

THE LYMPHATIC SYSTEM. 



117 



FIG. 139. 



Perivascular lymphatic 

 (6) enclosing a small ar- 

 tery (a), from the silvered 

 mesentery of frog : c , 

 branching lymphatic cap- 

 illary. 



these latter consist of a fold of endothelium, strengthened often by a 

 minute quantity of elastic tissue. 



The relation of the lymph-spaces to the capillary blood-vessels on 

 the one hand and to the~lympjiaticvessels oh the other is very inti- 

 mate ; in certain localities, as in the omentum, 

 indirect communication between the blood-vessels 

 and lymhatics is established by means of the 

 spaces of the groundwork of the dense connec- 

 tive tissue (Klein). Many nerve-trunks are en- 

 closed by perineurial lymphatic channels, into 

 which the lymph-spaces of the surrounding tissue 

 open. The blood-vessels of the central nervous 

 system, especially of the retina, likewise are 

 surrounded by distinct perivascular lymph- 

 sheaths, formed by the enlargement and con- 

 fluence of the clefts within the adventitia of the 

 vessels. In some membranous structures, notably 

 the amphibian mesentery, the vessels lie encased 

 within distinct endothelial tubes. 



Lymphatic vessels of large__size have walls 

 of considerable thickness, resembling those of 

 the^veins. In such vessels three coats are recog- 

 nizable the inner, or endothelial, the middle, or muscular, and 

 the outer, or connective tissue. The 

 thoracic duct possesses a well-developed ,r FIG. 140, 



intima, composed of a considerable layer .*" 

 of subendothelial connective tissue con- ^ 

 taining a net-work of longitudinally dis- r v t 

 posed elastic fibres. The muscular tissu 

 of the media is supplemented by bundles 

 of involuntary muscle extending length- 

 wise within the outer coat, which in the 

 vessel under consideration is particularly 

 robust. 



The lymph contained within the lym- 

 phatic vessels, like the blood, consists of 

 two parts the clear, straw-colored 

 plasma, or liquor lymphae, and the 

 cellular elements, the lymph-cor- 

 puscles. The cells of the lymph are 

 small nucleated masses of active proto- 

 plasm, when at rest presenting a spherical 

 form and measuring about .01 mm. in 

 diameter ; in their usual condition of activity, however, their outlines 



Transverse section of human tho- 

 racic duct : i, m, and o, respectively 

 the inner, middle, and outer tunics ; 

 .r, endothelial lining, beneath which 

 lies the fibrous stratum containing 

 net- work of longitudinal elastic fibres 

 (y) ; z, longitudinally disposed bun- 

 dles of muscular tissue within adven- 

 titia ; v, capillary blood-vessels. 



