464 THE BLOOD-VASCULAR SYSTEM. 



muscular fibres. These segments are connected by their bases to the fibrous ring 

 surrounding the auriculo-ventricular orifice, and by their sides with one another, so 

 as to form a continuous annular membrane, which is attached round the margin of 

 the auriculo-ventricular opening, their free margins and ventricular surfaces affording 

 attachment to a number of delicate tendinous cords, the chordce tendinece. The 

 largest and most movable segment is placed toward the left side of the auriculo- 

 ventricular opening, interposed between that opening and the infundibulum ; hence 

 it is called the left or infundibular cusp. Another segment corresponds to the 

 right part of the front of the ventricle, the right or marginal cusp; and a third to 

 its posterior wall, the posterior or septal cusp. The central part of each segment 

 is thick and strong : the lateral margins are thin and translucent. The chordae 

 tendineae are connected with the adjacent margins of the principal segments of the 

 valve, and are further attached to each segment in the following manner : 1. Three 

 or four reach the attached margin of each segment, where they are continuous with 

 the auriculo-ventricular tendinous ring. 2. Others, four to six in number, are 

 attached to the central thickened part of each segment. 3. The most numerous 

 and finest are connected with the marginal portion of each segment. 



The columncB carnece are the rounded muscular columns which project from 

 nearly the whole of the inner surface of the ventricle, excepting near the opening 

 of the pulmonary artery, where the wall is smooth. They may be classified, accord- 

 ing to their mode of connection with the ventricle, into three sets. The first set 

 merely form prominent ridges on the inner surface of the ventricle, being attached 

 by their entire length on one side, as well as by their extremities. The second set 

 are attached by their two extremities, but are free in the rest of their extent; while 

 the third set (musculi papillares) are attached by one extremity to the wall of the 

 heart, the opposite extremity giving attachment to the chordce tendinece. There 

 are two papillary muscles, anterior and posterior: of these, the anterior is the larger; 

 its chordae tendineoe are connected with the right and left segments of the valve. 

 The posterior is not always single, but sometimes consists of two or three muscular 

 columns ; . its chordae tendineae are connected with the posterior and the right seg- 

 ments. In addition to these, some few chordae may be seen springing directly from 

 the ventricular septum, or from small eminences on it, and passing to the left and 

 posterior segments. A fleshy band, well marked in the ox and some other animals, 

 is frequently seen passing from the base of the anterior papillary muscle to the 

 interventricular septum. From its attachments it may assist in preventing over- 

 distention of the auricle, and so has been named the moderator band. 



The right auriculo-ventricular orifice allows the blood to pass freely from the 

 right auricle into the right ventricle, and it will be noted that the surface of the 

 tricuspid valve next the blood-current is quite smooth. When the right ventricle 

 contracts to force the blood into the pulmonary artery, the segments of the tricuspid 

 valve come together and close the auriculo-ventricular opening, and so prevent the 

 blood from passing back into the auricle. The papillary muscles and chordse ten- 

 dineas moor the segments of the valve, and prevent their being forced through into 

 the auricle by the weight of blood behind them. 



The semilunar valves, three in number, 1 guard the orifice of the pulmonary 

 artery. They consist of three semicircular folds, two anterior (right and left) and 

 one posterior, formed by a duplicature of the lining membrane, strengthened by 

 fibrous tissue. They are attached, by their convex margins, to the wall of the 

 artery, at its junction with the ventricle, the straight border being free, and 

 directed upward in the lumen of the vessel. The free margin of each is some- 

 what thicker than the rest of the valve, is strengthened by a^bundle of tendinous 

 fibres, and presents, at its middle, a small projecting thickened nodule, called cor- 

 pus Arantii, and consisting of bundles of interlacing connective-tissue fibres with 

 branched connective-tissue cells and some few elastic fibres. From this nodule 



1 The pulmonary semilunar valves have been found to be two in number instead of three (Dr. 

 Hand, of St. Paul, Minn., in North Western Med. and Swg. Journ., July, 1873), and the same variety 

 is more frequently noticed in the aortic semilunar valves. 



