COMPARATIVE ANATOMY 57 



Ventricles. — The two ventricles constitute the greater 

 visible portion of the heart. The left is much firmer than the 

 right, due to its greater muscular development, by which differ- 

 ence the boundaries between the two ventricles may be fairly 

 well traced. The apex or free tip of the heart lies wholly within 

 the boundaries of the left ventricle. 



Auricles.— The auricles are thin-walled sacs anterior to the 

 ventricles. Externally the division between right and left 

 auricles is only slightly indicated. A large part of each auricle 

 is taken up by a flap, the auricular appendage. 



Venae Cavae. — Two large veins enter the right auricle. 

 The posterior or inferior vena cava brings blood to the heart from 

 the posterior part of the body; the anterior or superior vena cava 

 brings blood from the shoulders and head. 



Aorta. — This is the large artery emerging from the left 

 ventricle. It passes between the auricular appendages and 

 soon after leaving the heart bends to the left and passes down 

 the back as the dorsal aorta. It is the beginning of the arterial 

 system. 



Pulmonary Vessels. — The pulmonary artery leaves the 

 right ventricle to the left of the aorta and carries blood to the 

 lungs. There are several short pulmonary veins entering the left 

 auricle that bring blood from the lungs back to the heart. 



Coronary Vessels. — These are the vessels that take blood 

 to and from the tissues of the heart itself. The left coronary 

 artery extends along the division line between the ventricles 

 on the ventral side, the right coronary artery between the ven- 

 tricles on the dorsal side. Trace these arteries to their points 

 of origin. The left and the right coronary veins parallel the 

 coronary arteries. The coronary sinus is a vein partially 

 encircling the heart between the auricles and ventricles on the 

 dorsal side. It may be covered with fat. It receives the blood 

 from both right and left coronary veins and empties into the 

 right auricle. 



Veins of the Cat 



Considerable variation occurs among blood vessels, particu- 

 larly as to their points of origin. A blood vessel is therefore 



