563 



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

 one to the auricular appendix. 



The Left Auricle (atrium sinistrum) 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 auricular appendix. 



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

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

 by the auricular septum (septum atriorum); behind, it receives on either side two 

 pulmonary veins. 



The left auricular appendix (auricula sinistra) 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 artery. 



The interior of the left auricle presents the following parts for examination : 



The openings of the four pulmonary veins. 

 Auriculoventricular opening. 

 Musculi pectinati. 

 Foramina Thebesii. 



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

 surface of the left auricle two on either side of its middle line. They are not 

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

 opening. 



The left auriculoventricular opening, or mitral orifice (ostium venosum ventriculi 

 sinistri), is the aperture of communication between the left auricle and the left 

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



The musculi pectinati, fewer and smaller than in the right auricle, are confined 

 to the inner surface of the auricular appendix. 



On the auricular septum 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 of the right auricle. The inner surface of the left auricle 

 also shows foramina Thebesii and venae minimae cordis. 



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 auriculoventricular 



groove. 



The Left Ventricle (ventriculus sinister) 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 its postero-inferior surface. It also forms the apex of the 

 heart by its projection beyond the right ventricle. Its walls are much thicker 

 than those of the right side, the proportion being as 3 to 1. 



Its interior (Fig. 419) presents the following parts for examination : 



f Auriculoventricular. ( Mitral or Bicuspid. 



Openings j ^.^ ^Valves | Semilunal , 



Chordae tendineae. Columnae carneae. 



The left auriculoventricular opening or the mitral orifice (ostium venosum ven- 

 triculi sinistri) is placed below and to the left of the aortic orifice. It is a little 

 smaller than the corresponding aperture of the opposite side, admitting only two 



