536 LEFT AURICLE. 



of muscular columns interlacing in almost every direction. They are 

 divided into three sets, according to the manner of their connection. 

 1. The greater number are attached by the whole of one side, and 

 merely form convexities into the cavity of the ventricle. 2. Others 

 are connected by both extremities, being free in the middle. 3. A 

 few (columnae papillares) are attached by one extremity to the walls 

 of the heart, and by the other give insertion to the chordae tendineae. 



The Semilunar valves, three in number, are situated around the 

 commencement of the pulmonary artery, being formed by a folding of 

 its lining membrane, strengthened by a thin layer of fibrous tissue. 

 They are attached by their convex borders, and free by the concave 

 which are directed upwards in the course of the vessel, so that, during 

 the current of the blood along the artery they are pressed against the 

 sides of the cylinder ; but if any attempt at regurgitation ensue they 

 are immediately expanded, and effectually close the entrance of the 

 tube. The margins of the valves are thicker than the rest of their 

 extent, and each valve presents in the centre of this margin a small 

 fibro-cartilaginous tubercle, called corpus Arantii* which locks in 

 with the two others during the closure of the valves, and secures the 

 triangular space that would otherwise be left by the approximation of 

 three semilunar folds. 



Between the semilunar valves and the cylinder of the artery are 

 three pouches, called the pulmonary sinuses (sinuses of Valsalva). 

 Similar sinuses are situated behind the valves at the commencement of 

 the aorta, and are larger and more capacious than those of the pulmo- 

 nary artery. 



The Pulmonary artery commences by a scalloped border, corre- 

 sponding with the three valves which are attached along its edge. It is 

 connected to the ventricle by muscular fibres, and by the lining mem- 

 brane of the heart. 



The LEFT AURICLE is somewhat smaller, but thicker, than the 

 right ; of a cuboid form, and situated more posteriorly. The appendix 

 auriculas is constricted at its junction with the auricle, and has a fo- 

 liated appearance ; it is directed forwards towards the root of the 

 pulmonary artery, to which the auriculae of both sides appear to 

 converge. 



The left auricle is to be laid open by a - 1 - shaped incision, the hori- 

 zontal section being made along the border which is attached to the 

 base of the ventricle. It presents for examination five openings, and 

 the muscular structure of the appendix ; these are, 



Four pulmonary veins, 

 Auri culo- ven trie ular openin g, 

 Musculi pectinati. 



* Julius Caesar Arantius, Professor of Medicine in Bologna. He was a dis- 

 ciple of Vesalius, one of the founders of modern anatomy. His treatise " De 

 Humano Foctu" was published at Rome, in 1564. 



