52 DISSECTION OP THE DOG 



a prominent, semilunar crest, the crista terminalis, which corresponds in 

 position to a shallow and sometimes very imperfectly defined groove, the 

 sulcus terminalis, on the exterior of the atrium. 



The openings by which blood enters the atrium are as follows : (1) The 

 opening of the caudal vena cava ; (2) the opening of the cranial vena cava ; 

 (3) the opening of the coronary sinus ; (4) the foramina of the small veins of the 

 heart (foramina venarum minimarum [Thebesii]). 



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

 Between them is a semilunar ridge projecting from the wall of the cavity, and 

 known as the intervenous tubercle of Lower (tuberculum intervenosum [Loweri]). 

 The coronary sinus opens immediately ventral to the caudal caval opening, 

 and between this and the orifice which leads from the atrium into the right 

 ventricle. A variable fold of endocardium and subendocardial tissue, the 

 valve of the vena cava (valvula venae cavse [Eustachii]), exists between the mouth 

 of the coronary sinus and the caval opening. The foramina of the small cardiac 

 veins are scattered over the wall of the atrium. 



The blood passes from the atrium into the right ventricle by a large, rounded 

 orifice, which occupies the whole of the floor of the main part of the atrium. 



The septum between the right and left atria (septum atriorum) is not 

 equally thick throughout. The thinnest part is at the bottom of a poorly 

 defined depression, the fossa ovalis, which marks the position of an embryonic 

 connection between the two cavities. 



Dissection. — The right ventricle must now be opened. Make an incision 

 through the wall of the ventricle parallel to, and a short distance from, the 

 coronary sulcus. Care must be taken not to insert the knife too deeply, 

 or the tricuspid valve may be injured. The incision should be carried 

 well towards the commencement of the pulmonary artery. A second 

 incision must begin where the first finished, i.e. near the pulmonary 

 artery, and should be carried parallel to, and a short distance from, the 

 longitudinal sulcus as far as the right border of the heart. 



Venteiculits dexter. — The cavity of the right ventricle has a triangular 

 outline. The base of the triangle is formed by the right border of the heart, 

 and its apex by that part of the ventricle which is known as the conus arteriosus 

 from its conical shape and from its communication with the pulmonary artery. 

 The cavity of the ventricle does not reach the apex of the heart, but stops 

 short on a level with the point at which the longitudinal sulcus of the exterior 

 crosses the right border. 



It will be noticed that the septum between the two ventricles (septum 

 ventriculorum) is convex towards the right, thus causing the cavity of the right 

 ventricle to present a crescentic outline were a transverse section to be made. 



The whole of the interior of the wall of the ventricle is irregular owing to 

 the presence of fleshy ridges (trabecules carneae) of variable size and form. 



