THE HEART. 1091 



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

 straight border being free, and directed somewhat upward in the lumen of the 

 1. 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 thickened nodule called corpus Arantii. 1 From this nodule tendinous 

 fibres radiate through the valve to its attached margin, and these fibres form a con- 

 stituent part of its substance throughout its whole extent, excepting two narrow 

 lunated portions (lunulce) placed on each 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 the cylinder and the course of the blood along the tube is unin- 

 terrupted : 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 corpora Arantii filling up the small 

 triangular space that would be otherwise left by the approximation of the three 

 semilunar valves. 



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 (*!nu*e% of Vahalva). 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 toward the heart, finds its way into these sinuses, 

 and so shuts down the valve-flaps. 



Iu order to examine the interior of the left auricle, make an incision on the posterior surface 

 of the auricle from the pulmonary veins on one side to those on the other, the incision being 

 carried a little way into the vessels. Make another incision from the middle of the horizontal 

 one to the appendix. 



The Left Auricle is rather smaller than the right ; its walls thicker, measuring 

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

 atrium or sinus and an appendix auriculce. 



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 : behind, it receives on each side two pulmonary veins, being free in the rest 

 of its extent. 



The appendix auriculce is somewhat constricted at its junction with the auricle; 

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

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

 is forward and toward the right side, overlapping the root of the pulmonary artery. 



Within the auricle the following parts present themselves for examination : 



The openings of the four pulmonary veins. 

 Auriculo-ventricular opening. 

 Musculi pectinati. 



The pulmonary veins, four in number, open, two into the right, and two into 

 the left side of the auricle. The two left veins frequently terminate by a common 

 opening. They are not provided with valves. 



The auriculo-ventricular opening is the large oval aperture of communication 

 between the auricle and ventricle. It is rather smaller than the corresponding 

 opening on the opposite side (see note, page 1090). 



1 In former editions, as well as in other text-books on anatomy, these little nodules have been 

 described as fibro-cartilaginous in structure. At my request, Dr. Le Cronier Lancaster, Demonstrator 

 of Anatomy at St. George's Hospital, has investigated this subject, and reports that the "corpora 

 Arantii " appear to consist of bundles of interlacing connective-tissue fibres with branched connective- 

 tissue cells, and some few elastic fibres. Occasionally a rounded cell, with indistinct capsule, resem- 

 bling a cartilage-cell wa.< seen ; but there were not many of them. At the free edge of the corpus the 

 structure is denser, there being a larger proportion of fibres to cells than in the central portion. He 

 thinks the structure of the corpus should be put down as fibrous and not fibro-cartilaginous. 



