ANATOMY OF THE LACTEAL AND LYMPHATIC VESSELS. 281 



inch (19 to 25 mm.) from its termination, which effectually prevent the 

 entrance of blood from the venous system. 



The foregoing sketch of the descriptive 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 finally 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 they be filled with the 

 opaque chyle. 



From the difficulty in studying the lymphatics at their origin, except by 

 means of injections or by reagents which stain the vessels, investigations into 

 the structure of the smallest vessels have not been very satisfactory. It is 

 supposed, however, that the 

 vessels here consist of a single 

 coat, resembling, in this re- 

 gard, the capillary blood-ves- 

 sels. Belaieff has described in 

 the capillary lymphatics of the 

 penis a lining of endothelial 

 cells arranged in a single layer. 

 These cells are oval, polygonal, 

 fusiform or dentated, with 

 their long diameter in the di- 

 rection of the axis of the ves- 

 sels. 



In all but the capillary lym- 

 phatics, although the walls are 

 very thin, three distinct coats 

 can be distinguished. The in- 

 ternal coat consists of an elas- 

 tic membrane lined with ob- 

 long, endothelial cells. This 

 coat readily gives way when 

 the vessels are forcibly dis- 

 tended. The middle coat is 

 composed of longitudinal fibres 

 of connective tissue, with deli- 

 cate elastic fibres, and non- 

 striated muscular fibres ar- 

 ranged transversely. The external coat is composed of the same structures 

 as the middle coat, but most of the fibres are arranged longitudinally. In 

 this coat the muscular fibres do not form a continuous sheet, but are col- 

 lected into separate fasciculi, which have a direction either longitudinal or 



FIG. 90. Thoracic duct (Mascagni). 



1, thoracic duct ; 2, great lymphatic duct ; 3, receptaculum 

 chyli : 4, curve of the thoracic duct just before it empties 

 into the venous system. 



