PHARVNX AND LARYNX. 39 
BeOS SHCTION- OMY LAE TiRART- AND (GREAT 
BLOOD-VESSELS OF THE THORAX. 
1. The Pericardium is the membranous sac in which, 
the heart is enclosed. Like the pleura and the perito- 
neum it isa double-walled sac. One layer, the visceral, 
is attached closely to the muscular substance of the 
heart ; the other, the parietal, lies loosely round the 
heart. The two layers meet at the roots of the great 
blood-vessels, and hold between them a little serous 
liquid, the pericardial liquid. Notice the attachment 
of the pericardium, the parietal layer, to the dia- 
phragm. 
Cut away the pericardium, and clean as carefully as 
possible the roots of the large veins and artertes arts- 
ong from the heart. 
The greater portion of the surface of the heart 
facing ventrally is made up of the right ventricle. 
Anterior and to the right lies the right auricle. The 
tip of the heart—the apex—is formed by the left 
ventricle alone. The left ventricle makes up also the 
greater portion of the dorsal surface of the heart, 
The position of the internal septum between the two 
ventricles is marked externally by a groove, contain- 
ing a coronary artery and vein, which begins down 
toward the apex on the ventral surface, and runs 
obliquely around to the dorsal side. 
Determine the positions of the right and left ven- 
tricles, and the right and left auricles. Each of the 
auricles consists of a main cavity, the a¢rzwm, into 
which the veins open, and an auricular appendrx, 
usually the most conspicuous portion of the auricle. 
