THE HEART. 465 



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, the lunulce, placed one on each side of the nodule im- 

 mediately adjoining 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 opened, and the course of the blood along the tube is uninter- 

 rupted ; but during the ventricular diastole, when the current of blood along the 

 pulmonary artery is checked arid 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 fill- 

 ing up the small triangular space that would be otherwise left by the approxima- 

 tion 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 (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 toward the heart, finds its way into these sinuses, 

 and so shuts down the valve-flaps. 



In 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, 

 or sinus, and an appendix auriculae. 



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 auricula? is somewhat constricted at its junction Avith 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 pulmo nary 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 auricula-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 463). 



The musculi pectinati are fewer in number and smaller than on the right side ; 

 they are confined to the inner surface of the appendix. 



On the inner surface of the septum auricularum may be seen a lunated 

 impression bounded below by a crescentic ridge the concavity of which is turned 

 upward. The depression is just above the fossa ovalis in the right auricle. 



To examine the interior of the left ventricle, make an incision a little to the left of the 

 anterior interventricular groove from the base to the apex of the heart, and carry it up from 

 thence, a little to the left of the posterior interventricular groove, nearly as far as the auriculo- 

 ventricular groove. 



The Left Ventricle is longer and more conical in shape than the right ventricle, 

 and on transverse section its cavity presents an oval or nearly circular outline. It 

 forms a small part of the anterior surface of the heart, and a considerable part of 



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