248 



ABSORPTION LYMPH AND CHYLE 



arch of the aorta, and empties at the junction of the left subclavian with 

 the internal jugular vein. It diminishes in size from. the receptaculum 

 to its middle 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 usually is a valvular fold, but this is not constant. There 

 is always, however, a pair of semilunar valves in the duct, three-quarters 



of an inch to an inch (19 to 

 25 millimeters) from its ter- 

 mination, which prevents 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 liquids, not 

 only from the intestinal canal 

 during digestion, but from 

 nearly every tissue and organ 

 in the body, and that these 

 are finally received into the 

 venous circulation. 



Structtire of the Lymphatic 

 and Lacteal Vessels. The 

 lymphatic vessels, even those 

 of largest size, are remarka- 

 ble for the delicacy and trans- 

 parency of their walls. This 

 Fig. 58. TAoratic ducf (Mascagni). is illustrated in the case of 



I, thoracic duct ; 2, great lymphatic duct ; 3, receptacu- the lacteals, which are hardly 

 lum chyli ; 4, curve of the thoracic duct just before it empties M i ,1 



into the venous system. visible in the transparent 



mesentery, unless filled with 



chyle. On account of 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 regard, the capillary 

 bloodvessels. 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 direction of the axis of the vessels. 



