286 ESSENTIALS OF ZOOLOGY 



ventral side upwards, it will be easy to identify the two auricles 

 and the two ventricles ; also to see that the pulmonary artery 

 arises from the base of the right ventricle. Just behind it is the 

 thick elastic aorta. The right auricle will be seen to have 

 passing into it the anterior and posterior venae cavae, and the 

 left auricle the opening of the pulmonary vein. 



By opening the right auricle the thin smooth-walled cavity 

 will be found to present the openings of the veins, and also the 

 coronary sinus, a remnant of the left anterior vena cava which 

 receives the coronary veins. In the rabbit the left anterior 

 caval vein persists and the coronary vein opens into it ter- 

 minally. The auricular appendage presents folded walls due 

 to strands of muscle, called the musculi pectinati. The left 

 auricle is similar in structure and presents the opening of the 

 pulmonary vein. The auricles open by wide apertures into the 

 respective ventricles, and the openings are guarded by valves. 

 These and the ventricular cavities may be best explored by 

 cutting the wall on each side of the septum, carrying the 

 incision from the front to the back of the heart in each case. 

 The valves will be seen to be membranous and attached to 

 the wall of the ventricle by chordae tendineae. The walls of 

 the ventricles are much thicker than those of the auricles, and 

 the wall of the left than that of the right. The internal and 

 external walls of the right ventricle are connected by a 

 muscular moderator band. Internally, the wall is raised 

 into folds by muscular bands, and the chordae tendineae are 

 attached to prominent eminences, called papillary muscles. 

 There are three valves guarding the right auriculo-ventricular 

 aperture, and two that of the left. The former is termed the 

 tricuspid, and the latter the bicuspid, valve. The right 

 ventricle is discharged by the pulmonary artery, and the left 

 ventricle by the aorta. At the base of each of these are three 

 pocket valves with the openings of the pockets directed to the 

 vessel. Each pocket terminates in a margin which is thinner 

 than the rest of the membranous valve, and in the centre of 

 each margin is a hard round body, or nodule, called the corpus 

 Arantii. These valves are termed semilunar valves. In the 

 sinuses formed between the wall of the aorta and two of the 

 pockets are the openings of the two coronary arteries. These 



