THE CHAMBERS OF THE HEART. 699 



wall of the conus arteriosus. The ring surrounding the left auriculo-ventricular 

 orifice is somewhat thicker than that of the right, and is fused with the systemic aortic 

 ring throughout about the medial third of its circumference, whereas the correspond- 

 ing fusion of the right ring is of much less extent. In the angle formed by the 

 junction of the right auriculo-ventricular ring at the side with the systemic aortic 

 ring in front a special thickening of the fibrous tissue occurs, so that it becomes of 

 almost cartilaginous consistency, and a similar, although smaller, thickening also 

 occurs in the angle formed by the junction of the anterior walls of the left auriculo- 

 ventricular and systemic aortic rings. These thickenings form what are termed the 

 right and left auriculo-ventricular nodes (trigona fibrosa), and they are of interest as 

 being occasion9.11y the seat of a calcareous deposit or of a fatty infiltration, a condi- 

 tion which may be shared by fibre-like prolongations of the nodes i^fila coro7iaria) 

 which extend into adjacent portions of the auriculo-ventricular rings. 



The Auriculo- Ventricular Valves. — Attached by its base to each auriculo- 

 ventricular fibrous ring, and projecting downward into the cavity of the correspond- 

 ing ventricle, is a valve having the general form of a membranous cone, whose walls 

 are of thin but strong fibrous tissue covered on both sides by the endocardium. Each 

 cone, however, is divided by deep incisions into triangular segments, of which there 

 are three in the valve of the right ventricle, whence it is usually termed the tricuspid 

 valve, while two incisions divide the left valve into two segments and procure for it 

 the name of the bicuspid or mitral valve, the latter term being suggested by its 

 resemblance to a bishop's mitre. Of the three segments of the tricuspid valve, 

 one (ciispis anterior), larger than the others and also known as the infundibular 

 cusp is attached to the anterior border of the auriculo-ventricular orifice ; a second 

 one (cuspis posterior) is attached to the posterior border ; while the third or septal 

 (ciispis medialis) occupies the interval between the medial edges of the other two, 

 and is attached to that portion of the auriculo-ventricular fibrous ring which is 

 united to the right auriculo-ventricular node and to the upper part of the ventricular 

 septum. In the mitral valve one segment (cuspis posterior) is attached to the 

 posterior border of the auriculo-ventricular fibrous ring, while the other (cuspis 

 anterior) or aortic cusp is situated anteriorly, and depends from that portion of 

 the ring which is united to the ring surrounding the systemic aortic orifice, and 

 consequently appears to be a downward prolongation from the posterior border of 

 that orifice. It is to be noted that the depths of the incisions separating the seg- 

 ments of both valves vary considerably, and additional incisions may occur, resulting 

 in the formation of additional segments. Not infrequently a small accessory segment 

 occupies the apex of one or more of the incisions. , 



These valves, while permitting the free passage of blood from the auricles into the 

 ventricles, prevent its passage in the reverse direction during the contraction of the 

 ventricles ; for the pressure of the blood within the ventricles forces the segments up- 

 ward so that they completely occlude the auriculo-ventricular orifices, the chordae 

 tendineae which are attached to them, and which are rendered taut by the contraction 

 of the papillary muscles, preventing them from being forced back into the auricles. 

 The musculi papillares of each ventricle are arranged in two groups, one consisting 

 of small papillae, situated near the upper portion of the ventricle behind the segments 

 of the valves, and the other, composed of larger conical muscles, situated nearer the 

 apex. The chordae tendineae which arise from the upper group are short, and are 

 attached to the ventricular surface of the valve near its base ; those which arise from 

 the lower group are much larger, and are attached to the edges of the valve and to 

 its ventricular surface near its free edge. The papillary muscles belonging to this 

 lower group tend to be arranged in sets corresponding in position with the incisions 

 which separate the segments of the valves, and there are, accordingly, three sets in 

 the right ventricle and two in the left ; but this arrangement is not quite definite, and 

 there is also considerable variation in the number of papillary muscles in each set, 

 only one being present in some cases and several in others. However that may be, 

 the chordae tendineae arising from the apices of the muscles of each set diverge as 

 they pass upward and are attached to both the adjacent segments of the valve. When 

 distinct accessory segments occur, they also receive the insertion of some of the 

 chordae tendineae. 



