118 ANATOMY OF UPPER EXTREMITY, ETC. 



The columnae carnese are the muscular columns, inter- 

 lacing in all directions, which give the ventricle its charac- 

 teristic appearance. 



The chordae tendineae are small tendinous bands attached 

 to certain of the columnae carnese, and extending from them 

 to the free edge of the tricuspid valve ; they interlace with 

 each other, and several of them converge to one column 

 for attachment. 



The tricuspid valve consists of three or more folds of 

 the lining membrane of the heart, strengthened by fibrous 

 tissue, and attached by their base to the auriculo-ventricu- 

 lar orifice, and by their free edge, which is usually a little 

 thicker than elsewhere, to the chordae tendineae. This valve 

 obstructs the regurgitation of the blood from the ventricle 

 to the auricle, the chordae tendineae preventing its flaps from 

 being pressed through into the auricle. 



The infundibulum, or conus arteriosus, is that dilated 

 portion of the ventricle from which the pulmonary artery 

 arises; it is, as it were, separated from the rest of the 

 ventricle by a sort of constriction ; it has fewer columnae 

 carneae than other parts of the cavity. 



The lining membrane, at the commencement of the pul- 

 monary artery, forms three crescentic folds, calle'd sigmoid, 

 or semilunar valves. Attached by their base, they are 

 free along their concave margin, and look upward in the 

 course of the vessel. The margin of these valves is often 

 perforated by small openings, and each valve contains in 

 the centre of its concavity a little fibrous nodule, called the 

 corpus Arantii; this is sometimes directly at the edge, at 

 other times a little distant from it. Behind each of these 

 valves the pulmonary artery forms dilatations like those 

 similarly placed in the aorta, but not nearly so well marked. 



The left auricle may be opened by a transverse incision along its 

 ventricular border. 



N 



The LEFT AURICLE is smaller than the right, but has, 

 however, somewhat thicker walls. It has four openings for 

 the pulmonary veins, two upon each side ; occasionally, two 

 of them coalesce. The septum auriculae has, upon this side, 

 no trace of the fossa ovalis, except when an opening exists, 

 and then a small valvular fold may be observed. The mus- 

 culi pectinati are found only in the appendix. The auriculo- 

 ventricular- orifice is smaller than that of the right side. 



