SECT. 217.] STRUCTURE OF LYMPHATIC VESSELS. 503 



phatics has been hut little investigated. According to Briicke, an 

 epithelial layer can be detected over the nuclei in those chyle- 

 ussels of the intestinal walls which already possess valves, and 

 which measure 002 of a millimetre ; while no epithelium is found 

 in the smaller valvclcss branches : these soon lose their walls, 

 and communicate freely with the spaces of the tissue before 

 described. It was not possible, on either the one or the other of 

 these vessels, to distinguish a special wall apart from the connective 

 tissue of the tunica adventitia surrounding them; indeed, layers of 

 connective tissue appeared to form the whole wall of the vessel as 

 far as the epithelium, though it should be mentioned, that in the 

 valved vessels of the submucous tissue smooth muscular fibres were 

 also found (Sitzu?ic/sher. d. Wien. Akad. vom Marz, 1853). 



The finest lymphatic vessels which have come under my own ob- 

 servation have measured ^"\ \'" to %'", and, except in the thickness 

 of the individual layers, have completely agreed with larger vessels 

 which measured \" to iV". These medium-sized lymphatics 

 possess three coats. The tunica intima consists of an epithelium 

 whose cells are elongated, though still rather short, and of a simple 

 elastic reticulate coat, which is rarely double, and has its fibres dis- 

 posed in the longitudinal direction ; with reference to the thickness 

 of its fibres, and the narrowness of its meshes, these are points 

 subject to considerable variation, yet it never Fig.206. 



becomes a thick fibrous coat or true elastic 

 membrane (according to Weyrich, this coat is 

 absent in the lymphatics of the mesentery, 

 while, on the other hand, I have always found 

 it in those of the lumbar plexus and of the 

 extremities). Next follows a thicker tunica 

 media, composed of transverse smooth muscular thro^guThTcoats of the 

 fibres, with fine elastic fibres likewise disposed 2pU%£T2 

 transversely ; lastly, a tunica adventitia, made S^^'iS^SfJto 

 up of longitudinal connective tissue, with scanty connective S" of'the 

 networks of fine elastic fibre, and a larger or ™^ e muscles ° of 1 '"the 

 smaller number of smooth muscular fasciculi, ^^rith^tM^S- 

 running obliquely and longitudinally. I have tudi " al muscnl « fibres - 

 found these latter in the lymphatics of the extremities, on vessels 

 of - 1 X ~" in diameter, and regard them as a good mark of distinction 

 between lymphatics and small veins (see my Micr. Anat., ii. 1, 

 p. 236). 



The thoracic duct differs in some respects from the smaller 

 lymphatics. The epithelium itself presents no peculiarity, but is 



