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=.