This section, therefore, shows two completely closed sacs without any
visible communication. In the female, however, the great sac is not
absolutely closed, for the Fallopian tubes open into it by their minute
_ostia abdominalia_, while at the other ends they communicate with the
cavity of the uterus and so with the vagina and exterior.
A horizontal section through the upper part of the first lumbar vertebra
will, if a fortunate one (see fig. 2), pass through the foramen of
Winslow and show the communication of the two sacs. A starting-point may
be made from the mid-ventral line and the parietal peritoneum traced
round the left side of the body wall until the outer edge of the left
kidney (K) is reached; here it passes in front of the kidney and is soon
reflected off on to the spleen, which it nearly surrounds; just before
it reaches the hilum of that organ, where the vessels enter, it is
reflected on to the front of the stomach (St), forming the anterior
layer of the _gastro-splenic omentum_; it soon reaches the lesser
curvature of the stomach and then becomes the anterior layer of the
lesser omentum, which continues until the bile duct (B.D) and portal
vein (P.V) are reached at its right free extremity; here it turns
completely round these structures and runs to the left again, as the
posterior layer of the lesser omentum, behind the stomach (St) and then
to the spleen (Sp) as the posterior layer of the gastro-splenic omentum.
From the spleen it runs to the right once more, in front of the pancreas
(P), until the inferior vena cava (V.C) is reached, and this point is
just behind the portal vein and is the place where the lesser and
greater sacs communicate, known as the foramen of Winslow. From this
opening the lesser sac runs to the left, while all the rest of the
peritoneal cavity in the section is greater sac. From the front of the
vena cava the parietal peritoneum passes in front of the right kidney
(K) and round the right abdominal wall to the mid-ventral line. The
right part of this section is filled by the liver (L), which is
completely surrounded by a visceral layer of peritoneum, and no
reflection is usually seen at this level between it and the parietal
layer. Some of the viscera, such as the kidneys and pancreas, are
retro-peritoneal; others, such as the small intestines and transverse
colon, are surrounded, except at one point where they are attached to
the dorsal wall by a _mesentery_ or _mesocolon_ as the reflections are
called; others again are completely surrounded, and of these the caecum
is an example; while some, like the liver and bladder, have large
uncovered areas, and the reflections of the membrane form ligaments
which allow considerable freedom of movement.
The _tunica vaginalis_ is the remains of a process of the peritoneum
(_processus vaginalis_) which descends into the scrotum during foetal
life some little time before the testis itself descends. After the
descent of the testis the upper part usually becomes obliterated, while
the lower part forms a serous sac which nearly surrounds the testis, but
does not quite do so. Posteriorly the epididymis is in close contact
with the testis, and here the visceral layer is not in contact; there
is, however, a pocket called the _digital fossa_ which squeezes in from
the outer side between the testis and epididymis. The parietal layer
lines the inner wall of its own side of the scrotum.
For a full description of the topography of the serous membranes see
any of the standard text-books of anatomy, by Gray, Quain, Cunningham
or Macalister. Special details will be found in Sir F. Treves'
_Anatomy of the Intestinal Canal and Peritoneum_ (London, 1885); C. B.
Lockwood, _Hunterian Lectures on Hernia_ (London, 1889); C. Addison,
"Topographical Anatomy of the Abdominal Viscera in Man," _Jour.
Anat._, vols. 34, 35; F. Dixon and A. Birmingham, "Peritoneum of the
Pelvic Cavity," _Jour. Anat._ vol. 34, p. 127; W. Waldeyer, "Das
Becken" (1899), and "Topographical Sketch of the Lateral Wall of the
Pelvic Cavity," _Jour. Anat._ vol. 32; B. Moynihan, _Retroperitoneal
Hernia_ (London, 1899). A complete bibliography of the subject up to
1895 will be found in _Quain's Anatomy_, vol. 3, part 4, p. 69.
[Illustration: After Young and Robinson, Cunningham's _Text-Book of
Anatomy_.
FIG. 3.--Diagram of Longitudinal Section, showing the different areas of
the Blastodermic Vesicle.
_a_, Pericardium. _e_, Placental area.
_b_, Bucco-pharyngeal area. _d_, Entoderm.
_c_, Ectoderm.]
_Embryology._--As the mesoderm is gradually spreading over the embryo it
splits into two layers, the outer of which is known as the
_somatopleure_ and lines the parietal or ectodermal wall, while the
inner lines the entoderm and is called the _splanchnopleure_; between
the two is the coelom. The pericardial area is early differentiated from
the rest of the coelom and at first lies in front of the neural and
bucco-pharyngeal area; here the mesoderm stretches right across the
mid-line, which it does not in front and behind. As the head fold of the
embryo is formed the pericardium is gradually turned right over, so that
the dorsal side becomes the ventral and the anterior limit the
posterior; this will be evident on referring to the two accompanying
diagrams.
[Illustration: After Young and Robinson, Cunningham's _Text-Book of
Anatomy_.
FIG. 4.--Diagram of a Developing Ovum, seen in Longitudinal Section.
_f_, Spinal cord. _i_, Brain.
_g_, Notochord. _k_, Extra embryonic coelom.
_h_, Dorsal wall of alimentary canal. Other numbers as in fig. 3.]
The two primitive aortae lie at first in the ventral wall of the
pericardium, but with the folding over they come to lie in the dorsal
wall and gradually bulge into the cavity as they coalesce to form the
heart, so that the heart drops into the dorsal side of the pericardium
and draws down a fold of the membrane called the _dorsal mesocardium_.
In mammals A. Robinson (_Jour. Anat. and Phys._, xxxvii. 1) has shown
that no ventral mesocardium exists, though in more lowly vertebrates it
is present. Laterally the pericardial cavity communicates with the
general cavity of the coelom, but with the growth of the Cuvierian ducts
(see development of veins) these communications disappear. Originally
the mesocardium runs the whole length of the pericardium from before
backward, but later on the middle part becomes obliterated, and so the
two separate reflections from the parietal to the visceral layer,
already noticed, are accounted for.
Just behind the pericardium and in front of the umbilicus, which at
first are close together, the mesoderm forms a mass which is called the
_septum transversum_, and into this the developing lungs push bag-like
protrusions of the coelom, consisting of visceral and parietal layers,
and these eventually lose their connexion with the rest of the coelom,
as the diaphragm develops, and become the pleural cavities. After the
pericardium and pleurae have been separated off the remainder of the
coelom becomes the peritoneum. At first the stomach and intestine form a
straight tube, which is connected to the dorsum of the embryo by a
_dorsal mesentery_ and to the mid-ventral wall in front of the umbilicus
by a _ventral mesentery_. Into the ventral mesentery the liver grows as
diverticula from the duodenum, so that some of the mesentery remains as
the _falciform ligament_ of the liver and some as the lesser omentum.
Into the dorsal mesentery the pancreas grows, also as diverticula, from
the duodenum, while the spleen is developed from the mesoderm contained
in the same fold. As the stomach turns over so that its left side
becomes ventral, the dorsal mesentery attached to it becomes pulled out,
in such a way that part of it forms the great omentum and part the
gastro-splenic omentum. After the caecum is formed as a diverticulum
from the intestine it is situated close to the liver and gradually
travels down into the right iliac fossa. This passage to the right is
accompanied by a throwing over of the duodenal loop to the right, so
that the right side of its mesentery becomes pressed against the dorsal
wall of the abdomen and obliterated. This accounts for the fact that the
pancreas and duodenum are only covered by peritoneum on their anterior
surfaces in man. The formation of the lesser sac is due to the turning
over of the stomach to the right, with the result that a cave, known
sometimes as the _bursa omentalis_, is formed behind it. Originally, of
course, the whole colon had a _dorsal mesocolon_ continuous with the
mesentery, but in the region of the ascending and descending colon this
usually disappears and these parts of the gut are uncovered by
peritoneum posteriorly. The transverse mesocolon persists and at first
is quite free from the great omentum, but later, in man, the two
structures fuse[1] and the fourth layer of the great omentum becomes
continuous with the posterior layer of the transverse mesocolon.
For further details see Quain's _Anatomy_ (London, 1908).
_Comparative Anatomy._--In the Amphioxus the coelom is developed in the
embryo as a series of bilateral pouches, called _enterocoeles_, from the
sides of the alimentary canal; these are therefore entodermal in their
origin, as in Sagitta and the Echinodermata among the invertebrates. In
the adult the development of the atrium causes a considerable reduction
of the coelom, represented by two dorsal coelomic canals communicating
with a ventral canal by means of branchial canals which run down the
outer side of the primary gill bars. Into the dorsal canals the
nephridia open. In the intestinal region the coelom is only present on
the left side.
In the higher vertebrates (_Craniata_) the coelom is developed by a
splitting of the mesoderm into two layers, and a pericardium is
constricted off from the general cavity. In all cases the ova burst into
the coelom before making their way to the exterior, and in some cases,
_e.g._ amphioxus, lamprey (Cyclostomata), eels and mud-fish (Dipnoi),
the sperm cells do so too. The Cyclostomata have a pair of _genital
pores_ which lead from the coelom into the urino-genital sinus, and so
to the exterior.
In the Elasmobranch fish there is a _pericardio-peritoneal canal_
forming a communication between these two parts of the coelom; also a
large common opening for the two oviducts in the region of the liver,
and two openings, called _abdominal pores_, on to the surface close to
the cloacal aperture. In the Teleostomi (Teleostean and Ganoid fish)
abdominal pores are rare, but in most Teleostei (bony fish) the ova pass
directly down oviducts, as they do in Arthropods, without entering the
peritoneal cavity; there is little doubt, however, that these oviducts
are originally coelomic in origin. In the Dipnoi (mud-fish) abdominal
pores are found, and probably serve as a passage for the sperm cells,
since there are no vasa deferentia. In fishes a complete dorsal
mesentery is seldom found in the adult; in many cases it only remains as
a tube surrounding the vessels passing to the alimentary canal.
In the Amphibia, Reptilia and Aves, one cavity acts as pleura and
peritoneum, though in the latter the lungs are not completely surrounded
by a serous membrane. In many lizards the comparatively straight
intestine, with its continuous dorsal mesentery and ventral mesentery in
the anterior part of the abdomen, is very like a stage in the
development of the human and other mammalian embryos. In the mammalia
the diaphragm is complete (see DIAPHRAGM) and divides the
pleuro-peritoneal cavity into its two constituent parts. In the lower
mammals the derivatives of the original dorsal mesentery do not undergo
as much fusion and obliteration as they do in adult man; the ascending
and descending mesocolon is retained, and the transverse mesocolon
contracts no adhesion to the great omentum. It is a common thing,
however, to find a fenestrated arrangement of the great omentum which
shows that its layers have been completely obliterated in many places.
In those animals, such as the rabbit, in which the tests are sometimes
in the scrotum and sometimes in the abdomen, the communication between
the peritoneum and the tunica vaginalis remains throughout life.
For further details and literature up to 1902, see R. Wiedersheim's
_Vergleichende Anatomie der Wirbeltiere_ (Jena, 1902). (F. G. P.)