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. 118, 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. 
1. 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 
ofthe head,—-tongue, pharynx, etc., and enter a large lymphatic 
