THE LUNGS 



333 



partition is approximately of the same thickness as the rest of the 

 wall of the left ventricle. 



(a) 



(b) 



The bicuspid or left atrioventricular valve (valvula bicuspi- 

 dalis) is similar in general structure to the tricuspid valve 

 of the right ventricle, but is more nearly circular in form, 

 with stout, closely grouped papillary muscles. 

 The semilunar valves of the aorta are three in number, 

 and are similar to those of the pulmonary artery. Opposite 



ao 



Fig. 113. Diagram of a transverse section of the thorax 

 of a rabbit, passing through the posterior tip of the heart, 

 am, anterior mediastinum; ao, aorta; cp, costal pleura; 

 ep, epicardioum; f, fold of mediastinal pleura containing 

 the inferior caval vein; Ipl, left pulmonary ligament; m, 

 medial lobule of inferior lobe of the right lung; ml, middle 

 lobe the left lung; mp, mediastinal pleura ;_ oe, oesophagus; 

 p. pericardium; pm, posterior mediastinum; pp, pul- 

 monary pleura; rpl, right pulmonary ligament; rv, right 

 ventricle; s, sternum; vc, inferior caval vein. 



each, the wall of the aorta is slightly excavated so that the 

 valve will not adhere to it. The cavity enclosed by each 

 valve is known as an aortic sinus (of Valsalva) . 



4. Examination of the lungs and their connections (Figs. 57, 114). 

 The removal of the ventral w^all of the thorax opens the pleural 



cavities by taking away a considerable portion of the costal pleura, 



which is adherent to the internal surfaces of the ribs. The chief 



features may be made out as follows: 



