340 PRACTICAL ANATOMY. 



are thicker; it is a cuboidal sinus, with an auricular 

 extension to the right and across the root of the pulmo- 

 nary artery ; it presents the openings of the pulmonary 

 veins and the mitral opening. 



The four openings of the pulmonary veins are at the 

 upper posterior part of the auricle ; they have no valves, 

 but the circular fibres of the auricle are disposed around 

 them as a thickened layer, and prevent, in a measure, 

 the reflux of blood. 



The left auricula-ventricular or mitral opening will 

 admit two fingers ; accurately, it measures three and eleven- 

 twelfths inches. It is strengthened by a fibrous ring, 

 which surrounds it and affords attachment to the mitral 

 valve and the layers of the muscular fibres of the left 

 ventricle. In the interior of the appendix auriculae are 

 the musculi pectinati, arranged as on the right side ; on 

 the septum auricularum is the reverse of the annulus 

 and fossa ovalis. 



The Left Ventricle. 



The left ventricle, longer, but less capacious, than the 

 right, forms the posterior part and apex of the heart. 

 Its walls are from one- third to one-half inch thick, and 

 thickest about the middle of the ventricle. It presents 

 for examination the 



Auriculo-ventricular or mitral opening, which is to 

 the left of the aortic opening. It will admit two fingers, 

 and measures accurately three and eleven-twenty-fourths 

 inches. It is strengthened by a strong ring of fibrous 

 tissue, to which are attached the mitral valves and the 

 muscular layers of the heart. 



The aortic opening, at the summit of the left conus 

 arteriosus, will admit the index finger, and measures accu- 

 rately three and one-sixth inches. It is surrounded by a 

 strong ring of fibrous tissue, which affords attachment 



