The Thorax. 177 



are usually to be found, the third being destroyed on opening 

 the vessel. 



In the atria : 



(a) The .respective positions of the pulmonary and systemic 

 veins at their points of entrance. 



(b) The complete separation of the two chambers. In the par- 

 ' tition separating them may be seen a thin fibrous portion 



denoting the position of the embryonic foramen ovale. 

 Open the left ventricle by a ventral longitudinal incision, cutting well 

 through the tip of the ventricle and extending the incision across the 

 pulmonary artery and into the aorta. On account of the great thick- 

 ness of the wall the internal structure is not so easily examined as in the 

 right ventricle. 



(a) The bicuspid valve (valvula bicuspidalis) is similar in general 

 structure to the tricuspid valve of the right ventricle. 



(b) The semilunar valves of the aorta are three in number, and 

 are similar to those of the pulmonary artery. 



4. Examination of the lungs and their connections. 



The removal of the ventral wall 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 : 



(a) Each pleural cavity (cavum pleurae) is a closed serous sac, 

 the lining membrane, or pleura, being distributed over the 

 costal surface as the costal pleura, partly over the anterior 

 surface of the diaphragm as the diaphragmatic pleura, and 

 over the surface of the lung as the pulmonary pleura. Pos- 

 teriorly the pulmonary pleura passes from the medial margin 

 of the left lung to the medial lobule of the right and thence 

 backward to the diaphragm, forming a broad sheet of attach- 

 ment, the pulmonary ligament (lig. pulmonale). 



(b) The lungs (pulmones) are paired expansible structures, the 

 surfaces of which are free, except posteriorly, where they 

 are attached to the diaphragm through the pulmonary liga- 

 ment, and medially where they are connected with the 

 pulmonary bloodvessels and the respiratory tubes. 



(c) Each lung is divided into three portions, the superior, middle, 

 and inferior lobes. On account of the smaller size of the 

 left lung, the superior lobe is imperfectly developed. On 

 the right side the inferior lobe is divided into two portions, 

 the medial and lateral lobules, the inferior caval vein passing 

 between them. 



(d) The trachea divides at its posterior end into two portions, the 

 right and left bronchi, one for each lung. Each bronchus is 

 again divided into smaller portions, the bronchial rami, which 

 penetrate the substance of the organ and redivide into smaller 

 tubes. 



