700 



HUMAN ANATOMY. 



The Semilunar Valves. — Although really belonging to the pulmonary and 

 systemic aortae, it is con\enient to consider these valves along with the heart, since 

 they prevent the regurgitation of the blood contained in the aortse into the \entricles 

 at the completion of their contraction. 



The segments guarding these \'alves are three in number in each aorta and are 

 attached to the fibrous ring of the aortic orifices. Each segment is a crescentic 

 pouch-like structure, whose cavity is directed away from the heart, so that any ten- 

 dency for the blood to return from the aortae into the ventricles will result in the fill- 

 ing of the pouches so that the three are brought into apposition and effectually close 

 the orifice. Their efiiciency is increased by ( i } the occurrence at the middle of the 

 free edge of each segment of a small fibro-cartilaginous nodule, the nodidc of Aran- 

 tius, which fills the small gap which might otherwise be left at the point of meeting of 

 the free edges of all three segments ; and by (2) the aorta being pouched out behind 

 each segment to form a small pocket, a sinus of Valsalva, greater opportunity being 

 thus allowed for the blood to enter the cavities of the valves and so force their free 

 edges together. 



The segments of the semilunar valves of the systemic aorta (valviilae semilunares 

 aortae) are somewhat stronger than those of the pulmonary aorta (valvulae semilu- 



FlG. 662. 



Post, leaflet of pulm. valve 



Left coronary artery, 

 behind left posterior leaflet 



Anterior cusp of left 



auriculo-ventricular valve 



Posterior cusp — '. 



Fibrous ring surrounding 



mitral valve - — 



ght coronary artery, 

 behind anterior leaflet 



Right posterior leaflet of 

 aortic valve 



Anterior cusp of right 



auriculo-\entricular valve 



Medial cusp 



\r- Posterior cusp 



T — • Fibrous ring surrounding 

 tricuspid valve 



Valves of heart viewed from above, after removal of auricles and greater part of aortse. 



nares a pulmonalis), and are arranged, if considered with reference to the planes of 

 the body, the heart being in situ, so that one is situated anteriorly and the other 

 two right and left posteriorly. In the pulmonary aorta one valve segment will be 

 posterior and the others right and left anteriorly. If, however, the heart be held 

 so that its ventricular septum lies in the sagittal plane, then the valve segments differ 

 by 60° from the relative position given above, those of the pulmonary artery being- 

 arranged so that one lies anteriorly and the other two right and left posteriorly, 

 while in the systemic aorta one is posterior and the other two right and left anter- 

 iorly, an arrangement to be expected from the manner of development of the 

 valves (page 710). 



The Architecture of the Heart Muscle. — The musculature of the ^valls 

 of the auricles is relatively very thin, and it is difficult to distinguish any definite 

 arrangement of its fibres in layers. Groups of fibers can, however, be distin- 

 guished, and of these certain are confined to each auricle, while others are common 

 to the two. 



Of the fasciculi proper to each auricle two principal groups can be recognized. 



I. Annular fasciculi, which surround the orifices of the veins entering the 

 auricles, and represent the continuation of the circular muscle layer of the veins into 

 the auricular walls. 



