DISSECTION OF THE DOG 



49 



The exterior of the heart is divided into areas corresponding to the four 

 cavities in the interior. Forming the base are the two atria, separated from 

 the much larger ventricles by the coronary sulcus (sulcus coronarius), which is 

 continuous round the whole heart except where interrupted by the pulmonary 

 artery. Owing to the presence of fat and blood-vessels, the depth of the 

 sulcus cannot be. properly estimated in the undissected heart. 



Externally the distinction between the right and left atria is very im- 



a. subclavia sinistra 



" Incisura [apicis] cordis 

 Apex cordis 



Fig. 20. — Outline of the heart as seen from the left. 



perfectly marked, except on the left side. Caudally there is only a faint line 

 of demarcation between the two chambers in the form of a shallow groove 

 between the pulmonary veins and the caudal vena cava. Consequently the 

 common atrial mass is crescentic in form, the horns of the crescent being 

 two free projections, the auriculae cordis, separated from each other by the 

 pulmonary artery and the aorta. 



The ventricular part of the heart is responsible for the conical shape of the 

 organ as a whole. Its base is connected with the atria, while its apex 

 forms the apex of the entire organ. The common mass of the two ventricles 

 presents two surfaces and two borders. The sterno-costal face (facies sterno- 



