212 VERTEBRATE ZOOLOGY 



The medial walls of the pleural sacs do not extend to the median 

 plane of the body but inclose between themselves a large space 

 called the mediastinum, which occupies the medial area of the 

 thoracic cavity. Within this space lie the median, unpaired organs 

 of the thorax : the heart and its great vessels, the oesophagus, the 

 trachea, and the thymus. 



By lifting up the hinder end of the breastbone a view of the 

 thoracic cavity will be obtained. The large heart will be seen in- 

 closed in the membranous pericardium, in which much fat is often 

 embedded. Dorsal to the heart are two thin membranes passing 

 diagonally to the right and left, to the dorsal body wall. These 

 are the medial walls of the pleural sacs, the space between them 

 being the mediastinum, in which the heart lies. Ventral to the 

 heart the walls of the mediastinum are much nearer together than 

 dorsally, and appear as a single membrane which joins the peri- 

 cardium with the body wall in the midventral plane. Do not 

 disturb any of these organs. 



Exercise 5. Draw a figure representing a diagrammatic cross section 

 through the thoracic cavity, showing the relative positions of the 

 two pleural sacs, the mediastinum, the pericardium, and the lungs. 



The abdominal cavity contains the greater part of the digestive 

 organs, the reproductive organs, and other important viscera. It 

 is lined by a thin membrane called the peritoneum, which forms 

 a sac inclosing nearly the entire abdominal space. Just as the 

 pleural sacs are closed sacs, so the peritoneal sac, in the male 

 animal, is a closed one, and in the female it is closed except 

 that the reproductive system furnishes a pair of openings to the 

 exterior through which ova and young find an exit. Also, just 

 as the lungs and other organs project into the pleural sacs, covered, 

 however, by folds of their walls, so the various organs of the 

 abdominal cavity project into it, carrying folds of the peritoneal 

 wall with them. Thus all the abdominal organs will be found 

 attached to the abdominal wall by these peritoneal folds, which 

 are called mesenteries, in the broad use of the term. 



Immediately back of the diaphragm are the stomach and the 

 liver, as we have already seen. The mesentery of the latter is 



