The general disposition of the peritoneum has been described, and other
facts in regard to it were mentioned in the description of the viscera.
It is now desirable to study it as a continuous whole.[103] (Figs. 256,
257, 278, 279.)
We may consider the peritoneum as consisting of two sacs—a greater and a
lesser. The =greater sac= lines the greater part of the abdominal
cavity, and covers most of the viscera which have a peritoneal
investment. The =lesser sac= is an introversion or recess of the greater
sac, formed during the development of the viscera. The two sacs
communicate by a relatively narrow passage, termed the =epiploic
foramen= of Winslow (Foramen epiploicum). This opening is situated on
the visceral surface of the liver above the portal fissure. It can be
entered by passing the finger along the caudate lobe of the liver toward
its root. Its dorsal (or anterior) wall is formed by the caudate lobe
and the posterior vena cava. Its ventral (or posterior) wall consists of
the pancreas, the gastro-pancreatic fold, and the portal vein. The walls
are normally in contact, and the passage merely a potential one. It is
usually about four inches (ca. 10 cm.) in length. It is narrowest at the
right extremity, where it is about an inch (ca. 2.5 to 3 cm.) wide.[104]
If the finger is passed into the foramen from right to left, it enters
the cavity of the lesser sac. If now an opening is made in the great
omentum and the other hand introduced through it, the fingers of the two
hands touch each other over the lesser curvature of the stomach. The
formation and boundaries of the lesser sac should now be examined by
spreading out the great omentum. It will be found that the latter now
incloses a considerable cavity behind the stomach; this is termed the
=omental cavity= (Bursa omentalis). Passing forward over the lesser
curvature of the stomach, we enter another space, the =vestibule= of the
omental cavity (Vestibulum bursæ omentalis). This space is closed on the
left by the gastro-phrenic ligament, below and on the right by the
lesser omentum, and dorsally by the gastro-pancreatic fold, which is
attached to the dorsal border of the liver and to the posterior vena
cava. Above the œsophageal notch the fingers can be passed around the
border of the liver and the vena cava till the coronary ligament is
encountered. Thus the vestibule is closed except (1) on the right, where
it communicates with the cavity of the greater sac by the epiploic
foramen; and (2) behind, where it communicates with the cavity of the
omentum.
[Illustration:
FIG. 278.—DIAGRAM OF GENERAL ARRANGEMENT OF PERITONEUM (OF MARE) IN
SAGITTAL TRACING.
_a_, Pouch between rectum and roof of pelvis, continuous with _b_,
recto-genital pouch; _c_, vesico-genital pouch; _d_, pouch below
bladder and its lateral ligaments; _f_, lesser omentum; _Int._,
small intestine. The arrow points to the epiploic foramen (of
Winslow).
]
The general arrangement of the great omentum has already been indicated.
We may now trace its line of attachment, which would correspond to the
mouth of the sac. Beginning at the ventral part of the great curvature
of the stomach, the line passes to the ventral face of the pylorus, then
crosses obliquely the first part of the duodenum to the point where the
pancreas is adherent to it. Here it passes to the anterior face of the
terminal transverse part of the great colon, runs along this
transversely (from right to left), and continues for some ten or twelve
inches (ca. 25 to 30 cm.) on the small colon. It then forms an acute
angle, passes inward and forward along the small colon to the dorsal
part of the hilus of the spleen, where it blends with the suspensory
ligament of the latter, and forms a recess (Recessus lienalis) behind
the saccus cæcus of the stomach. It now passes along the hilus of the
spleen, and is continued to the great curvature of the stomach by the
gastro-splenic omentum. It is convenient to regard the spleen as being
intercalated in the left part of the great omentum; on this basis the
gastro-splenic omentum would be that part of the great omentum which
connects the hilus of the spleen with the great curvature of the
stomach. The great omentum is relatively small in the horse, and is
usually not visible when the abdomen is opened. It is generally folded
up in the space between the visceral surface of the stomach and the
intestine.[105]
The lesser sac furnishes the peritoneal covering for: (1) the visceral
surface of the stomach and a small area of the first curve of the
duodenum; (2) a large part of the dorsal surface of the pancreas and
portal vein; (3) a small part of the visceral surface of the liver
above the attachment of the lesser omentum and the portal fossa; (4)
the posterior vena cava, from the level of the epiploic foramen [of
Winslow] to its passage through the diaphragm (in so far as it is not
embedded); (5) the part of the parietal surface of the liver between
the right and middle divisions of the coronary ligament; (6) the
corresponding part of the diaphragm, and the right part of the right
crus of the same; (7) part of the anterior surface of the terminal
part of the great colon, and the origin of the small colon; (8) the
left extremity of the pancreas (inconstant); (9) the spleen.
We may now trace the peritoneum in a longitudinal direction, beginning
in front. It is reflected from the ventral abdominal wall and the
diaphragm upon the liver, forming the ligaments and serous coat of the
gland. It leaves the visceral surface of the liver as lesser omentum,
and the crura of the diaphragm as the gastro-phrenic ligament, reaches
the saccus cæcus and lesser curvature of the stomach and the first curve
of the duodenum, covers these organs, and is continued by the great
omentum.
[Illustration:
FIG. 279.—DIAGRAM OF ABDOMINAL PERITONEUM IN FRONTAL (HORIZONTAL)
TRACING.
_D_, Duodenum. The arrow indicates the epiploic foramen (of Winslow).
By an oversight the leader line to the great omentum is omitted, and
the coronary ligament of the liver is erroneously marked lesser
omentum; the latter extends from liver to stomach, but is not
marked.
]
On the left it passes from the left crus of the diaphragm and the left
kidney to form the suspensory ligament of the spleen, clothes that
organ, and leaves it to be continued by the great omentum.
On the right it passes from the right crus of the diaphragm and the
dorsal border of the liver to the concave border of the duodenum,
forming the gastro-pancreatic fold (second part of the mesoduodenum),
and covering part of the dorsal surface of the pancreas. From the margin
of the pancreas, the right kidney, and a small area of the sublumbar
region behind the latter, it passes on to the base of the cæcum and the
terminal part of the great colon. From these it passes on the right to
the duodenum, forming the third part of the mesoduodenum. On the left it
clothes part of the ventral surface and the outer border of the left
kidney, from which it passes to the base of the spleen, forming the
ventral layer of the suspensory ligament of the latter. Behind the
terminal part of the great colon it is reflected around the great
mesenteric artery to form the great mesentery. Behind this it is
reflected almost transversely from the roof of the cavity and from the
origin of the small colon on to the duodenum, forming the terminal part
of the mesoduodenum. The line of origin of the colic mesentery begins on
the inner part of the ventral surface of the left kidney, and extends to
the sacral promontory, where the mesorectum begins. At the termination
of the latter the peritoneum is reflected from the rectum on the dorsal
and lateral walls of the pelvic cavity. Below the rectum it forms the
urogenital fold, and passes on to the dorsal surface of the bladder,
covers its anterior part, and is reflected on to the body-wall laterally
and ventrally, forming the lateral and middle ligaments of the bladder.
In the female the broad ligaments of the uterus replace the urogenital
fold, with which they are homologous.
In the new-born foal certain folds are specially large. The falciform
ligament of the liver extends to the umbilical opening, and contains in
its free edge the large umbilical vein. The bladder—at this time an
abdominal organ—has a ventral median fold, which connects it and the
urachus with the abdominal floor. This is flanked on either side by a
fold which also extends to the umbilicus, and contains the large
umbilical artery.