PHARYNX AND LARYNX. 39 



£. DISSECTION OF THE HEART AND GREAT 

 BLOOD-VESSELS OF THE THORAX. 



I. The Pericardium is the membranous sac in which 

 the heart is enclosed. Like the pleura and the perito- 

 neum it is a 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 arteries aris- 

 ing f^om 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 atrium, into 

 which the veins open, and an a2tricular appendix, 

 usually the most conspicuous portion of the auricle. 



