COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 253 



surface of the masseter. Other tributaries of the anterior facial vein come from 

 the nearby lymph and salivary glands. The posterior facial vein may next be 

 followed. It passes to the parotid gland where it receives a superficial vein, the 

 posterior auricular vein, from the back of the ear and head. The main vein 

 beyond the entrance of this branch lies imbedded in the parotid gland which 

 may be dissected from it. The vein is accompanied by the facial nerve. At the 

 base of the ear it is formed by the inferior ophthalmic vein from the orbit, the 

 temporal veins from the temporal region, and the anterior auricular vein from 

 the region in front of the ear. 



The internal jugular vein extends the length of the neck, receiving but few 

 small branches, of which the chief ones are those from the thyroid gland. It 

 may be traced to the occipital region of the skull, from which it emerges by way 

 of the jugular foramen; it collects part of the blood from the brain. As already 

 stated, its size and place of junction with the external jugular are highly variable. 



The left precaval vein is identical in its tributaries with the right, except 

 that there is no azygos vein on the left side. 



Draw the branches of the precaval as far as found. 



Cat: Turn the apex of the heart to the left and note the large vein which 

 enters the anterior margin of the right auricle. This is the precaval vein. Note 

 that there is no such vein on the left side. Instead there is a vein called the 

 coronary sinus, which runs along the dorsal surface of the heart in the groove 

 between the auricles and ventricles. The coronary sinus will be found by clean- 

 ing out the fat from this groove. Note the numerous coronary veins which come 

 from the heart wall and enter the sinus. The sinus itself opens into the left 

 posterior corner of the right auricle. It represents the reduced proximal part 

 of a former left precaval vein. The distal part of this vein is still present and, 

 as we shall see shortly, is united with the right precaval. Again pressing the 

 heart to the left, clean the base of the precaval and note the large vein which 

 passes in front of the root of the right lung and joins the precaval as the latter 

 enters the auricle. This tributary of the precaval is the azygos vein. Trace it 

 posteriorly, pressing the right lung to the left. It passes along the dorsal thoracic 

 wall near the mid-dorsal line and receives at regular intervals the intercostal 

 veins. These course along the posterior borders of the ribs. The most anterior 

 of the intercostal veins join into a common trunk which enters the azygos shortly 

 caudad to the entrance of the latter into the precaval. The azygos also receives 

 small branches from the esophagus and bronchi. 



Trace the precaval anteriorly. It receives small branches from the thymus 

 gland and then receives a tributary of moderate size, the common stem of the 

 internal mammary veins, which comes from the midventral wall of the chest. On 

 following this posteriorly it is soon seen to be formed by the union of two veins, 

 the internal mammary veins, which run posteriorly in the chest wall one to each 

 side of the midventral line, and are extended onto the abdomen as the superior 



