THE CAT 227 



the pericardium and the thymus gland, but without cutting any of 

 the blood vessels. Study the heart and the thoracic blood vessels. 



The heart is a large, conical body and lies nearly in the middle 

 of the thorax ; it is composed of two ventricles and two auricles. 

 The posterior and much larger portion is formed by the ventricles. 

 The walls of the right ventricle are thin and flabby ; those of the 

 left ventricle are thick and firm and include the apex of the heart. 

 The auricles lie at the forward end of the heart and appear as a 

 pair of thin-walled sacs. The greater part of each auricle is 

 dorsal in position, with an extension on the ventral side called 

 the right and the left auricular appendix respectively. 



The two auricles may be distinguished from each other by their 

 difference in color, the right auricle being much darker than the 

 left because of the dark-colored venous blood it contains. The 

 septum separating the two ventricles is indicated on the ventral 

 surface by the position of the coronary artery, which emerges from 

 beneath the left auricle and runs to the right of the apex on the 

 outer surface of the heart. 



Study the great blood vessels of the thorax and their relation 

 to the heart. Thoroughly dissect the fat and other loose tissues 

 from them, being very careful not to cut them. Note the numerous 

 nerves which accompany and surround many of the vessels at the 

 forward end of the heart ; these are branches of the sympathetic 

 and vagus nerves. 



Extending forward from the anterior end of the heart are 

 three large vessels. The most conspicuous is the great precaval 

 vein, which is dark red in color and lies at the right of the median 

 plane. The other two will be seen emerging from between the 

 right and left auricles; they are the pulmonary artery and the 

 aorta. The former springs from the right ventricle, on the ventral 

 side of the heart, and is seen passing diagonally forward to the 

 animal's left; it divides almost immediately into the right and 

 left pulmonary arteries, which carry venous blood to the right and 

 left lungs. These arteries may be brought into view by turning 

 the heart forward. 



The aorta, which carries arterial blood to the entire body, issues 

 from the left ventricle just dorsal to the pulmonary artery. It 



