208 The Alligator and Its Allies 



on account of the large size of that organ, of con- 

 siderable caliber. 



The Anterior Vencz Cavce and their Branches. 

 The entrance of the precaval veins into the 

 heart was mentioned above; their branches, in 

 order from the heart cephalad, will now be de- 

 scribed. Since the two precavas are alike, it 

 will be necessary to describe the branches of only 

 one side of the body. After leaving the heart, the 

 precava may be traced forward, for a short dis- 

 tance, at the side of the trachea and oesophagus, as 

 a wide, thin-walled trunk, Fig. 60, vca. The first 

 tributaries that it receives are the internal mam- 

 mary and vertebral veins, which join it at the base 

 of the neck at almost the same place. 



The internal mammary, Fig. 60, im, is a rather 

 small vein, bringing blood from the ventral wall of 

 the thorax. It may be followed along the inner 

 surface of the ribs, near the sternum, in company 

 with its corresponding artery. 



The vertebral vein, Fig. 60, v, is also of small dia- 

 meter and extends to the dorsal body wall near the 

 spinal column, from which region it returns blood 

 to the anterior vena cava; it is drawn too large in 

 the figure. 



Just cephalad to the vertebral and internal 

 mammary, the internal jugular, j, enters the pre- 

 cava. The internal jugular may be followed 

 directly forward, close to the side of the trachea 

 and oesophagus, from which it receives numerous 



