HEART LEFT AURICLE. 695 



The columnar carnese are the rounded muscular columns which project from 

 nearly the whole of the inner surface of the ventricle, excepting near the opening 

 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. Tho second set are attached by 

 their two extremities, but are free in the rest of their extent ; whilst the third set 

 (columnse papillares), three or four in number, are attached by one extremity to 

 the Avail 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 they are pressed during the passage of the blood along its canal. 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 consti- 

 tuent 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 contact with one another by their opposed surfaces, the three fibro-cartilagi- 

 nous 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 LEFT AURICLE is rather smaller but thicker than the right, measuring about 

 one line and a half; it consists 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 auricu- 

 larum ; and behind, it receives on each side the pulmonary veins, being free in 

 the rest of its extent. 



The appendix auriculas is somewhat constricted at its junction with the auricle: 

 it is longer, narrower, and more curved than that of the right side, and its mar- 

 gins more deeply indented, presenting a kind of foliated appearance. Its direction 

 is forwards towards the right side, overlapping the root of the pulmonary 

 artery. 



In order to examine its interior, a horizontal incision should be made along the attached border 

 of the auricle to the ventricle ; and, from the middle of this, a second incision should be carried 

 upwards. 



The following parts then- present themselves for examination : 



The openings of the four pulmonary veins. 

 Auriculo-ventricular opening. 

 Musculi pectinati. 



