96 THE CIRCULATION. 



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 constructed 

 than those of the pulmonary artery. Like the tricuspid and 

 mitral valves, they are formed by a duplicature 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 bor- 

 der, 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 semilunar valves are 

 placed side by side around the arterial orifice of each ventricle, 

 so as to form three little pouches, which can be thrown back 

 and flattened by the blood passing out of the ventricle, but 

 which belly out immediately so as to prevent any return (6, 

 Fig. 34). This will be again referred to immediately. 



The muscular fibres of the heart, unlike those of most in- 

 voluntary muscles, present a striated appearance under the 

 microscope. (See chapter on Motion.) 



THE ACTION OF THE HEART. 



The heart's action in propelling the blood consists in the 

 successive alternate contractions and dilatations of the muscu- 

 lar walls of its two auricles and two ventricles. The auricles 

 contract simultaneously; so do the ventricles; their dilatations 

 also are severally simultaneous ; and the contractions of the 

 one pair of cavities are synchronous with the dilatations of the 

 other. 



The description of the action of the heart may best be corn- 



principal anterior columna carnea or musculus papillaris attached to it; 5, 5, mus- 

 culi papillares; 5', the left side of the septum, between the two ventricles, within 

 the cavity of the left ventricle ; 6, 6', the mitral valve ; 7, placed in the interior of 

 the aorta near its commencement and above the three segments of its semilunar 

 valve, which are hanging loosely together ; 7', the exterior of the great aortic sinus; 

 8, the root of the pulmonary artery and its semilunar valves ; 8', the separated por- 

 tion of the pulmonary artery remaining attached to the aorta by 9, the cord of the 

 ductus arteriosus ; 10, the arteries rising from the summit of the aortic arch. (From 

 Quain's Anatomy.) 



