THE RIGHT VENTRICLE. 359 



partition which corresponds to the beginning of the aorta from the left ventricle 

 (fig. 317). The inner surface is marked by muscular bundles, columns earner, some 

 of which are attached by each extremity to the wall of the ventricle and are free in 

 the middle (trabeculw) ; others are only sculptured in relief, as it were, being con- 

 tinuous with the wall of the ventricle in their whole length ; and a third set, forming 

 two principal bundles, an anterior and a posterior, named musculi papillares, are 

 attached at their base to the ventricular wall, and by the other end are prolonged 

 into small tendinous cords (chordae tendinece), through which they are connected 

 with the segments of the auriculo-ventricular valve. The inside~ of the conus 

 arteriosus is smooth, and free from columnae carneae. 



The valve guarding the right auriculo-ventricular opening is composed of three 

 triangular segments or flaps, and is hence named tricuspid. The flaps are mainly 

 formed of fibrous tissue covered by endocardium. At their bases they are continuous 

 with one another, so as to form an annular membrane attached around the margin of 

 the auricular opening : they are directed downwards, and are retained in position 

 within the ventricle by the chordae tendineae, which are attached to their ventricular 



Fig. 311. CROSS SECTION OF THE VENTRICULAR 



PART OP THE HEART AT TWO-THIRDS FROM 

 THE APEX, LOOKING DOWNWARDS INTO THE 



CAVITIES. (Allen Thomson. ) f 



1, 1', wall of right ventricle ; 2, 2', wall 

 of left ; 3, 3', septum ; 4, principal papillary 

 muscle of right ventricle ; 4', some columnae 

 carness on the septum near the front ; 4", 

 posterior papillary muscles ; 5, anterior or 

 left, 5', posterior or right papillary muscles of 

 left ventricle ; 6, deepest part of cavity of 

 right ventricle ; 7, that of left ventricle at apex 

 of heart. 



surfaces and free margins. The middle part of each segment is thicker than the 

 rest, while the marginal part is thin, transparent, and jagged at the edges (compare 

 fig. 315, B, e, e'). Of the three segments, one (fig. 313, 5), which is usually the 

 largest, is situated at the anterior and left part of the opening, and projects down- 

 wards between the latter and the infundibulum, whence it is called the infundibular 

 or left flap ; the second, or right (5') corresponds to the free margin of the ventricle ; 

 and the third, named posterior or septal (5"), is placed internally and posteriorly, 

 against the ventricular septum. 



The anterior papillary muscle (311, 4) is the larger and more constant: its 

 chordae tendineae pass to the cleft between the infundibular and right segments, to 

 be attached to both. The posterior papillary muscle (4") is more irregular, and is 

 often represented by two or three smaller ones : the chordae tendineae proceeding 

 therefrom are attached in like manner to the right and septal segments ; while 

 others forming a third set spring directly from the surface of the septum, sometimes 

 from small eminences upon it (4'), and pass upwards to be attached to the adjacent 

 borders of the left and posterior segments. In the angles between each pair of the 

 principal segments of the auriculo-ventricular valves there may be found, bat not 

 constantly, as many small intermediate lobes. On the other hand, the division 

 between the infundibular and right flaps is often indistinct. 



During the contraction of the ventricle, the segments of the valve are applied to 

 the opening leading from the auricle, and prevent the blood from rushing back into 

 that cavity. Being retained by the chordae tendineae, the expanded flaps of the 

 valve resist the pressure of the blood, which would otherwise force them back 



