VERTEBRATES, 



Make a longitudinal lateral cut through the middle of the 

 ribs, and remove the ventral wall of the thorax, dissecting 

 it carefully away from its attachments to the internal 

 organs. 



The diaphragm is the transverse muscular partition 

 found attached to the posterior borders of the ribs. It is 

 the boundary between thorax and abdomen. Lift up its 

 severed ventral margin, and note its shape, convex toward 

 the thorax, and concave toward the abdomen. Observe a 

 thin tendon at its center. The muscular libers composing 

 it arise from the walls of the body cavity, and converge 

 toward their insertion into this central tendon. When 

 these contract, they draw the tendon posteriorly, increas- 

 ing the capacity of the thorax. 



Internal Features. The large, dark-colored liver lies 

 close against the posterior side of the diaphragm. The 

 capacious stomach lies partly concealed by the liver, and 

 posteriorly nearly the entire cavity of the abdomen is 

 filled with intestine, slung from the dorsal side of the 

 cavity in folds of the mesentery. 



The pink lungs, lying free in the thoracic cavity, rest 

 against the anterior surface of the diaphragm. The heart, 

 within its pericardium, lies between the lungs, its apex 

 normally resting upon the diaphragm. 



Note the following parallelisms in the linings of thoracic 

 and abdominal cavities. The abdomen is lined with a 

 thin, transparent, and closely adherent membrane (the 

 peritoneum), which, on the dorsal side, is reflected ventrally 

 in a double fold (the mesentery) enveloping the viscera. 

 The thoracic cavity is lined with a similar membrane (the 

 pleura), which is also reflected ventrally from the dorsal 

 side to form a double fold (the mediastinum), which, at the 

 base of each lung, is reflected completely over it, so as to 

 inclose it as within a bag. Between the two lungs there 



