308 



ABSORPTION. 



line for the inferior half of its length. It then inclines to the left side, forms a 

 semicircular curve something like the 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 re- 

 united a short distance above. At 

 its opening into the venous system, 

 there is generally a valvular fold, 

 but, according to Sappey, this is not 

 constant. 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 probable that the lymphatic 

 and lacteal vessels have no direct con- 

 nection with the blood-vessels, except 

 by the two openings by which they 

 discharge their contents into the ven- 

 ous 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 ca- 

 nal 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 they be filled with 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 been very few and are not very satisfactory. It is supposed, how- 

 ever, that the vessels here consist of a single amorphous coat, resembling, in this regard, 

 the capillary blood-vessels. Dr. Belaieff describes, in the capillary lymphatics of the 

 penis, a lining of epithelial cells arranged in a single layer. These cells are oval, polygo- 

 nal, fusiform or dentated, with their long diameter in the direction of the axis of the 

 vessels. 



In all but the capillary lymphatics, although the walls are excessively thin, three dis- 

 tinct coats can be distinguished. The internal coat consists of an elastic membrane lined 

 with oblong epithelial cells. This coat readily gives way when the vessels are forcibly 

 distended. The middle coat is composed of longitudinal fibres of the white fibrous tissue, 

 with delicate elastic fibres and unstriped muscular fibres arranged transversely. The 

 external coat is composed of the same structures as the middle coat ; but the fibres are 

 arranged, for the most part, longitudinally. In this coat, the muscular fibres do not form 

 a continuous sheet, but are collected into separate fasciculi, which have a direction either 



FIG. 89. Thoracic duct. (Mascagni.) 



1, thoracic duct; 2, great lymphatic duct; 8, receptaculum chyli; 

 4, curve of the thoracic duct just before it empties into the 

 venous system. 



