LEFT AURICLE AND VENTRICLE. 475 



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 Jlrantii,* 

 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. Similar sinuses are 

 situated behind the valves at the commencement of the aorta, and 

 are much larger and more capacious than those of the pulmonary 

 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 

 membrane of the heart. 



The LEFT AURICLE is somewhat smaller than the right; of a 

 cuboid form, and situated more posteriorly. The appendix auriculce 

 is constricted at its junction with the auricle, and has an arborescent 

 appearance ; it is directed forwards towards the root of the pul- 

 monary artery, to which the auriculae of both sides appear to con- 

 verge. 



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

 horizontal section being made along the border which is attached to 

 the base of the ventricle. 



It presents for examination five openings, and the muscular struc- 

 ture of the appendix ; these are, 



Four pulmonary veins, 

 Auriculo-ventricular opening, 

 Musculi pectinati. 



The Pulmonary veins, two from the right and two from the left 

 lung, open into the corresponding sides of the auricle. The two left 

 pulmonary veins terminate frequently by a common opening. 



The Auriculo-ventricular opening is the aperture of communica- 

 tion between the auricle and ventricle. 



The Musculi pectinati are fewer in number than in the right 

 auricle, and are situated only in the appendix auriculas. 



LEFT VENTRICLE. The left ventricle is to be opened, by making 

 an incision a little to the left of the septum ventriculorum, and con- 

 tinuing it around the apex of the heart, to the auriculo-ventricular 

 opening behind. 



The left ventricle is conical, both in external figure and in the 

 form of its internal cavity. It forms the apex of the heart, by pro- 

 jecting beyond the right ventricle, while the latter has the advantage 



* Julius Csesar Arantius, Professor of Medicine in Bologna. He was a disciple of 

 Vesalius, one of the founders of modern anatomy. His treatise " De Human! Fcetu," 

 was published at Rome, in 1564. 





