1090 THE THORAX. 



And a muscular and tendinous apparatus connected with the tricuspid valve : 

 Columns carneae. Chordae tendineae. 



The auriculo-ventricular orifice is the large oval aperture of communication 

 between the auricle and ventricle. It is situated at the base of the ventricle, near 

 the right border of the heart. The opening is about an inch in diameter, 1 oval 

 from side to side, surrounded by a fibrous ring, covered by the lining membrane 

 of the heart, and rather larger than the corresponding aperture on the left side. It 

 is guarded by the tricuspid valve. 



The opening of the pulmonary artery is circular in form, and situated at the 

 summit of the conus arteriosus, close to the septum ventriculorum. It is placed on 

 the left side of the auriculo-ventricular opening, upon the anterior aspect of the 

 heart, and, when viewed from above, on cross-section, the aortic opening is seen 

 intervening. Its orifice is guarded by the pulmonary semilunar valves. 



The tricuspid valve consists of three segments of a triangular or trapezoidal 

 shape, formed by a duplicature of the lining membrane of the heart, reflected on 

 both sides of a layer of fibrous tissue, which contains, according to Kurschner and 

 Senac, muscular fibres. These segments are connected by their bases to the auriculo- 

 ventricular orifice, and to 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 ten- 

 dinous cords, the chordae tendinece. The largest and most movable segment, placed 

 toward the left and anterior side of the auriculo-ventricular opening, is directed 

 downward between that opening and the infundibulum (left or infundibular flap). 

 Another segment corresponds to the front and right of the ventricle (right flap), 

 and a third to its posterior wall (posterior or septal flap]. The central part of each 

 segment is thick and strong ; the lateral margins are thin and indented. The 

 chordce tendinece are connected with the segments of the valve 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 columnce carnece are muscular columns which project from the inner sur- 

 face, excepting near the opening of the pulmonary artery, where the wall becomes 

 smooth. They may be classified into three sets : The first merely form prominent 

 ridges ; the second set (trabeculce) are attached by their two extremities only ; 

 whilst 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. The chordce tendinece of 

 the former go to the left and right segments. Those of the latter, which is often 

 replaced by two or three smaller ones, pass to the right and septal segments. There 

 is still another set of chordae which arise directly from the septum and pass to the 

 septal and left segments. 



The semilunar valves, three in number, 2 guard the orifice of the pulmonary 

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

 one posterior, formed by a fibrous membrane, covered above by the inner coat of 

 the artery and below by a reflection of the endocardium. They are attached by 



1 In the Pathological Transactions, vol. vi. p. 119, Dr. Peacock has given some careful researches 

 upon the weight and dimensions of the heart in health and disease. He states, as the result of his 

 investigations, that in the healthy adult heart the right auriculo-ventricular aperture has a mean cir- 

 cumference of 54.4 lines, or 4|f inches ; the left auriculo-ventricular aperture, a mean circumference 

 of 44.3 lines, or Sff inches; the pulmonic orifice, of 40 lines, or 3|f inches; and the aortic orifice, 

 of 35.5 lines, or 3fa inches; but the dimensions of the orifices varied greatly in different cases, 

 the auriculo-ventricular aperture having a range of from 45 to 60 lines, and the others in the same 

 proportion. 



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

 Hand, of St. Paul, Minn., in the North- Western Med. and Surg. Jour., July, 1873), and the same 

 variation is more frequently noticed in the aortic semilunar valves. 



