102 THORAX 



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 commence- 

 ment the wall of the pulmonary artery shows three distinct 

 bulgings ; they 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, with 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 lunulae of adjacent cusps are coin- 

 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 

 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 stem of the pulmonary artery 

 is about 50 mm. (two inches) long. It lies within the fibrous 

 pericardium, enclosed, with the ascending part of the aorta, 

 in a common sheath of the serous pericardium. It 

 commences at the upper end of the conus arteriosus, 

 posterior to the sternal extremity of the third left costal 

 cartilage ; and it runs, backwards and upwards, into the con- 

 cavity of the aortic arch, where it bifurcates into two branches, 



