THE VASCULAR SYSTEM. 165 



three main trunks. In fact, the vessels bringing the 

 lymph to the glands may be said to terminate there. 

 They are called afferent lymphatics. The lymph moves 

 free through the tissue of the gland, and on the opposite 

 side is taken up by the efferent lymphatics. 



The right and left tracheal trunks lie on their respective 

 sides of the trachea and empty into the external jugular 

 vein just before its junction with the subclavian. The 

 opening is guarded by a valve which prevents the blood 

 from entering the lymphatic trunk. 



The thoracic duct (Fig. 78) 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 

 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 of the thoracic duct is expanded, forming 

 the receptaculum chyli. Numerous valves are present 

 in the duct, and give to it a moniliform appearance. It 

 receives all the lymph from the left abdominal wall, both 

 pelvic extremities, a large part of the thoracic wall, and 

 the thoracic and abdominal viscera. 



DUCTLESS GLANDS. 



The spleen, thyroid gland, thymus gland, and suprarenal 

 bodies are ductless glands whose functions are imper- 

 fectly know r n. The largest of these is the spleen, which 

 is of a deep red color in a fresh specimen, and lies in 

 the abdominal cavity on the left side caudad to the 

 stomach (Fig. 54). It is about five centimeters long, 

 two centimeters wide, and less than a centimeter thick. 

 It is composed of lymphoid tissue of two forms, sup- 

 ported by connective-tissue trabeculae which are merely 



