The greater part of the viscera are situated in the body cavity or
cœlom. This is divided by the diaphragm into two parts, the =thoracic
cavity= and the =abdominal cavity=. Each is lined by a serous membrane,
in which the part covering the outer wall of the cavity is distinguished
as the =parietal layer= from the part covering the viscera, which is
known as the =visceral layer=.
The thoracic cavity is bounded by the thoracic vertebræ, the ribs, the
sternum, and the diaphragm. The cranial opening of the cavity is filled
by the trachea and œsophagus as they enter from the neck region. The
thoracic cavity is lined by two thin layers of tissue, the outer one of
which is the =fascia endothoracica=, while the inner is the =pleura=.
The fascia endothoracica is a sheet of connective tissue which lines the
entire inner surface of the thoracic cavity, descending from the dorsal
median line to the heart, and passing into the fibrous layer of the
pericardium. The pleura is a thin membrane covering the fascia
endothoracica and corresponding to the peritoneum of the abdominal
cavity. It forms two sacs, the =pleuræ=, lining respectively the right
and left halves of the thoracic cavity. Each of these two sacs is
closed, the viscera being suspended within them by folds of the
membrane, so that the cavity is everywhere separated from the viscera by
a sheet of the pleura. That portion of the pleura which lines the
thoracic wall is known as the parietal layer; it may be divided into
that covering the ribs (=costal= pleura), and that covering the
diaphragm. That portion which covers the viscera is the visceral layer,
or, since it covers chiefly the lungs, it may be distinguished as the
=pulmonary= pleura. The medial walls of the two pleural sacs come in
contact in the median plane, forming a median vertical partition
passing lengthwise of the thoracic cavity. This partition is known as
the =mediastinal septum=. The space between the two layers which make up
the mediastinal septum is known as the mediastinum, or mediastinal
cavity; it contains numerous organs of the thorax. Three parts are
usually distinguished in this cavity: a =ventral= mediastinal cavity,
containing chiefly blood-vessels and the thymus gland; a =middle=
mediastinal cavity, enclosing the heart and the anterior and posterior
venæ cavæ; and a =dorsal= mediastinal cavity, containing the trachea,
the œsophagus, and the aorta.
The abdominal cavity lies caudad of the diaphragm; in it are sometimes
distinguished the =abdominal cavity proper=, extending as far caudad as
the cranial edge of the pubis, and the =pelvic= cavity, lying caudad of
this, in the region surrounded by the innominate bones and the sacrum.
The two cavities are not distinctly marked off, so that it is convenient
to consider the abdominal cavity as undivided. Both parts are lined by
the =peritoneum=.
The =peritoneum= is a thin transparent sheet of connective tissue
supporting on its surface a layer of flattened epithelial cells, the
peritoneal epithelium. It forms a sac which lines the entire abdominal
cavity. This sac is closed in the male; in the female, however, it
communicates with the exterior through the uterine (or Fallopian) tubes
and uteri. All the organs of the abdominal cavity are outside the sac.
In the course of their development these organs have encroached on the
peritoneal sac. Each has grown against the outer wall of the sac to a
greater or less extent and has forced a part of this wall ahead of it
into the cavity. In some cases the encroachment has gone so far that the
organ in question lies apparently within the peritoneal cavity,
suspended from the wall of the sac by a fold of that wall. The wall may
thus be divided into three portions. One of these, the =parietal= layer,
lines the wall of the body cavity. The second (the =mesentery= in case
of the alimentary canal, or a =ligament= in the case of another organ)
suspends the organ from the body wall. Between the layers of each
mesentery or ligament blood-vessels may pass to the organs. The third
portion or =visceral= layer covers the organ in question, forming its
=serous= covering.
The reflections of the peritoneum to form the mesenteries and ligaments
may be thus described:
Caudally the peritoneum covers the whole surface of the bladder and is
reflected from its ventral wall to the linea alba as the =suspensory
ligament= of the bladder. Farther craniad the peritoneum suspends the
rectum and colon from the middorsal line, forming the =mesorectum= and
=mesocolon=. The mesocolon continues craniad to the level of the caudal
end of the right kidney and is broadest at its cranial end. At this end
the mesocolon passes into the =mesentery= proper, which suspends the
small intestine and is very broad and much folded. Its dorsal border is
attached to the median line opposite the caudal end of the kidneys and
is very short compared to its very long ventral or intestinal border.
Toward the cranial end the mesentery of the jejunum passes gradually
into the very much shorter duodenal mesentery. This is drawn out at the
caudal end of the duodenum into a fold, the =duodenorenal= ligament
which attaches the duodenum to the kidney.
The =mesogastrium= or peritoneal fold for the stomach passes from that
part of the median dorsal line lying between the kidneys and the
diaphragm, to the greater curvature of the stomach. It does not pass
directly to the stomach, but passes first ventrad of the small intestine
as far as the pelvis. Thence it turns craniad to reach the greater
curvature of the stomach. The fold thus formed is called the =great
omentum=. It forms the dorsal and ventral walls of a sac, the =omental
sac=, the cavity of which is called the =lesser peritoneal cavity=. The
descending limb of the fold forms the dorsal wall of the sac, and its
ascending limb forms the ventral wall. Each of these walls is double
like a mesentery, so that the great omentum consists of four sheets of
peritoneum. Between the two sheets forming the descending limb lies the
left half of the pancreas, which passes thence to the right into the
duodenal mesentery. A transverse fold passes from the descending limb of
the omentum along the cranial border of the pancreas to the duodenal
mesentery. Farther to the right the descending limb of the omentum,
which is here shorter, encloses the spleen and holds it in position
parallel to the greater curvature of the stomach and about one
centimeter from it. This part of the great omentum is sometimes called
the =gastrosplenic= or =gastrolienal= omentum.
The great omental sac communicates with the peritoneal sac by an
opening, the =foramen epiploicum= or =foramen of Winslow=. This opening
lies caudad and dextrad of the caudate lobe of the liver. This lobe
itself lies within the sac. Along the ventral border of the foramen
epiploicum pass the common bile-duct from the liver and the portal vein
to the liver.
The size of the great omental sac is increased by the =lesser omentum=.
This is a double sheet of peritoneum which stretches horizontally from
the liver to the duodenum and the lesser curvature of the stomach. It
covers ventrad the caudate lobe of the liver. Its right border is at the
foramen epiploicum. The part of it which stretches to the duodenum is
called the =duodenohepatic= ligament and contains the bile-ducts and the
portal vein. The part that stretches to the stomach is called the
=gastrohepatic= ligament. The peritoneum covers the caudal and cranial
surfaces of the liver and is reflected to the adjacent parts to form the
ligaments of the liver.
The =suspensory= ligament of the liver passes from the caudal surface of
the diaphragm and the median ventral line for about one or two
centimeters caudad of the diaphragm, to the liver, and extends between
its two halves. The =ligamentum teres= or round ligament is the
thickened free caudal border of the suspensory ligament. It is the
remains of the fœtal umbilical vein. From the dorsal border of the liver
the peritoneum which covers its cranial surface turns ventrad onto the
caudal surface of the diaphragm, while that which covers its caudal
surface turns dorsad onto the caudal surface of the diaphragm to reach
the dorsal body wall. Between these two sheets a small linear part of
the surface of the liver is closely applied to the diaphragm without
intervening peritoneum. The two sheets which bound this area constitute
the =coronary= ligament of the liver. This ligament is broader where it
passes from the left lateral lobe to the diaphragm and is called the
=triangular= ligament. (There is perhaps a corresponding right
triangular ligament, from the cranial division of the right lateral
lobe.) The caudal division of the right lateral lobe is held to the
kidney of that side by the =hepatorenal= ligament.