a90 CIRCULATORY APPARATUS. 



valve {Eustachian valve) circumscribing the interauricular opening in the 

 foetus.^ 



In the wall are small orifices {foramma Thebesii) which lead to anastomosing 

 vessels (vence cordis mitiimce) in its substance. 



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

 orifices, Llie largest of which is the opening 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 tvall, 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, areolae 

 separated by muscular columns. 



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

 the reliefs sculptured on the inner face of that cavity. In some points it is 

 about ^ 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 layer with a free concave border, which forms 

 a kind of valve of very variable extent. Behind this vena cava is a thick 

 eminence — the tuberculum 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 musculo -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. Red-blood (ok 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, the transverse section of 

 which is irregularly circular. Its walls attain a thickness of from H 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 reticulated cul-de-sac, which occupies the point 

 of the heart. The base is perforated by the auiiculo-ventricular and the aortic 

 openings. The aurindo-ventriculnr opening — precisely similar to that of the right 

 ventricle — is provided with a circular membrane, the mitral (or bicuspid) valve, 

 because it is cut into several festoons, of which two are the principal — the one 



• Zangger and Zundel have observed instances of persistent foramen ovale in Foals, and 

 even in Horses. Goubaux lias colkcted fifteen cases of this kind, thirteen of which occurred 

 in Bovines-aged from four mouths to twenty years— one in the Sheep, and another in the Dog. 

 ThiM anatomist also met witli an abnormal communication between the two ventricles in an 

 adult Horse. Chatiii has studied a similar anomalv in a young Hemionus. 



' * The presence of a Eustachian valve in the Horse has been denied.) 



