THE HEART. 279 
3. The left auricle (atrium sinistrum) (Fig. 115, d; Fig. 
116, 6). The auricular appendage (Fig. 115, @) is bent at 
right angles and its apex turned caudad. The cavity is thus 
irregular. Muscular trabeculae occur only in the auricular 
appendage. When the auricular septum is examined by trans- 
mitted 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, 4, 7). Each group opens into a 
more or less pronounced sinus or extension of the auricular 
cavity. The sinuses may be called, on account of their posi- 
tion, dorsal (g), sinistral (2), and dextral (2). 
4. The left ventricle (Fig. 115, 4) 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 trabeculz of the left ventricle. There are two very large 
columnz carnez, 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 chorde tendinee, which come from their 
free borders as well as from their outer surfaces, are attached 
to the columnz carne, chordz 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 
