214 THE CIRCULATORY SYSTEM [CH. XIX. 



name musculi papillares has been given, are attached to the wall of 

 the ventricle by one extremity only, the other projecting, papilla- 

 like, into the cavity of the ventricle (5, fig. 192), and having attached 

 to it chordae tendineae. Of the tendinous cords, besides those which 

 pass to the margins of the valves, there are some of especial strength, 

 which pass to the edges of the middle and thicker portions of the 

 cusps. The ends of these cords are spread out in the substance of 

 the valve, giving its middle part its peculiar strength and toughness. 

 Moreover, the musculi papillares are so placed that, from the 

 summit of each, tendinous cords proceed to the adjacent halves of 

 two of the principal divisions, and to one intermediate or smaller 

 division, of the valve. 



The preceding description applies equally to the mitral and 

 tricuspid valve ; but it should be added that the mitral is considerably 

 thicker and stronger than the tricuspid, in accordance with the 

 greater force which it is called upon to resist. 



The semilunar valves guard the orifices of the pulmonary artery 

 and of the aorta. They are nearly alike on both sides of the heart ; 

 but the aortic valves are more strongly constructed than the pul- 

 monary valves, in accordance with the greater pressure which they 

 have to withstand. Each valve consists of three parts which are 

 of semilunar shape, the convex margin of each 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, so as to form 

 a little pouch like a watch-pocket (7, fig. 192). In the centre of 

 the free edge of the pouch, 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 endo- 

 cardium. Thus constructed and attached, the three semilunar 

 pouches are placed side by side around the arterial orifice of each 

 ventricle ; they are separated by the blood passing out of the ventricle, 

 but immediately afterwards are pressed together so as to prevent any 

 return. Opposite each of the semilunar cusps, both in the aorta 

 and pulmonary artery, there is a bulging outwards of the wall of the 

 vessel : these bulgings are called the sinuses of Valsalva. 



Course of the Circulation, 



The blood is conveyed away from the left ventricle (as in the 

 diagram, fig. 195) by the aorta to the arteries, and returned to the 

 right auricle by the veins, the arteries and veins being continuous 

 with each other at the far end by means of the capillaries. 



From the right auricle the blood passes to the right ventricle, then 



