THE VASCULAR SYSTEM 53 



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 ex- 

 treme 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 glamd 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 jugidar veins. The right and left internal 

 mammary veins pass from the ventral thoracic wall, each 

 one parallel to and near the corresponding internal mam- 

 mary 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 jugidar and cephalic veins unite near the 



