436 ABSORPTION. 



the junction of the left subclavian with the internal jugular 

 vein. It diminishes in size from' the receptaculum to its mid- 

 dle portion, and becomes larger again near its termination. 

 It occasionally bifurcates near the middle of the thorax, but 

 the branches become reunited a short distance above. At 

 its opening into the venous system, there is generally a val- 

 vular fold, but, according to Sappey, this is not constant. 1 

 There is always, however, a pair of semilunar valves in the 

 duct, from three-quarters of an inch to an inch from its 

 termination, which effectually prevent the entrance of blood 

 from the venous system. 



It is now generally admitted that the lymphatic and lac- 

 teal vessels have no connection with the blood-vessels, except 

 by the two openings by which they discharge their contents 

 into the venous system. The foregoing sketch of the de- 

 scriptive anatomy of what has been called the absorbent 

 system of vessels shows that they may collect fluids, not only 

 from the intestinal canal during digestion, but from nearly 

 every tissue and organ in the body ; and that these fluids 

 are received into the venous circulation. 



Structure of the Lacteal and Lymphatic Vessels. The 

 lymphatic vessels, even those of largest size, are remarkable 

 for the delicacy and transparency of their walls. This is well 

 illustrated in the case of the lacteals, which are hardly visible 

 in the transparent mesentery, unless filled with opaque chyle. 



From the difficulty in studying the lymphatics at their 

 origin, except by means of injections, or reagents which stain 

 the vessels, investigations into the structure of the smallest 

 vessels have been very few and not very satisfactory. It is 

 supposed, however, that the vessels here consist of a single 

 amorphous coat, resembling, in this regard, the capillary 

 blood-vessels. Dr. Belaieff describes in the capillary lym- 

 phatics of the penis, a lining of epithelial cells arranged in 

 a single layer. These.cells are oval, polygonal, fusiform or 



. - 1 SAPPEY, op. tit., tome i., p. 621. 



