290 VERTEBRATE ZOOLOGY 



Remove the diaphragm and the liver from the body. Make a 

 hole in the pericardium and blow into it with a blowpipe to deter- 

 mine its extent. Remove the pericardium and the thymus, 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 ventri- 

 cles ; the walls of the right ventricle are thin and flabby, whereas 

 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, dark-colored sacs ; the 

 greater part of each auricle is dorsal in position, with an exten- 

 sion onto the ventral side called the right and the left auricular ap- 

 pendix respectively. 



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 numer- 

 ous nerves which accompany and surround many of the vessels at 

 the forward end of the heart; these are branches of the sym- 

 pathetic and vagus nerves. 



Extending forward from the anterior end of the heart will be 

 seen four large vessels. The most conspicuous of these are the 

 dark-red right and left precaval veins, which lie respectively at 

 the animal's right and left, bringing venous blood from the entire 

 forward part of the body to the right auricle; between these 

 veins are the aorta and the pulmonary artery. 



Observe the entrance of the right precaval into the right auricle ; 

 turn the heart forward and observe the entrance of the left pre- 

 caval. Note the azygos vein, which joins the right precaval near 

 the point where this enters the right auricle, and trace its course 

 posteriorly along the dorsal wall of the thorax; note the sev- 

 eral pairs of intercostal veins which join it at regular intervals. 

 Note the minute coronary veins which bring blood from the wall 

 of the heart to the left precaval vein and the right auricle. 

 Note the large dark-colored postcaval vein and trace it to the 

 diaphragm. 



