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 venae cavae ; and a dorsal medi- 

 astinal cavity, containing the trachea, the oesophagus, 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) 



