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.
 | ||
| After Young and Robinson, Cunningham’s Text-Book of Anatomy. | ||
| Fig. 3.—Diagram of Longitudinal Section, showing the differentareas of the Blastodermic Vesicle. | ||
| a, Pericardium. | c, Ectoderm. | d, Entoderm. |
| b, Bucco-pharyngeal area. | e, Placental area. | |
 | |
| 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. |
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.
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).