Make a second longitudinal incision in the thoracic wall seven or eight millimeters to the right of the sternum. Separate the median strip, containing the sternum, from the diaphragm and turn the strip forward. The right lung will be exposed. Observe that the dorsal edge of the extreme posterior lobe on the right side is attached by a mesentery to the esophagus, which, in turn, is similarly suspended from the dorsal wall of the thoracic cavity.
Observe, also, the thymus gland anterior to the heart. This gland has no duct. Its secretion enters the blood. It is therefore an endocrine gland, or gland of internal secretion.
The veins of mammals show considerable variation. Fluctuations from the relations described in this text should be noted. While dissecting the veins, care should be taken to avoid injuring the arteries.
Venous blood from the regions of the body anterior to the heart drains into the right and the left superior vena cava (precaval veins), which enter the right atrium of the heart. Trace one of the superior venae cavae cephalad and note that it is formed by the confluence of the subclavian and internal jugular veins. The right and left internal mammary veins pass from the ventral thoracic wall, each one parallel to and near the corresponding internal mammary artery, to the precaval vein of the same side. Both mammary arteries and veins will be seen in the median strip which has been reflected forward. The azygous vein empties into the left precava near its union with the heart. Follow the azygous dorsally at the left of the aorta, thence caudally parallel to the aorta, noting its tributaries. Is there a corresponding branch of the right precaval vein? Trace the internal jugular vein anteriorly to the thyroid gland.
The external jugular and cephalic veins unite near the
