210 THE CIRCULATORY SYSTEM. [CH. xvm, 



tendinous rings, their ventricular surface and borders are fastened 

 by slender tendinous fibres, the chordce tendinece, to the internal 

 surface of the walls of the ventricles, the muscular fibres of which 

 project into the ventricular cavity in the form of bundles or columns 

 the colwmnas carnece. These columns are not all alike, for while 

 some are attached along their whole length on one side, and by 

 their extremities, others are attached only by their 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. 209), and having attached to it chordae tendinese. 

 Of the tendinous cords, 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 one between each two 

 principal 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 con- 

 siderably 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 Qf 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 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. 209). 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 



