242 ANATOMY IN A NUTSHELL. 



folds of fibrous membrane, covered by endocardium below and by the inner 

 arterial coat above. The convex margin of each valve is attached to the inner 

 coal of the artery where it joins the ventricle, while its double crescentic border 

 is free and is directed upward. The corpus arantii are small nodules situated 

 at the center of each free margin. Tendinous fibers radiate from the corpus 

 arantii to the attached margin of the valve, passing throughout the entire 

 valve excepting a small portion on each side of the corpus arantii called the 

 lunula. At this point the valve consists of lining membrane only. When the 

 valve closes the surfaces of the lunula come in contact and the copora arantii 

 fill the interval at the center. Sinus of valsalva is a pouch behind each valve. 

 8. The moderator band is a muscular beam running across the left ventricle 

 obliquely. 



LESSON LXVI 



The left auricle like the right auricle has two parts, a sinus or atrium and 



an auricular appendix. This auricle is smaller but has thicker walls than the 

 right one. The atrium lies behind the aorta and the pulmonary artery and is 

 separated from the right auricle by the auricular septum internally. The left 

 auricular appendix is longer and narrower, and more curved than the right 

 one, and it projects forward to the right over the root of the pulmonary artery. 

 The left auricle presents the following points: 



1 . The openings of the four pulmonary veins, two on each side. These 

 veins have no valves and carry pure blood. There may be five pulmonary veins 

 or only three. 



2. The auriculo-ventricular opening is smaller than the one on the right 

 side. It is surrounded by a fibrous ring covered by endocardium and guarded 

 by the mitral valve. 



3. The musculi pectinati are fewer and smaller than those on the right side 

 and are confined to the auricular appendix. 



4. On the auricular septum, just above the fossa ovalis of the right auricle, 

 there is an impression which is bounded below by a crescentic ridge whose con- 

 cavity looks upward. 



The left ventricle is longer than the right and its walls are about three times 

 as thick. It forms most of the posterior surface of the heart, all the apex, and 

 a small part of the left side of the anterior surface. 



The left ventricle presents the following points: 



1. The auriculo-ventricular opening which is below and to the left of the 

 aortic opening is described with the left auricle. 



2. The aortic opening is circular and is guarded by the semilunar valves. 

 It i> situated in front and to the right of the auriculo-ventricular opening. 



3. The mitral or bicuspid valve is attached to the margin of the auriculo- 

 ventricular opening similar to the tricuspid valve on the right side of the heart, 

 luit is stronger and thicker than the tricuspid valve. Its segments or flaps are 

 covered by < ndocardium ami contain a few muscle fibers. The larger flap lies 

 to the right and in front between the auriculo-ventricular opening and the 



