THE VASCULAR SYSTEM 59 



the thoracic cavity and becomes the right axillary 

 artery. 



The right internal mammary artery comes off from the 

 posterior surface of the subclavian just median to the first 

 rib. Its course is described in connection with the branches 

 of the left subclavian. 



The branches of the right axillary artery closely resemble 



those of the left. Verify this by a dissection of the vessels 



of the right arm. 



Exercise XV. Sketch and label the arteries anterior to the 

 heart. This drawing will be combined later (Exercise XXVI) 

 with a similar sketch of the arteries posterior to the heart. 



THE HEART 



Study the attachments of the pericardium, the trans- 

 parent membrane which surrounds the heart. If the 

 thorax has been opened with sufficient care the pericardium 

 will be unruptured. Insert a blowpipe through a small 

 incision in the pericardium and inflate it with air. The 

 pericardial cavity^ the space between the pericardium and 

 heart, is a part of the body cavity or coelom. The two 

 other components of the coelom are the thoracic and ab- 

 dominal cavities. 



Remove the pericardium and study the heart in position. 

 Note the attachments of the aortic arch^ p^ulmonary ar- 

 teries and veins, the superior venae cavae, and the inferior 

 vena cava. The heart contains four chambers — two an- 

 terior atria and two posterior ventricles. The atrial and 

 ventricular portions of the organ may be identified easily 

 in a preserved animal. The atria are very dark in color 

 because they contain masses of clotted blood, while the 

 ventricles are decidedly lighter. The coronary sulcus is 

 the deep furrow on the heart's surface which separates the 

 atria from the ventricles. The right and left auricles (au- 



