218 THE VISCERA. 
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 venz cavz; and a dorsal medi- 
astinal cavity, containing the trachea, the cesophagus, 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 con- 
sider 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) 
