Coelom And Serous Membranes
COELOM AND SEROUS MEMBRANES. In human anatomy the body-cavity or coelom is divided into the pericardium, the two pleurae, the peritoneum and the two tunicae vaginales.
The pericardium is a closed sac which occupies the central part of the thorax and contains the heart. Like all the serous membranes it has a visceral and a parietal layer, the former of which is closely applied to the heart and consists of endothelial cells with a slight fibrous backing: to it is due the glossy appearance of a freshly removed heart. The parietal layer is double; externally there is a strong fibrous protective coat which is continuous with the other fibrous structures in the neighbourhood, especially with the sheaths of the great vessels at the root of the heart, with prolongations of the fascia of the neck, and with the central tendon of the diaphragm, while internally is the serous layer which is reflected from the surface of the heart, where the great vessels enter, so that everywhere the two layers of the serous membrane are in contact, and the only thing within the cavity is a drop or two of the fluid secreted by the serous walls. When the parietal layer is laid open and the heart removed by cutting through the great vessels, it will be seen that there are two lines of reflection of the serous layer, one common to the aorta and pulmonary artery, the other to all the pulmonary veins and the two venae cavae.
The pleurae very closely resemble the pericardium except that the fibrous outer coat of the parietal layer is not nearly as strong; it is closely attached to the inner surface of the chest walls and mesially to the outer layer of the pericardium; above it is thickened by a fibrous contribution from the scalene muscles, and this forms the dome of the pleura which fits into the concavity of the first rib and contains the apex of the lung. The reflection of the serous layer of the pleura, from the parietal to the visceral part, takes place at the root of the lung, where the great vessels enter, and continues for some distance below this as the ligamentum latum pulmonis. The upper limit of the pleural cavity reaches about half an inch above the inner third of the clavicle, while, below, it may be marked out by a line drawn from the twelfth thoracic spine to the tenth rib in the mid axillary line, the eighth rib in the nipple line, and the sixth rib at its junction with the sternum. There is probably very little difference in the lower level of the pleurae on the two sides.
The peritoneum is a more extensive and complicated membrane than either the pericardium or pleura; it surrounds the abdominal and pelvic viscera, and, like the other sacs, has a parietal and visceral layer. The line of reflection of these, though a continuous one, is very tortuous. The peritoneum consists of a greater and lesser sac which communicate through an opening known as the foramen of Winslow, and the most satisfactory way of understanding these is to follow the reflections first in a vertical median (sagittal) section and then in a horizontal one, the body being supposed to be in the upright position. If a median sagittal section be studied first, and a start be made at the umbilicus (see fig. 1), the parietal peritoneum is seen to run upward, lining the anterior abdominal wall, and then to pass along the under surface of the diaphragm till its posterior third is reached; here there is a reflection on to the liver (L), forming the anterior layer of the coronary ligament of that viscus, while the membrane now becomes visceral and envelops the front of the liver as far back as the transverse fissure on its lower surface; here it is reflected on to the stomach (St) forming the anterior layer of the gastro-hepatic or lesser omentum. It now covers the front of the stomach, and at the lower border runs down as the anterior layer of an apron-like fold, the great omentum, which in some cases reaches as low as the pubes; then it turns up again as the posterior or fourth layer of the great omentum until the transverse colon (C) is reached, the posterior surface of which it covers and is reflected, as the posterior layer of the transverse meso-colon, to the lower part of the pancreas (P); after this it turns down and covers the anterior surface of the third part of the duodenum (D) till the posterior wall of the abdomen is reached, from which it is reflected on to the small intestine (I) as the anterior layer of the mesentery, a fold varying from 5 to 8 in. between its attachments. After surrounding the small intestine it becomes the posterior layer of the mesentery and so again reaches the posterior abdominal wall, down which it runs until the rectum (R) is reached. The anterior surface of this tube is covered by peritoneum to a point about 3 in. from the anus, where it is reflected on to the uterus and vagina (V) in the female and then on to the bladder (B); in the male, on the other hand, the reflection is directly from the rectum to the bladder. At the apex of the bladder, after covering the upper surface of that organ, it is lifted off by the urachus and runs up the anterior abdominal wall to the umbilicus, from which the start was made. All this is the greater sac. The tracing of the lesser sac may be conveniently started at the transverse fissure of the liver, whence the membrane runs down to the stomach (St) as the posterior layer of the lesser omentum, lines the posterior surface of the stomach, passes down as the second layer of the great omentum and up again as the third layer, covers the anterior surface of the transverse colon (C) and then reaches the pancreas (P) as the anterior layer of the transverse mesocolon. After this it covers the front of the pancreas and in the middle line of the body runs up below the diaphragm to within an inch of the anterior layer of the coronary ligament of the liver; here it is reflected on to the top of the Spigelian lobe of the liver to form the posterior layer of the coronary ligament, covers the whole Spigelian lobe, and so reaches the transverse fissure, the starting-point.
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.
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  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.)
 Some authorities hold that this alteration is not brought about by fusion, but by a dragging away of the posterior layer of the great omentum from the dorsal wall of the abdomen.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)