DISSECTION OF THE BACK AND THORAX. 



127 



sinus conveys the blood from the wall of the heart itself, and its mouth 

 will be found under that of the posterior vena cava. 4, The vena 

 azygos sometimes has an independent opening into the auricle, and it 

 then discharges itself by the roof of the sinus, behind the mouth of the 

 anterior cava. At other times it opens into the anterior cava. 5. The 

 vence cordis minima are small veins of the wall of the right auricle, 

 which, instead of discharging themselves by the coronary sinus, open 

 directly on the wall by minute 

 mouths named the foramina 

 Thebesii. Of all these orifices, 

 that of the coronary sinus is 

 the only one provided with a 

 valve. It is a thin fold of the 

 living membrane, termed the 

 valve of Thehesius. 



The inner wall of the sinus 

 venosus is formed by the 

 auricular septum, which is the 

 partition between the two 

 auricles. On this the follow- 

 ing objects are to be noticed : — 

 1. Between the orifices of the 

 anterior and posterior cavse is 

 a muscular prominence— the 

 tubercle of Lower. 2. Above 

 and in front of the opening of 

 the posterior cava is a depres- 

 sion of the septum that looks 

 like another venous orifice. 

 This is the fossa oralis, and it 

 marks the former position of 

 the foramen ovale — an aperture 

 which, in the foetus, estab- 

 lished a communication be- 

 tween the right and left auricles. The raised border which surrounds 

 the fossa is termed the annulus ovalis. In the foetus of many animals 

 but not of the horse, a valve, termed the Eustachian valve, directs the 

 blood from the posterior cava through the foramen ovale. After birth 

 the foramen ovale in nearly every case becomes completely closed, but 

 sometimes an oblique slit remains, which, however, does not necessarily 

 permit any blood to pass through the septum. 



The blood which passes through the right auricle is venous in char- 

 acter. It has been circulating among the tissues, and it is poured into 

 the cavity at the venous orifices already enumerated. When the auricle 



Fig. 9. 

 Diagram of the two Cavities of the right 



RIDE OF THE HEART (Ellis). 



a. Anterior cava; b. Posterior cava; c. Right 

 anricnlo - ventricular opening; d. Fossa ovalis; 

 e. Opening of the coronary sinus ; /. Foramina 

 Thebesii. the opening of veins ; g. Aperture of the 

 pulmonary artery ; h. Auricular appendix. 



