THORAX AND ABDOMEN OF THE HORSE 31 



The constant openings by which blood enters the right atrium 

 are : — (1) The opening of the caudal vena cava; (2) the opening of the 

 cranial vena cava ; (3) the opening of the coronary sinus ; and (4) the 

 openings of the small veins of the heart. In addition to these there 

 may be two other venous openings, namely, those of the vena azygos and 

 the middle cardiac vein. 



The caval openings occupy the caudal and cranial ends of the atrium. 

 Between them is a semilunar ridge projecting from the roof of the cavity 

 and known from its position as the intervenous tubercle (tuberculum 

 intervenosum). It will be observed that the term " tubercle " is not 

 very appropriate as descriptive of the object in the heart of the horse. It 

 is obvious that the ridge will serve the mechanical purpose of directing 

 the two caval streams towards the exit from the atrium. The coronary 

 sinus debouches immediately ventral to the termination of the caudal 

 vena cava, and between this and the opening that leads from the atrium 

 into the ventricle. The opening is provided with a small valve (valvula 

 sinus coronarii). If the middle cardiac vein does not terminate in the 

 coronary sinus, its mouth will be found close to that of the sinus. Four 

 or five openings of the small cardiac veins are hidden in the depressions 

 between the pectinate muscles. 



Occasionally in young animals a variable fold of endocardium and 

 subendocardial tissue, the valve of the vena cava (valvula venae cavae), 

 is present between the caval opening and the mouth of the coronary 

 sinus. 



The blood passes from the atrium into the right ventricle by a 

 rounded atrio-ventricular orifice, about 5 cm. in diameter, which 

 occupies the entire floor of the main part of the atrium. 



The septum between the right and left atria (septum atriarum) is 

 not equally thick throughout. The thinnest part is at the bottom of a 

 well-defined fossa^ ovalis that marks the position of an embryonic 

 connection between the two cavities. The mouth of the fossa looks 

 towards the caudal caval opening and is partly surrounded by a 

 prominent margin, the limhus^ fossce ovalis. The interatrial com- 

 munication (foramen ovale) of the embryo may be regarded as a means 

 by which most of the blood brought to the heart by the caudal vena 

 cava may be quickly restored to the systemic circulation instead of being 

 compelled to follow a route impeded by the as yet undistended lungs. 

 Another provision of a like nature exists in the ductus arteriosus that 

 connects the pulmonary artery with the aorta of the embryo.^ 



1 Fossa [L.], a ditch, trench, fosse. 



^ Limbus [L.], a border. 



^ See the appendix on the fuetal circulation, p. 192. 



