THE HEART 



929 



Openings. — Of tlie tliree tliit'f (j})t'nings, that of tlie superior cava appears at 

 tlie iip])er and l»ack ]»art of tlic cavity; whilst at tlic lower ami hack part is the 

 opening of the inferior cava. The right auriculo-ventricular opening, which 

 leads into the ventricle, is ])laced helow and in front; whilst alcove and in front the 

 cavity is })rol()nged into the auricular a})j)endix. 



The orifice of the coronary sinus lies between the lower eaval and auriculo- 

 ventricular oi)enings. J5(sidcs these there are about a score of small scattered 

 orifices, known as the foramina Thebesii. 



The superior caval opening is valveless, and is directed downwards and for- 

 wards towards the auriculo-ventricular opening. 



The inferior caval opening is somewhat larger, and is directed u]iwards ami 

 inwards. It is usually guarded l)y a semilunar fold, the Eustachian valve, whidi 

 is much larifer in f(etal life, and which then serv-es to direct the current of hlood 



Fig. 548. — Tkaxsvekse Section passing through the Aukicles of the Heart, showing 



THE Al'RICri.O-VENTRIClLAR ORIFICES AND THE SEMILUNAR VaLVES OF THE PULMONARY 



Artery and Aorta. Seen from above. 



(The portions of the auricles removed are seeu in Fig. oo2.) 



Right aurieulo-ventrieular orifice and tricuspid valve 



LEFT ANTERIOR SEGMENT OF PULMONARY 

 SEMILUNAR VALVE 



CHORD<€ TENDiNE>E 



I y ^^ "'G.HJ.fOSTERIOR SEGMENT OF 



AORTIC SEMILUNAR VALVE 



Left auriculo-ventricular orifice and mitral valve 



tlirough a foramen in the wall tlividing the two auricles. It is attached by its 

 convex margin to the front and left side of the vein, its free concave edge looking 

 upwards and to the right. The left cornu or horn of the crescent is continuous 

 with the anterior edge of the annulus ovalis, whilst the right horn is lost on the 

 auricular wall. The Eustachian valve contains interlacing muscular filu-es; it is 

 often very incomplete and sometimes perforated. 



The coronary sinus returns the blood from the heart substance, and is gunrded 

 by a semilunar or sometimes doul)le valve, known as the coronary valve, or valve 

 of Thebesius. Like the Eustachian, it is formed of a fold of the endocardium, 

 and serves to direct the blood current, but does not prevent regurgitation. It is 

 sometimes perforated, and occasionally presents the most delicate lacework. 



The foramina Thebesii. — The greater number of these small orifices end 

 blindly, but the rest are the terminations of minute veins from the muscular sub- 

 stance (venae minimae cordis . One of these, more constant tlian tlie others, 

 59 



