230 VERTEBRATE ZOOLOGY 



variation. The vertebral veins return blood from the cranial 

 cavity; they lie, one on each side, in the vertebrarterial canals 

 of the six anterior cervical vertebrae ; the costocervical veins bring 

 blood from the muscles of the back and neck. 



The subclavian forms the proximal portion of the system of 

 veins which brings blood from the shoulder and the foreleg; 

 lateral to the wall of the thorax it becomes the axillary vein, which 

 receives several small tributaries and is formed by the union of 

 two veins, the subscapular and the brachial. The former of these 

 brings blood from the shoulder. The brachial vein comes from 

 the leg. Follow these veins and their branches on the right side 

 of the body, being careful not to injure the arteries which ac- 

 company them. 



The external jugular vein lies just beneath the skin, on the side 

 of the neck. It receives, near its base, the internal jugular vein. 

 Follow this large vein and the external jugular forward on the 

 right side of the body. The internal jugular lies at the side of the 

 trachea alongside a prominent nerve, the vagus, and the carotid 

 artery ; it arises at the base of the skull and returns blood from 

 the brain. The external jugular is formed at the base of the head 

 by the union of the anterior facial vein, which arises beneath the 

 eye and brings blood from the lower portion of the face, with the 

 posterior facial vein, which collects blood from the upper and 

 hinder portions of the head. Near their origin the right and left 

 external jugulars are connected by the transverse vein, which 

 passes across the throat. 



Near the base of the internal jugular the left-hand external 

 jugular receives the thoracic duct. This is the main lymph canal 

 of the body; it comes from the region of the large mesenteric 

 lymph glands and lies along the left side of the aorta and the 

 oesophagus. It will be seen with difficulty, if at all. 



Note the thyroid gland. It is a large double gland near the 

 forward end of the neck; it consists of two elongated lobes 

 lying on each side of the trachea, and a ventrally situated con- 

 necting lobe. 



Exercise 19. Draw a diagram showing the veins of the precaval sys- 

 tem so far as observed. 



