104 HANDBOOK OF PHYSIOLOGY. 



extremities ; and a third set, to which the 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. 89), and having attached to it chordae tendineae. Of the tendi- 

 nous chords, besides those which pass from the walls of the ventricle 

 and the musculi papillares to the margins of the valves, there are some 

 of especial strength, which pass from the same parts to the edges of the 

 middle and thicker portions of the cusps before referred to. The ends 

 of these cords are spread out in the substance of the valve, giving its 

 middle piece its peculiar strength and toughness ; and from the sides 

 numerous other more slender and branching cords are given off, which 

 are 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 princi- 

 pal ones. 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, three in number, guard the orifices of the pul- 

 monary artery and of the aorta. They are nearly alike on both sides of 

 the heart ; but the aortic valves are altogether thicker and more strongly 

 constructed than the pulmonary valves, in accordance with the greater 

 pressure which they have to withstand. 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, so that each valve forms a little pouch like a watch- 

 pocket (7, Fig. 90). In the centre of the free edge of the valve, which 

 contains a fine cord of fibrous tissue, is a small fibrous nodule, I\\Q -cor- 

 pus Arantii, and from this and from the attached border fine fibres ex- 

 tend into every part of the mid substance of the valve, except a small 

 lunated space just within the free edge, on each side of tine corpus Aran- 

 tii. 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 separated by the blood 

 passing out of the ventricle, but which immediately afterwards are 

 pressed together so as to prevent any return (7, Fig. 89, and 7, Fig. 90). 

 This will be again referred to. 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. 



