THE VASCULAR SYSTEM. 153 
fore its junction with the subclavian. The opening is 
guarded by a valve which prevents the blood from enter- 
ing the lymphatic trunk. 
The thoracic duct (Fig. 80) is the largest vessel of the 
lymphatic system, but is usually invisible unless injected. 
It extends from the region of the kidneys on the dorsal 
Fic. 81. PHotocraru or LyMpPHATIC CAPILLARIES INJECTED WITH BERLIN 
Biue. They lie immediately beneath the skin on the outer aspect 
of the ear. 
a, Point of injection; the black area is Berlin blue lying in the connec- 
tive-tissue spaces from which the capillaries arise; , injection es- 
caped from ruptured vessel. 
aspect of the body cavity, along the left side of the spinal 
column, to its opening into the external jugular vein near 
where the latter joins the subclavian vein. The caudal end 
