198 OUTLINES OP EQUINE ANATOMY. 



ventricular opening, consisting of a smTonnding fibrous 

 ring, from whicli hang three flaps, composed of double 

 layers of endocardmm (the serous membrane lining the 

 heart, which resembles the internal coat of blood-vessels), 

 which serve to regulate the passage of the blood from one 

 cavity to the other. They are triangular in shape, having 

 their outer boundary attached to the circular fibrous ring, 

 with its terminating angles confluent with the other two 

 sides which form the free margin to which powerful fibrous 

 cords (cliordce tendinece) are attached. These chordae ten- 

 dinese nin to large projections from the inner surface of 

 the muscular wall of the ventricle, which receive the 

 special name musculi loapillares ; they are three in number, 

 one projecting from the outer wall, two from the septum 

 ventriculorum. From one musculus papillaris chordae ten- 

 dinese run to each of the three flaps of the tricuspid valve. 

 The wall of the right ventricle is thicker than that of 

 the auricle, and presents on its internal surface three 

 forms of muscular prominences (carnese columnae). The 

 mnsculi impillares, having one end free, constitute the 

 first set; the moderator lands run from one side of the 

 cavity to the other, often consist simply of yellow elastic 

 tissue covered by serous membrane, they form the second ; 

 while the third or true carnece columnce are attached by 

 both ends and by one margin, thus resembling the musculi 

 pectinati of the auricle. The right ventricle does not 

 extend to the apex of the heart, which is wholly formed by 

 the left, but superiorly it forms a funnel-shaped cavity 

 (termed conus arteriosus), behind the posterior flap of the 

 tricuspid valve, from the upper extremity of which the 

 pulmonary artery commences. The communication of the 

 ventricle with this vessel is guarded by three crescentic 

 flaps which constitute the semilunar pidmonary valves. They 

 have a free and an attached margin. The free margin 

 centrally presents a point on which we sometimes see a 

 small nodule {corpusculum arantii), which serves to ensure 

 complete closure of the opening by the valves. The 

 attached margin is continuous with the pulmonary fibrous 

 ring and its extremities meet the other flaps. The 

 pulmonary artery runs backwards and in passing under 

 the posterior aorta is connected to it by an imi)ervious cord, 

 the remains of the ductus arteriosus of the foetus. After this 

 it bifurcates, its larger division passing to the right lung, 



