i08 THE CIRCULATION. 



attached all over the ventricular surface of the adjacent 

 border-pieces of the principal portions of the valves, as 

 well as to those smaller portions which have been mentioned 

 as lying between each two principal ones. Moreover, the 

 musculi papillares are so placed that from the summit of 

 each tendinous cords may proceed to the adjacent halves of 

 two of the principal divisions, and to one intermediate or 

 smaller division, of the valve. 



It has been already said that while the ventricles com- 

 municate, on the one hand, with the auricles, they communi- 

 cate, on the other, with the large arteries which convey the 

 blood away from the heart; the right ventricle with the pul- 

 monary artery (6, fig. 34), which conveys blood to the lungs, 

 and the left ventricle with the aorta, which distributes it 

 to the general system (7, fig. 35). And as the auriculo- 

 ventricular orifice is guarded by valves, so are also the 

 mouths of the pulmonary artery and aorta (figs. 34, 35). 



The valves, three in number, which guard the orifice of 

 each of these two arteries, are called the semilunar valves. 

 They are nearly alike on both sides of the heart ; but those 

 of the aorta are altogether thicker and more strongly con- 

 structed than those of the pulmonary artery. Like the 

 tricuspid and mitral valves, they are formed by a dupli- 

 cature of the lining membrane of the heart, strengthened 

 by fibrous tissue. Each valve is of semilunar shape, its 

 convex margin being attached to a fibrous ring at the 

 place of junction of the artery to the ventricle, and the 

 concave or nearly straight border being free (fig. 35). In 

 the centre of the free edge of the valve, which contains a 

 fine cord of fibrous tissue, is a small fibrous nodule, the 

 corpus Arantii, and from this and from the attached border 

 fine fibres extend into every part of the mid substance of 

 the valve, except a small lunated space just within the 

 free edge, on each side of the corpus Arantii. Here the 

 valve is thinnest, and composed of little more than the 

 endocardium. Thus constructed and attached, the three 



