276 VERTEBRATE ZOOLOGY 



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

 a sac inclosing nearly the entire abdominal space. Into this space 

 the various organs of the cavity project, 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 

 called the suspensory, or falciform, ligament ; it is a median verti- 

 cal membrane, which appears between the right and left halves of 

 the liver and joins it with the diaphragm. The green gall bladder 

 will be seen on the right side of the liver. 



Exercise 4. Draw a semidiagrammatic sketch of the abdominal cavity 

 in an outline of the body, together with the organs which appear 

 in a ventral view. Carefully label all. 



Open the thoracic cavity in the following way: With scissors 

 make a longitudinal incision along each side of the breastbone, 

 through the body wall, to the forward end of the trunk. Cut the 

 diaphragm where it joins the ventral body wall on each side. Lift 

 up the hinder end of the breastbone and, without further cutting 

 or disturbing anything, look into the cavity which is thus opened. 



It will be seen, in the first place, that the entire body cavity is 

 made up of two divisions, the abdominal cavity and the thoracic 

 cavity, which are separated by the diaphragm. 



The thoracic cavity contains the lungs, the heart and the great 

 veins and arteries which enter and leave it, the posterior portion 

 of the oesophagus and the trachea, and the thymus. A thin mem- 

 brane called the pleura lines its inner surface. The space within 

 the thorax is not, however, a continuous one, but is divided 

 into three distinct compartments: the two pleural sacs, which 

 occupy the right and left sides of the thoracic cavity, and the 

 mediastinum, which is a median space between them. One of 

 the lungs projects into each pleural sac, covered by a fold of the 

 pleura; within the mediastinum lie the median organs of the 

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

 and the thymus. 



