THE LYMPHATIC SYSTEM. 331 



passage through the lymphatic glands the vessels break up into 

 fine channels which again unite on leaving the gland. In the 

 gland lymph-cells are added to the fluid, and probably other 

 changes are produced. 



Most or all of the lymphatic vessels of the body finally unite 

 into two trunks before entering the veins. The largest trunk 

 is the thoracic duct (Fig. 1 18, 5, page 282), which collects the 

 lymph from the entire caudal half of the body, and from the 

 left forelimb and the left side of the thorax, head, and neck. 

 It passes along the dorsosinistral side of the thoracic aorta, 

 lying against its surface, and enters the left external jugular 

 vein at its junction with the subclavian. The second main 

 trunk is the right lymphatic duct ; this collects the lymph 

 from the right side of the thorax, the right forelimb, and the 

 right side of the neck and head ; it enters the right external 

 jugular, 



i. Lymphatics of the Head. A number of lymphatic 

 vessels arise on the sides of the face, especially in the upper 

 and lower lips. They form a superficial network of vessels, 

 lying over the ventral half of the masseter muscle and an area 

 ventrad of it. These lymphatic vessels all enter two large 

 lymphatic glands (Fig. 131, 12, page 322) lying at the ventro- 

 caudal angle of the masseter muscle, covering the union of the 

 anterior and posterior facial veins. Into these glands pass also 

 a number of lymph vessels from the back of the head. From 

 these two lymphatic glands two or three small lymphatic 

 vessels pass caudad, lying on the surface of the external jugular 

 vein. Near the point of the shoulder, some distance craniad 

 of the junction of the external and internal jugular veins, these 

 vessels enter another small lymphatic gland lying on the dorsal 

 surface of the external jugular vein. Thence one or two 

 vessels continue caudad, still on the surface of the external 

 jugular, and finally unite with the deep lymphatics of the head 

 and neck and those of the arm to enter the caudal end of the 

 external jugular vein, usually (on the left side) after junction 

 with the thoracic duct, from the thorax. 



The deep lymphatics of the head come from the internal parts 

 of the head, tongue, pharynx, etc., and enter a large lymphatic 



