THE VASCULAR SYSTEM. 



141 



valve which prevents the blood from returning into the 

 auricle when the systole or contraction pushes the blood 

 into the pulmonary artery. The opening between the 

 left auricle and left ventricle 

 is guarded by the bicuspid or 

 mitral valve. These valves 

 may be displayed by cut- 

 ting away the caudal half of 

 the ventral wall of the ven- 

 tricles, when the chorda) 

 tendinese, delicate tendin- 

 ous cords, will be seen ex- 

 tending from the margins of 

 the translucent membran- 

 ous valves to the columnar 

 carneae or muscular projec- 

 tions on the walls of the 

 ventricles. The walls of the 

 auricles are thin in compari- 

 son with the w r alls of the left 

 ventricle, which are twice as 

 thick as the walls of the 

 right ventricle (Fig. 68). 



The auricles are very 

 small when not injected, 

 and may by the beginner be 

 cut away with the pericar- 

 dium and surrounding adi- 

 pose tissue. The right auri- 

 cle receives three veins, the 

 precava, postcava, and cor- 

 onary veins, all of which enter its dorsal aspect. The por- 

 tion of the auricle into which the above veins open is the 

 sinus venosus. At the dorsal part of the septum which 



FIG. 67. HEART VIEWED YEN- 

 TRALLY, WITH YKXTRAL HALF OF 

 THE AURICLES AND VENTRICLES 

 CUT AWAY AND THE AURICLES 

 DRAWN LATERAD. Partly dia- 

 grammatic. 



n, Orifice of the aorta; ao, aorta; 

 aao, arch of the aorta ; ap, orifice 

 of the precava ; as, orifice of the 

 postcava; h, left pulmonary- 

 veins; fan, left ventricle; mt, mi- 

 tral or bicuspid valve; la, left 

 auricle ; p, orifice of the pulmon- 

 ary veins ; pc, postcava ; prc, pre- 

 cava; ba, orifice of the pulmonary 

 artery; prv, right pulmonary 

 veins ; rv, right ventricle and one 

 of the right pulmonary veins; ra, 

 right auricle ; rp, right pulmonary 

 artery; tr, tricuspid valve; xp, 

 left pulmonary artery. 



