Xo. 172] A \ \'inM) <>!■ un I'l nliin \ , H I'S 323 



wI.otluT it he e.npty .listen.le.l. !t fonn. tl,e rinl.l anlerior 

 (|uarter of tlie heart, an.l lies somewhat ventral i.> the other parts 

 of that or<;aii. 



The left auricle or atrium L. : Form, the iHt anteri..r .,,iart<-r of 

 tJie heart, an.l i. it. lar^.-t .WsWum. thoiml, on a-.'ount of it> thin 

 disteusii.h. ualls this ehaml.er mav vary c.nsi.l.-rahly in siz,>. Its 

 walls are usually wrinkled and uneven, and its outline is more 

 rounded than that of tlie ventricle, though the entire outline cannot 

 he seen in either a dorsal or a ventral view, since the chamher is 

 partially covered dorsally hy the left auricle, and ventrally hy the 

 ventricle. At some point on its dorsal si(l(> the vein (/'///.) forin(>d 

 hy the union of the two pulmonary veins prol.al.iy enters it. I.iit, 

 on account of the very small size of thi> single pulmonary vein, its 

 exact point of entrance could not \)c .lettMinineil with certainty, 

 and so has not heen indicated in the fiuui(\ riie size of the pul- 

 monary veins in the figure has l)een e\auuerat. .1. The anterior 

 edge of the left auricle lies nearer the head than any other part of 



The right auricle (/,'. .I.i lie. dorsal an, I posterior t.. th(> ven- 



only the posterior half ..f tlii. ehaml.er .hou-^. ( )\\ing to its very 

 thin walls and to the l.-.rn.- ..p.-ninn- of the sinus venosus it has no 

 very definite shajn-. It is d, 'pressed in a (lorso-\ .Mitral direction, 

 and its greatest diameter is from si.li> to si.Ie. Into its antero- 

 lateral corners open the right and left anterior veiue cava' (/?. ('., 

 L. C), while posteriorly it is separated hy only a slight constric- 

 tion, externally, from the sinus venosus which in turn is continued 

 hack as the posterior vena cava. Extending loniiitndinall} across 

 the dorsal wall of the right auricle, and closely attaciieil to it. is 

 the pulmonary vein (PuL), formed hy the uni.)ii of the luo -mall 

 veins from the lungs. 



The sinus venosus tS. ]\) is merelv th<- tM.lai-v.l anterior .'iid 

 of the po.sterior or inferior vena ."iva. Its walls are extremely 

 thin, and its size an.l shape will .Icpen.l up.)n the ain..iint of llnid 

 it contains. Blood vessels to >!ipi)ly the walls of tin- heart may 

 be seen at several places, and arc shown in tlie figure at R. 



