THE THORAX 979 



right and posteriorly, whilst the pulmonary orifice is situated on 

 the left and anteriorly, being also on a higher level than the other. 

 Vence minima cordis are said to open into the right ventricle. 



The auriculo-ventricular, or tricuspid, orifice is, as already stated 

 in connection with the right auricle, oval, and it readily admits the 

 index, middle, and ring fingers held close together. It allows the 

 venous blood to flow from the right auricle into the right ventricle, 

 and, in order to prevent regurgitation of blood from the ventricle 

 into the auricle during the ventricular systole, it is guarded by an 

 important valve, called the right auriculo-ventricular or tricuspid 

 valve. This valve is composed of three segments or cusps, which 

 are covered with endocardium. These project into the cavity of 

 the ventricle, and are triangular. The bases of the cusps are con- 

 tinuous with one another, and form a ring, which is attached to 

 the margin of the auriculo-ventricular orifice. The largest cusp is 

 situated in front, and to the left, of the auriculo-ventricular orifice. 

 It inter\-enes between that orifice and the infrmdibulum, and is 

 known as the left or infundibular cusp. Another cus pis situated 

 behind the auriculo-ventricular orifice, and is in contact with the 

 interventricular septum. It is called the posterior or septal cusp. 

 The third cusp is situated to the right, near the margo acutus, and 

 is called the right or marginal cusp. In the angular intervals 

 between the basal parts of the three large cusps there are usually 

 three small cusps. 



Each cusp consists of two layers of endocardium, with fibrous 

 tissue between them, especially at their central parts. The middle 

 portion of each cusp is therefore thicker than the marginal portions, 

 these latter being thin and transparent. The margins themselves 

 are notched. The auricular surfaces of the cusps are smooth, but 

 their ventricular surfaces are roughened by the chordae tendineae, 

 which are also attached to the margins. 



The chordae tendinese are filiform, fibrous processes which are 

 connected, on the one hand, with the wall of the ventricle, and, on 

 the other, with the cusps of the auriculo-ventricular valve. Most 

 of them spring from the two papillary muscles, but a few of them 

 arise from the inter\'entricular septum and from small papillary 

 eminences upon it. Those which are connected with the anterior 

 papillary muscle pass to the internal between the infundibular and 

 marginal cusps ; those which are connected with the posterior 

 papillary muscle and its subdivisions pass to the interval between 

 the septal and marginal cusps ; and those which are connected 

 with the interventricular septum pass to the interval between the 

 infundibular and septal cusps. The connection of the chordae 

 tendineae with the cusps of the valve is of a threefold nature, as 

 follows : (i) the most numerous are connected with the marginal 

 parts of the cusps ; (2) others are connected with the thickened 

 central portion of each cusp ; and (3) a few pass to the basal portion 

 of each cusp, where they are connected with the fibrous ring around 

 the auriculo-ventricular orifice. 



