THORACIC CAVITY 75 



and the upper end of the muscular part of the septum, and then descends 

 along the left side of the septum. Both branches send off numerous rami- 

 fications which are distributed to the various parts of the walls of the 

 ventricles. 



The pulmonary orifice lies at the upper, anterior, and left 

 part of the ventricle, at the apex of the conus arteriosus. Its 

 centre is behind the third left costal cartilage immediately to 

 the left of the left border of the sternum, and its margin is 

 surrounded by a thin fibrous ring to which the bases of the 

 three semilunar cusps of the pulmonary valve are attached. 



Dissection. Note that immediately above its commencement the wall 

 of the pulmonary artery shows three distinct bulgings ; these are the 

 pulmonary sinuses (Valsalva) of which two are anterior, and the third is 

 situated posteriorly. Make a transverse incision across the wall of the 

 pulmonary artery immediately above the dilatations, and from each end of 

 the transverse incision make a vertical incision upwards towards the arch 

 of the aorta ; raise the flap so formed and examine the cusps of the valve 

 from above. 



The Pulmonary Valve. Each cusp of the valve is of semi- 

 lunar form. Its upper or arterial surface is concave, its 

 lower or ventricular surface is convex ; and it consists of a 

 layer of fibrous tissue covered, on each surface, by a layer of 

 endothelium. The fibrous basis of the cusp is not equally 

 thick in all parts. A stronger band runs round both the free 

 and the attached margin. The centre of the free margin is 

 thickened to form a small rounded mass the nodulus of the 

 valve and the small thin semilunar regions on each side of 

 the nodule are called the lunula of the valve. When the 

 ventricular contraction ceases, and the elastic reaction of the 

 wall of the pulmonary artery forces the blood backwards 

 towards the ventricle, the cusps of the valve are forced into 

 apposition ; the nodules meet in the centre of the lumen ; the 

 ventricular surfaces of. the lunulas of adjacent cusps are com- 

 pressed against each other, and their free margins project 

 upwards into the cavity of the artery, in the form of three 

 vertical ridges which radiate from the nodules to the wall of 

 the artery. Regurgitation of blood into the ventricle is thus 

 effectually prevented. 



The dissector may readily demonstrate the general appear- 

 ance of the cusps and their relationship to each other by 

 packing the concavity of each cusp with cotton wool. 



Arteria Pulmonalis. The pulmonary artery lies within 

 the fibrous pericardium, and is enclosed, with the ascending 



