THE HEART. 



337 



turn divides the heart into a right, or venous, and a 

 left, or arterial side. The right auricle is a cuboidal 

 blood-sinus, into which empty the descending and ascend- 

 ing venae cavae. It presents a principal sinus and an 

 appendix. Its \yalls are about as thick as stout wrap- 

 ping-paper, and present on their inner surface five open- 

 ings, two valves, and some relics of fcetal structure. 



The openings are : 



Superior Vena Cava. The large venous trunk 

 which drains the upper half of the body. It opens 

 into the upper anterior part of the auricle, the axis of 



v.p 



FIG. 145. ARRANGEMENT OF MUSCULAR FIBRES AROUND THE AURICLES 

 AND GREAT VEINS. 



I, muscular fibres on the left auricle ; v.p., fibres on the pulmonary veins ; V. the 

 left ventricle. II. muscular fibres on the superior vena cava ; a, opening of vena aiygos ; 

 v, auricle. 



its current being anterior to that of the ascending vena 

 cava and directed to the tricuspid opening. Around the 

 openings of the vena cava the circular fibres of the 

 auricles are reduplicated, but there are no valves. 



Inferior Vena Cava. Returns the blood of the lower 

 half of the body. The axis of its current lies behind 

 that of the superior vena cava and is directed against 

 the septum auricularum. 



The opening of the coronary sinus is to the right of 

 the tricuspid opening. It drains the blood from the 



22 



