THE HEART. 279 



3. The left auricle (atrium sinistrum) (Fig. 115, d] Fig. 

 116, /;). The auricular appendage (Fig. 115, d) is bent at 

 right angles and its ape.x turned caudad. The cavity is thus 

 iiregular. Muscular trabccuhu occur only in the auricular 

 ap])endagc. When the auricular septum is examined by trans- 

 mittetl light the position of the fossa ovalis is indicated at about 

 the middle of the septum as seen from the left side. Ventrad 

 of it is a fold of the septal wall. 



The pulmonary veins enter the dorsal wall of the auricle 

 in three groups (Fig. 116, g-, h, i). Each group opens into a 

 more or less pronounced sinus or extension of the auricular 

 cavity. The sinuses maybe called, on account of their posi- 

 tion, dorsal {g), sinistral [i), and dextral {h). 



4. The left ventricle (Fig. 115, /;) occupies rather more 

 than the left half of the base of the heart. Its walls are two 

 or three times as thick as those of the right ventricle. In- 

 ternally its walls present a few muscular bands, comparable to 

 the trabecular of the left ventricle. There are two very large 

 column?e carnea;, one dorsad and one ventrad. At the cranial 

 end it communicates near the lateral wall with the left auricle 

 by the auriculoventricular opening, and near the septum with 

 the aorta. The bicuspid valve (or mitral valve), which guards 

 the auriculoventricular opening, consists of two flaps, one septal 

 and one lateral. Their chorda: tendinea:, which come from their 

 free borders as well as from their outer surfaces, are attached 

 to the columnar carneae, chordae passing from both valves to 

 each columna carnea. 



The opening into the aorta is guarded by three aortic 

 semilunar valves, each of which partly conceals an aortic 

 sinus (or sinus of Valsalva). One valve is dorsal, one sinis- 

 tral, and one dextral. In the dextral sinus is the opening of 

 one of the coronary arteries, which carry blood to the walls of 

 the heart. 



Pericardium. 



The pericardium is a sac enclosing the heart. It lies in 

 the middle mediastinum, and the two halves of the mediastinal 

 septum with their fat may be dissected away from it. Its wall 

 is composed of two layers, an external firm fibrous layer and 



