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 transparent 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 abdominal cavities.
Remove the pericardium and study the heart in position. Note the attachments of the aortic arch, pulmonary arteries and veins, the superior venae cavae, and the inferior vena cava. The heart contains four chambers — two anterior '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-
