HEART LEFT AURICLE. 761 



nary artery. Another segment corresponds to the front of the ventricle ; and 

 a third to its posterior wall. The central part of each segment is thick and 

 strong ; the lateral margins are thin and indented. The chordae tendineee are 

 connected with the adjacent margins of the principal segments of the valve, and 

 are further attached to each segment 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 columnse carnese are the rounded muscular columns which project from 

 nearly the whole of the inner surface of the ventricle, excepting near the open- 

 ing of the pulmonary artery. They may be classified, according to their mode 

 of connection with the ventricle, into three sets. The first set merely form 

 prominent ridges on the inner surface of the ventricle, being attached by their 

 entire length on one side, as well as by their extremities. The second set are 

 attached by their two extremities, but are free in the rest of their extent; Avhilst 

 the third set (columnse papittares), three or four in number, are attached by one 

 extremity to the wall of the heart, the opposite extremity giving attachment to 

 the chordse tendinese. 



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

 artery. They consist of three semicircular folds, formed by a duplicature of the 

 lining membrane, strengthened by fibrous tissue. They are attached, by their 

 convex margins, to the wall of the artery, at its junction with the ventricle, the 

 straight border being free, and directed upwards in the course of the vessel, 

 against the sides of which the valve-flaps are pressed during the passage of the 

 blood along the artery. The free margin of each is somewhat thicker than the 

 rest of the valve, is strengthened by a bundle of tendinous fibres; and presents, 

 at its middle, a small projecting fibro-cartilaginous nodule, called corpus Arantii. 

 From this nodule, tendinous fibres radiate through the valve to its attached 

 margin, and these fibres form a constituent part of its substance throughout its 

 whole extent, excepting two narrow lunated portions, placed one on either side 

 of the nodule, immediately behind the free margin ; here, the valve is thin, and 

 formed merely by the lining membrane. During the passage of the blood 

 along the pulmonary artery, these valves are pressed against the sides of its 

 cylinder, and the course of the blood along the tube is uninterrupted; but 

 during the ventricular diastole, when the current of blood along the pulmonary 

 artery is checked, and partly thrown back by its elastic walls, these valves 

 become immediately expanded, and effectually close the entrance of the tube. 

 When the valves are closed, the lunated portions of each are brought into con- 

 tact with one another by their opposed surfaces, the three fibro-cartilaginous 

 nodules filling up the small triangular space that would be otherwise left by 

 the approximation of the three semilunar folds. 



Between the semilunar valves and the commencement of the pulmonary 

 artery are three pouches or dilatations, one behind each valve. These are the 

 pulmonary sinuses (sinuses of Valsalva). Similar sinuses exist between the 

 semilunar valves and the commencement of the aorta; they are larger than the 

 pulmonary sinuses. The blood, in its regurgitation towards the heart, finds its 

 way into these sinuses, and so shuts down the valve-flaps. 



The Left Auricle is rather smaller but thicker than the right, measuring 

 about one line and a half; it consists, like the right, of two parts, a principal 

 cavity or sinus, and an appendix auricula. 



The sinus is cuboidal in form, and concealed in front by the pulmonary 

 artery and aorta; internally, it is separated from the right auricle by the septum 

 auricularum; behind, it receives on each side the pulmonary veins, being free 

 in the rest of its extent. 



The appendix auriculae is somewhat constricted at its junction with the auri- 



