
the intestine that you encounter: descending colon! Follow it cranially if you want
to look at transverse colon. If you suspect (following your appreciation of the
signs of illness and the radiography of the animal) that the problem is in the
ascending duodenum, you simply look medial to the mesocolon of descending
colon. The ascending duodenum will always be there, fused to the mesocolon of
the descending colon.
If you want to find the descending duodenum……or the ascending colon and
cecum….. or the ileum, try this:
On the left side: descending colon is most superficial, next to the body wall,
and, deep to it, ascending duodenum.
On the right side, descending duodenum is most superficial, next to the body
wall, and deep to the mesoduodenum are ileum, ascending colon, and cecum.
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Is there a difference between obstruction of the small intestine vs. obstruction of
the large? Obstruction of the descending colon and rectum may not be detected
for weeks; unless the owner checks the animal’s passage of stool or, being
observant, notes the enlarging abdomen. Signs of obstruction of the small
intestine are fairly immediate (unwillingness to eat and vomiting within 24 hours,
usually a much shorter time) and much more critical.
Essential abdominal topography:
1. Stomach, a J-shaped sac lying chiefly to the left side of the body immediately
caudal to the diaphragm and liver.
Cardia: where the esophagus joins the stomach; cardiac ostium, the
opening between esophagus and stomach;
Fundus: the blind part of the stomach dorsal to the cardia;
Body: from fundus to the pyloric part;
Pyloric part: is made up of the pyloric antrum, pyloric canal, and pylorus.
The pyloric part is the narrow, tubular, right part of the stomach. Its wider part
nearest the body is the pyloric antrum; its more narrow part that joins the pylorus
is the pyloric canal.
Pylorus: The constricted part of the stomach that joins the cranial part of
the duodenum; pyloric sphincter, the strong smooth muscle pyloric sphincter at
the pylorus; pyloric ostium, the opening in the pylorus by which the ingesta enters
the duodenum.
Greater curvature, lesser curvature. These curvatures extend from the
cardia to the pylorus on opposing margins of the stomach. The greater omentum
attaches to the greater curvature; the lesser omentum, to the lesser curvature
and it extends also onto the cranial part of the duodenum.
2. The small intestine consists of duodenum, jejunum, and ileum. Its diameter is
smaller than the diameter of the large intestine.
Duodenum: it is comprised of cranial part, descending duodenum, caudal
flexure, and ascending duodenum. The ascending duodenum is continuous with
the jejunum at the duodeojejunal flexure.
Cranial part: the short segment between the pylorus of the stomach
and the cranial flexure.
Cranial flexure: the bend between the cranial part and the
descending duodenum;
Descending duodenum;
Major duodenal papilla. A mucosal papilla near the cranial
end of the descending duodenum. The common bile duct (ductus choledochus)
and pancreatic duct open here;
Minor duodenal papilla. It is 3 – 6 cm caudal to the major
duodenal papilla in the descending duodenum; the accessory pancreatic duct
opens here;
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Caudal flexure;
Ascending duodenum;
Jejunum has the longest mesentery and can be located in most parts of
the abdomen. It is the part of the gut most likely to be found in a scrotal or
umbilical hernia.
Ileum joins the ascending colon. The ileum is a part of the large loop of
jejunoileum. It can be found in a scrotal or umbilical hernia but, being shorter
than the jejunum, is less likely to be involved in a herniation. The ileum is joined
to the cecum by the ileocecal fold. If you incise the wall of the ascending colon
opposite to where the ileum joins it, the opening of the ileum, the ileal ostium, will
be seen to be on a raised area of the mucosa designated the ileal papilla.
3. The large intestine is of greater diameter than the small intestine. It consists of
cecum, colon, rectum, and anus.
The cecum empties into the ascending colon at the cecocolic ostium. In
the dog there is a fairly distinct sphincter around the ostium.
The colon is divided into ascending colon, transverse colon, and
descending colon. The rectum is the straight part of the large intestine that is
within the pelvis; it joins the anus. The anus is the part of the large intestine that
communicates with the exterior.
Note: The curvature of the colon permits a right colic flexure
(between ascending colon and transverse colon) and a left colic flexure (between
transverse colon and descending colon) to be distinguished. But the intestine is a
flexible, plastic tube and the position of these flexures (and therefore of the
boundaries of the three parts of the colon) can vary slightly.
Essential gastrointestinal topography:
1. Liver is caudal to the diaphragm; stomach is caudal to the liver.
2. On the right side, the mesoduodenum with descending duodenum is
most lateral, next to the right abdominal wall; medial to the mesoduodenum of
the descending duodenum are the cecum, terminal ileum, and ascending colon.
3. On the left side, mesocolon with descending colon is most lateral, next
to the left abdominal wall; medial to the mesocolon and fused with its medial side
is the ascending duodenum.
4. Coils of jejunum (and a little of the ileum) rest upon the collapsed
omental sac upon the ventral body wall (see the figure at the top of page 6).
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5. The sac formed by the greater omentum is attached cranially to the
greater curvature of the stomach and the dorsal body wall. Therefore, when
meeting the omentum on a ventral abdominal incision, the omentum is drawn
forward, toward its gastric attachment. If you pull it caudally, you will tear it from
its gastric attachment.
6. In the usual ventral median abdominal incision, you will encounter the
fat-filled remnant of the falciform ligament. You simply incise to one side of its
attachment to the ventral body wall in order to enter the peritoneal cavity.
Structure of the Peritoneum: The peritoneum is a membrane with an epithelial
part, the mesothelium, and a connective tissue part, the lamina propria. The
lamina propria attaches the peritoneum to the structures that it clothes. The
peritoneum that lines the walls of the abdomen and cranial pelvis is parietal
peritoneum. The diaphragm is the cranial wall of the abdomen and its caudal,
abdominal, surface is covered with parietal peritoneum.
Nomenclature of the Peritoneum: The peritoneum covering the
stomach, intestine, liver, spleen, ovary, uterus, etc., is visceral peritoneum. The
peritoneum extending between parietal peritoneum of the body wall and the
visceral peritoneum covering the abdominal and pelvic organs is always given a
particular name: greater omentum, mesentery (mesoduodenum, mesojejunum,
etc.), mesovarium, mesosalpynx, mesometrium, right triangular ligament of the
liver, etc. The peritoneum extending between parietal and visceral peritoneum is
sometimes referred to in general, lay terms as “connecting peritoneum.”
Function of the Peritoneum: The peritoneum, moistened with the
serous fluid that its mesothelium secretes, provides a smooth, relatively
frictionless surface for movement of the abdominal and pelvic viscera. The
serous fluid serves as a lubricant. Every time the diaphragm contracts, there is a
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slight movement of the viscera. Peristaltic movements of stomach and intestine
are falcilitated by the lubricating effect of the serous fluid. Ordinarily, the serous
fluid is present as a thin film. With inflammation of the peritoneum it is produced
in much greater quantity and contains inflammatory cells and a protein-containing
exudate.
Liver. The liver is the largest gland in the body.
Lobes of the liver: quadrate, caudate, left and right. The caudate lobe
consists of a caudate process and a papillary process. Left and right lobes
are each subdivided by fissures into left medial and left lateral lobes; right
medial and right lateral lobes.
Quadrate lobe: The quadrate lobe is between the fossa for the gall
bladder and the fissure for the round ligament of the liver. The fossa for the gall
bladder is the depression that seats the gall bladder. The fissure for the round
ligament is the first deep fissure to the left of the gall bladder.
Right lobe: to the right of the quadrate lobe and subdivided into
right medial and right lateral lobes.
Left lobe: to the left of the quadrate lobe and subdivided into left
medial and left lateral lobes.
Caudate lobe: dorsal on the caudal surface of the liver. It has
prominent caudate and papillary processes.
Fig. 14
Left figure is from Budras, Anatomy of
the Dog, 2007; Schlütersche
Verlagsgesellschaft, Hanover; right figure
is from Sisson.
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The porta and the portal circulation: The porta (also designated hilus) is the
“door” to the liver. It is central on the visceral face of the live and is the place
where the portal vein, hepatic branches of the hepatic artery, nerves and
lymphatics enter the liver and hepatic ducts conveying bile emerge from the liver.
The portal vein conveys venous blood from the stomach, intestine,
spleen and pancreas to the liver. The hepatic branches of the hepatic artery
convey oxygenated, arterial, blood to the liver. Branches of the portal vein and
hepatic artery empty into the same vessels, the fenestrated capillaries of the
liver, which are also called the hepatic sinusoids. Traversing the sinusoids, the
blood is acted upon by the liver cells and intravascular macrophages. Glucose,
albumin, and other constituents are added or removed to restore their
concentration in the blood to normal levels. Toxins are detoxified by the
hepatocytes. Bacteria are removed by the macrophages.
What about venous blood from the spleen? A function of the spleen
is the removal of “worn-out” red blood cells. The metabolic end-product of
hemoglobin metabolism is hemosiderin, which is transported from the spleen to
the liver where it is utilized in the production of bile acids that are excreted as the
main constituent of bile and serve to emulsify ingested fat.
What about venous blood from the pancreas? The pancreatic islets
secrete glucagon and insulin, hormones that determine blood sugar levels in
large part (but not entirely) by their action on the hepatocytes.
From the sinusoidal capillaries of the liver, the blood is collected
into venous channels that unite to form hepatic veins that empty into the caudal
vena cava as it passes medial to, and partly embedded within, the right lobe of
the liver.
A portal venous circulation is one in
which the blood passes through two sets
of capillaries instead of the usual single
capillary bed between artery and vein.
Thus blood that has traversed a first
capillary bed in stomach, intestine,
spleen, and pancreas then passes in the
portal vein to a second capillary bed, the
hepatic sinusoids. From the sinusoids
this blood is returned to the heart in the
caudal vena cava. Functionally, the
portal venous system restores the blood
draining the digestive system and spleen
to normal, homeostatic, levels before its
being distributed to the rest of the body
in the general circulation. The blood
delivered to the liver by hepatic17
branches
of the hepatic artery has, of course,
traversed a single set of capillaries, the
Fig. 15
Caudal vena cava. The caudal vena cava is dorsal in the abdomen, to the right
of the abdominal aorta. At the level of the first lumbar vertebra, it passes
cranioventrally on the medial side of the right lobe of the liver in which it is partly
embedded. At the level of the foramen venae cavae of the diaphragm, it passes
from the liver through the diaphragm and is continued within the thorax in the
dorsal margin of the plica venae cavae. It empties into the right atrium. As it
passes in relation to the liver, the hepatic veins empty into the caudal vena cava.
The hepatic veins are of variable size, the largest emptying into the caudal vena
cava as it departs the liver to pass in the foramen venae cavae.
The bile conducting system. Bile is produced by the hepatocytes and
conveyed within bile canaliculi to bile ducts that are within the liver. The bile
ducts of each liver lobe are collected to form a lobar hepatic duct. In the dog, the
lobar hepatic ducts discharge individually into the cystic duct, the large duct
leading from the gall bladder. After all lobar hepatic ducts have joined the cystic
duct, the cystic duct is continued by the common bile duct (ductus choledochus)
to the duodenum. The common bile duct opens on the major duodenal papilla.
The small pancreatic duct discharges with the common bile duct at the major
duodenal papilla.
Peritoneum and ligaments of the liver. The liver is interposed between
the abdominal face of the diaphragm and the stomach. Its diaphragmatic face is
apposed to the diaphragm and is in part adherent to the diaphragm. Parietal
peritoneum covering the abdominal surface of the diaphragm passes onto the
surface of the liver at the margin of this area of adhesion. Upon the liver, the
peritoneum is visceral peritoneum. The thin line of junction of the parietal
peritoneum with the visceral peritoneum, where it is at the margin of the area of
adhesion, is the coronary ligament of the liver (coronary = like a crown, signifying
that the line of reflection encircles the area of adhesion ”like a crown”). From the
coronary ligament, the right and left triangular ligaments are duplicatures of
peritoneum that extend to either side along the dorsal border of the
corresponding lobe of the liver. A part of the falciform ligament is evident usually
only as a short fold extending ventrally from the liver to the diaphragm close to
the place where the caudal vena cava departs the liver to pass through the
diaphragm.
Pancreas. The pancreas consists of right lobe, body, and left lobe. The right lobe
is in the mesoduodenum of the descending duodenum. From the cranial end of
the right lobe, the left lobe extends leftward in the dorsal part of the deep wall of
the greater omentum (the deep wall is the wall of the omental bursa that is
apposed to the viscera; the superficial wall is the wall of the omental bursa in
contact with the floor of the abdominal cavity). The left lobe lies within the deep
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wall just cranial to the transverse colon; its left tip is in relation to the dorsal
extremity of the spleen. The body of the pancreas is at the angle of junction of
right and left lobes and is in relation to the portal vein. It is just proximal to the
portal vein’s passing to the porta of the liver. The pancreatic duct opens with the
common bile duct at the major duodenal papilla. The accessory pancreatic duct
opens alone at the minor duodenal papilla. The pancreas is shown in the figure
below.
Fig. 17. The celiac artery
and its branches….
Arteries supplying the gastrointestinal tract and spleen. There are only three
ventral arteries of the abdominal aorta and all are involved in the blood supply to
the gastrointestinal tract. The arteries are: celiac, cranial mesenteric, and caudal
mesenteric.
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Celiac artery. It arises ventral to the first lumbar vertebra. It is about 2 cm
long and ends in three main branches: splenic, left gastric, and hepatic. Splenic
and left gastric often come off by a short common trunk.
The splenic artery is large. It passes left in the dorsal part of the
greater omentum, dividing into two branches that are united by an arciform
vessel that passes along the hilus of the spleen. The more ventral of the two
branches, or the arcade, gives off the left gastroepiploic artery that runs in the
greater omentum along the left ventral part of the greater curvature of the
stomach. This vessel anastomoses with the right gastroepiploic artery. Short
gastric branches of the splenic artery pass to the greater curvature of the
stomach and pancreatic branches supply the left lobe of the pancreas.
The left gastric artery passes cranioventrally, then to the right along
the lesser curvature of the stomach. Its branches supply the esophagus and
lesser curvature of the stomach. It anastomoses along the lesser curvature with
the right gastric artery.
The hepatic artery inclines right-cranioventrally in the dorsal part of
the deep wall of the greater omentum. It crosses the cranial surface of the left
lobe of the pancreas, reaches the portal vein as the portal vein passes through
the body of the pancreas, and continues to the right, passing ventral to the portal
vein. As it passes ventral to the portal vein, it gives off hepatic branches to the
liver---these pass to the porta of the liver with the portal vein. With the portal vein,
the hepatic branches lie within the lesser omentum. The hepatic artery ends by
dividing into the small right gastric artery and the large gastroduodenal artery.
The right gastric artery passes right along the lesser curvature of the stomach
and unites with the left gastric. The gastroduodenal artery gives off the cranial
pancreaticododenal artery onto the mesenteric border of the descending
duodenum and the right gastroepiploic artery, which passes caudal to the
duodenum at the cranial flexure. The right gastroepiploic artery passes left within
the greater omentum along the omental attachment to the greater curvature of
the stomach. It anastomoses with the left gastroepiploic artery of the splenic.
Cranial mesenteric artery. It arises from the aorta about 1 cm caudal to
the origin of the celiac. It passes ventrally with the mesentery, its proximal
branches passing to opposite ends of the gut: caudal pancreaticoduodenal artery
to the duodenum; ileocolic artery to ileum, cecum and colon. The continuing
cranial mesenteric artery passes within the mesojejunum, giving off jejunal
arteries to either side and one or two to the ileum.
The caudal pancreaticoduodenal artery passes in the
mesoduodenum to the ascending duodenum and caudal flexure, and
anastomoses with the cranial pancreaticoduodenal.
The ileocolic artery passes in the mesocolon to the transverse and
ascending colon, to the cecum and to the ileum.
The first branch of the ileocolic is almost always the middle
colic artery, which supplies the transverse colon and forms anastomotic arcades
with the right colic artery that next proceeds from the ileocolic. A left branch of
the middle colic passes along the mesenteric border of the descending colon. It
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anastomoses with the left colic artery, which is a branch of the caudal
mesenteric. The middle colic sometimes arises directly from the cranial
mesenteric. Note: In the writer’s opinion, it is clearest, and correct, to define the
middle colic artery always as the branch of the ileocolic (or sometimes of the
cranial mesenteric artery directly) that anastomoses with the left colic artery
branch of the caudal mesenteric.
Fig. 18. Right view at root of the mesentery. Proximal branches of the cranial
mesenteric artery; portal vein entering the porta of the liver; the omental
(epiploic) foramen.
The second and third (sometimes fourth also) branches of
the ileocolic are the single right colic artery and one or two colic branches. These
vessels pass to the mesenteric border of the ascending colon.
The ileocolic ends by dividing into a small mesenteric ileal
artery, which passes along the mesenteric border of the terminal ileum, and a
large cecal artery, which crosses the medial (deep) side of the ileocolic junction.
The cecal artery enters the ileocecal fold and gives off branches to both the
cecum and the terminal ileum. After the last cecal branch is given off, the
continuation of the cecal artery as the antimesenteric ileal artery passes within
the “tail” of the ileocecal fold upon the antimesenteric border of the ileum.
Note: There is always a small unnamed branch of the cecal artery that
passes on the lateral (superficial) side of the ileocolic junction. This branch anastomoses with the
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cecal artery within the ileocecal fold. The result is a vascular ring that surrounds the ileocolic
junction.
Caudal mesenteric artery. This artery arises from the aorta about at the
level of the fifth lumbar vertebra. It inclines caudally within the mesocolon and
ends at the mesenteric border by dividing into a cranially directed left colic artery
and a caudally directed cranial rectal artery. Colic lymph nodes are at the place
of division.
The left colic artery passes cranially along the mesenteric border of
the descending colon and anastomoses with the middle colic artery of the
ileocolic (or of the cranial mesenteric).
The cranial rectal artery passes caudally on the rectum,
anastomosing with branches of the middle and caudal rectal arteries.
Fig. 19. Union of the cranial and caudal mesenteric veins to form the portal vein.
Tributaries of the portal vein; veins satellite to the branches of the celiac,
cranial mesenteric, and caudal mesenteric arteries.
With a few exceptions, veins are satellite to branches of the celiac, cranial
mesenteric, and caudal mesenteric arteries; but, as explained below, no vein
accompanies the main trunk of the celiac, cranial mesenteric, and caudal
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mesenteric arteries to the caudal vena cava. Instead the large venous trunks are
collected to drain into the portal vein.
The caudal mesenteric vein arises from cranial rectal veins and, as a
continuous trunk, passes cranially alongside the left colic artery, receiving left
colic veins that proceed from the descending colon. Near its union with the
cranial mesenteric vein, it receives the ileocolic vein.
The cranial mesenteric vein accompanies the distal (jejunal) trunk of the
cranial mesenteric artery. It collects jejunal and ileal veins, receives the caudal
pancreaticoduodenal vein and joins the caudal mesenteric vein near the root of
the mesentery to form the portal vein.
From the root of the mesentery, the portal vein arches cranially over the
dorsal aspect of the transverse colon and the body of the pancreas. On the left, it
receives the large splenic vein. Near the porta of the liver it receives the
gastroduodenal vein ventrally and, joined by hepatic branches of the hepatic
artery passes in the lesser omentum to the porta of the liver.
The splenic vein accompanies the splenic artery. It receives the left gastric
vein and passes to the portal vein within the dorsal part of the deep wall of the
greater omentum.
No vein is satellite to the main trunk of the hepatic artery; it is its hepatic
branches that accompany the portal vein to the liver. The gastroduodenal vein
receives cranial pancreaticoduodenal and right gastroepiploic veins. It joins the
portal vein about 1 - 2 cm before the portal vein enters the porta of the liver.
Main thing to bear in mind: The portal vein is formed at the union of the cranial
mesenteric and caudal mesenteric veins. It then passes dorsal to the transverse
colon and turns cranioventrally to reach the porta of the liver. In passing to the
porta, the portal vein with the hepatic artery and its hepatic branches lies in the
lesser omentum, which forms the ventral margin of the omental foramen. Thus,
when your finger is in the omental foramen, it is ventrally in contact with the
lesser omentum with the portal vein, the hepatic artery and its branches to the
liver.
Note: The omental foramen is small after the liver is fully developed. It lies
always ventral to the caudate lobe of the liver. The ventral boundary of the
foramen is formed by the lesser omentum as explained above. The dorsal
boundary of the foramen is the caudate lobe and the caudal vena cava, which
passes in relation to the medial side of the caudate lobe.
Nerves supplying the abdominal viscera.
A brief introduction to nervous system function: Receptors of the body are
sensitive to various specific stimuli (heat, cold, pH, electromagnetic radiation of
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the visible spectrum, etc.). Stimulation of these receptors results in the
production of nerve impulses that pass by way of sensory, afferent, neurons to
the central nervous system (CNS), the brain and spinal cord. Within the brain and
spinal cord, the conduction of these impulses is directed into pathways provided
by interneurons (neurons that begin and end within the CNS). The interneuronal
pathways modulate the excitation arriving by way of the afferent neurons in that,
within the CNS, interneurons act to inhibit or facilitate the ongoing passage of
excitation. From the CNS there is an outflow of impulses resulting in the
contraction of muscle and the secretion of glands. “Activities” that we recognize
as thinking, consciousness (awareness of the stimulus), dreaming, attention, etc.
take place within the CNS itself. They are part of the circuitry within the CNS and
influence the outflow of impulses that result in the contraction of muscle and the
secretion of glands. The outflow from the CNS is by way of efferent nerves that
stimulate muscle or cause glands to secrete. The body thus responds to stimuli
by contraction of muscle and secretion of glands.
The outflow from the CNS to striated muscle fibers is always by way of a single
neuron that has its dendrites and cell body within the CNS. The outflow to
cardiac muscle, smooth muscle, and glands is called the autonomic nervous
system. For the most part, it occurs by way of a chain of two neurons. The cell
body and dendrites of the first neuron of the chain are in the CNS; the cell body
and dendrites of the second neuron of the chain are in a peripheral ganglion:
cervicothoracic ganglion, middle cervical ganglion, etc. Some gangia are
microscopic and lie within the walls of the organ innervated.
Fig. 20. Diagrammatic
representation of the
nervous system.
CNS, brain and
spinal cord
INPUT: afferent
neurons from
receptors carrying
impulses to the CNS
OUTPUT: Efferent
neurons pass to
striated muscle or to
24 effectors
autonomic
(heart muscle,
smooth muscle,
Interneurons within the
CNS, some excitatory,
some inhibitory.
Interneuronal pathways
ultimately lead to
efferent neurons.
2 neuron
chain to
autonomic
effectors
single neuron
to striated
muscle fibers
The abdominal viscera, containing no striated muscle but having lots of smooth
muscle and glands are innervated by the autonomic nervous system. They also
receive sensory fibers which, though they travel with autonomic fibers, are not
regarded as part of the autonomic nervous system. The autonomic nervous
system is in two divisions, sympathetic and parasympathetic which often have
opposing effects on the cells innervated. For example, sympathetic fibers relax
smooth muscle of the intestinal wall; except for its sphincters, which they
stimulate. Parasympathetic fibers generally have the reverse effect.
At the origin of the celiac and cranial mesenteric arteries are the
celiacomesenteric ganglia and plexus. Autonomic nerves passing to the ganglia
and plexus are (1) the greater and lesser splanchnic nerves, which contain
sympathetic and sensory fibers; and (2) a branch from the dorsal vagal trunk,
which contains parasympathetic and sensory fibers. From the gangia and plexus,
perivascular nerve fibers extend in plexuses that pass with the branches of
these two major arteries. It is in that way that innervation reaches the smooth
muscle and glands of the abdominal viscera with sympathetic and
parasympathetic fibers. Sensory fibers are also in the perivascular plexus. They
supply the viscera with sensory fibers.
On the cranial face of the caudal mesenteric artery is the caudal mesenteric
ganglion. It receives lumbar splanchnic nerves, which contain sympathetic and
sensory fibers. The cells of the ganglion give rise to R and L hypogastric nerves
that pass to the pelvis in the mesorectum. Parasympathetic and sensory fibers
pass in ventral branches of the second and third sacral spinal nerves and unite to
form the pelvic nerve. In the dog, the pelvic nerve passes ventrally just a little
caudal to the prostatic/vaginal vessels. Within the pelvis, fibers in the hypogastric
nerve mix with fibers in the pelvic nerve, forming the pelvic plexus. This plexus
provides sympathetic, parasympathetic and sensory fibers to the pelvic viscera. It
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is generally described as extending cranially in the mesocolon to the descending
colon and transverse colon.
The anus. This important part of the alimentary canal is subject to direct visual
inspection and palpation.
External anal sphincter: this is the striated muscle that gives voluntary
control over the passage of stool. It is well developed. It is innervated by the
caudal rectal nerve, a branch of the pudendal nerve.
Internal anal sphincter: a smooth muscle sphincter that is not well
defined on gross inspection. It is deep to the external sphincter.
Paranal sinuses (“anal sacs”). These are R and L saclike pouches,
about one centimeter in diameter situated between the internal and external anal
sphincter.. Each sinus stores the secretion of its glandular walls and discharges
by a single duct that is ventrolateral to the mucocutaneous junction of the anal
opening. The paranal sinuses function as marking devices by which the animal
designates its “territory.” The sacs are subject to infection and impaction and are
sometimes surgically removed. In the cutaneous area immediately surrounding
the anal opening are circumanal glands that may become tumorous.
Fig. 21. This figure is from Ellenberger – Baum,
1943; Springer Verlag.
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