506 THE CIRCULATORY APPARATUS. 



wall or roof, and the auriculo-ventricular opening, which occupies the whole 

 floor of the cavity. This orifice has been already described. 



The anterior cul-de-sac is in the appendix auriculaB ; it is divided by a 

 great number of muscular columns of the second and third orders (musculi 

 pectinati), into deep and complex areolse. 



The posterior wall responds to the interauricular septum ; it is smooth, 

 and usually marked by an oblique and more or less deep cul-de-sac (or de- 

 pression), the remains of Botal's foramen. This depression is surr9unded 

 by the ring of Vieussens (annulus ovalis) and is named the fossa ovalis ; it is 

 only separated from the left auricular cavity by a thin membrane, a vestige 

 of the valve circumscribing the interauricular opening in the foetus. 



The external wall is areolated, and perforated behind and below by two 

 orifices, the largest of which is the embouchure of the posterior vena 

 cava, the other the opening of the large coronary vein. Both are destitute 

 of valves, though these are found at a short distance in the coronary vein. 

 The bronchial vein sometimes opens separately beside the latter. 



The internal wall is smooth. 



The superior wall., or roof of the auricle, shows the openings of the 

 anterior vena cava and vena azygos ; the latter only is provided with valves, 

 which are, however, not always present. On this wall are also remarked, in 

 front, areolse separated by muscular columns. 



The thickness of the right auricular walls is very irregular, in conse- 

 quence of the reliefs sculptured on the inner face of that cavity. In some 

 points it is about the third of an inch, and in others, particularly in the 

 small culs-de-sac formed by the reticulations, it is sometimes so thin as 

 to appear exclusively formed by the union of the external and internal 

 serous membrane. 



(When the vena azygos opens behind, there is between it and the orifice 

 of the anterior vena cava, a muscular lamella with a free concave border, 

 which forms a kind of valve whose extent is very variable. Behind this vena 

 cava is a thick eminence, the tub'erculum Loweri ; this has the form of a 

 crescent, open in front, and elongated from right to left at the superior 

 border of the septum. The anterior, or left border of the fossa ovalis, is 

 thin and prominent, and constitutes the Eustachian valve : a muscular 

 membranous fold of a semilunar shape, with a concave free border directed 

 to the right and behind. It is of little use in animals, because of their 

 horizontal position. Immediately beneath the posterior vena cava, and 

 between it and the coronary vein, is a small membranous crescent the valve 

 of Thebesius.) 



C. EED-BLOOD (OB AORTIC) HEART. This is also called the posterior 

 heart, and more frequently the left heart, because it is situated behind and 

 to the left of the dark-blood heart. Its general disposition otherwise exactly 

 resembles that of the latter receptacle. 



LEFT VENTRICLE. This is a cylindro-conical cavity, whoso transverse 

 section gives an irregularly circular figure. Its walls attain a thickness of 

 from 1J to If inches, except towards the apex of the heart, where they are 

 extremely thin. They are less reticulated than those of the right ventricle, 

 and exhibit several columns of the second order, as well as two enormous 

 muscular pillars an external and internal, for the attachment of the tendons 

 of the auriculo-ventricular valve. The apex of the cavity forms a re- 

 ticulated cul-de sac, which occupies the point of the heart. The base is 

 perforated by the auriculo-ventricular and the aortic openings. The auriculo- 

 ventricular opening, precisely similar to that of the right ventricle, is 



