

THE HEART 



533 



ened by tendinous fibers, and the former presents, at its middle, a thickened nodule 

 (corpus Arantii). From this nodule tendinous fibers radiate through the segment 

 to its attached margin, but are absent from two narrow crescentic portions, the 

 lunulae, placed one on either side of the nodule immediately adjoining the free 

 margin. Between the semilunar valves and the wall of the pulmonary artery are 

 three pouches or sinuses (sinuses of Valsalva) . 



Left Atrium (atrium sinistum; left auricle). The left atrium is rather smaller 

 than the right, but its walls are thicker, measuring about 3 mm. ; it consists, like 

 the right, of two parts, a principal cavity and an auricula. 



The principal cavity is cuboidal in form, and concealed, in front, by the pul- 

 monary artery and aorta ; in front and to the right it is separated from the right 

 atrium by the atrial septum; opening into it on either side are the two pulmonary 

 veins. 



Auricula (auricula sinistra; left auricular appendix). The auricula is somewhat 

 constricted at its junction with the principal cavity; it is longer, narrower, and more 

 curved than that of the right side, and its margins are more deeply indented. It 

 is directed forward and toward the right and overlaps the root of the pulmonary 

 rtery. 



FIG. 496. Interior of left side of heart. 



The interior 

 examination: 



II 



of the left atrium (Fig. 496) presents the following parts for 



Openings of the four pulmonary veins. 

 Left atrio ventricular opening. 

 Musculi pectinati. 



The pulmonary veins, four in number, open into the upper part of the posterior 

 surface of the left atrium two on either side of its middle line: they are not 

 provided with valves. The two left veins frequently end by a common opening. 



The left atrioventricular opening is the aperture between the left atrium and 

 ventricle, and is rather smaller than the corresponding opening on the right side. 





