932 



THE ORCAXS OF CIRCILATIOX 



The Openings and their Valves 



The orifice of the pulmonary artery is circular and obliquely placed at the 

 suniiuit of the infundilmlum near the scptuui. It is guarded ])y three valves, the 

 pulmonary semilunar valves. Of these two are placed anteriorly and one pos- 

 teriorlv (tig. ")4S). liuiucdiately above and liehind each semilunar valve there is 

 a ])<incli or sinus. These collectively constitute the i)ulnionary sinuses, or sinuses 

 of Valsalva. 



Each valve is formed of a fibrous layer within a redu])lication of the lining 

 membrane, which is continuous on the upper surface with the innermost coats of 

 the artery, and on the opposite surface with endocardium of the ventricle. 



In the centre of the free straight edge of each valve there is a little fibro-carti- 

 laginous nodule, the corpus Arantii, and this margin is further strengthened by a 

 delicate tendinous band. Another fibrous band in like manner strengthens the 

 convex attached portion of the valve, and from this a third set of obli<juely inter- 

 lacing filires pass througliout the whole valve towards the nodule. Two narrow 

 crescent-s4iaped areas, the lunulae, near the free edge on each side of the nodide, 

 remain almost free from this fibrous invasion, and it is these thinner portions 

 which are in apposition during closure of the valve. A fibrous ring strengthens the 

 pulmonary orifice, giving attachment beloAv to the muscular fibres of the heart; whilst 

 above, opposite the sinuses of Valsalva, it is deeply hollowed into three semilunar 



Fig. 551. — Interior View of the Aortic Semilunar Valves. 



n 



Sinus of Valsalva 



Section of fibrous ring 



Free edge of valve 



Orifice of right coronary 

 artury 



Corpus Arantii 

 ~ Semilunar valve 



Decussating fibrous tissue 

 of valve 



notches. The valves are attached to the edges of these notches as Avell as to the 

 horns which project inwards and separate them from one another. (See fig. 551, of 

 the aortic valves, in wliich these characters are present and more strongly marked. ) 



The auriculo- ventricular opening is oval and guarded by the tricuspid valve. 

 The three triangular flaps of this valve are continuous with one another at their 

 broad ends, and so form a continuous fold around the orifice; but Ijeyond, they 

 ])roject with jagged and sharply dentated edges towards the apex of the ventricle. 



The chordie tendinea% which chiefly arise from the papillary muscles already 

 described, pass to their free borders and ventricular surfaces. The largest segment 

 of the valve is placed in front, between the auriculo-ventricular orifice and the 

 infnndibulum, the smallest behind near the septum, and tlic third, which is the 

 most movable, is situated on the right. 



Smaller segments intervene l)etween the larger flaps. The cliorda^ tendinea', 

 which arise in groups from the papillary muscles, divide as they pass to l)e attached 

 to the edges and ventricular surfaces of the neighbouring segments. Additional 

 cords are furnislied from the ventricular walls, and es{)ecially from the sei)tum to 

 the small segment, and soine of these are provided with little pajiillary muscles. 



The segments of this valve, except at the extremities and margins, contain 

 abundant fibrous and a small amount of muscular tissue. They are attached by 

 their thickened bases to a fibrous ring which surrounds and strengthens the orifice. 

 The surfaces whicli lof)k towards the o]ioning arc smooth, wliilst u])on the op])Osite 



