I. THE BODY CAVITY.
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.