FIG- 139. 



THE LYMPHATIC SYSTEM. 



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 o"he hand and to the lymphatic vessels on the other is very inti- 

 mate ; in certain localities, as in the omentum, 

 indirect communication between the blood-vessels 

 and lymphatics 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 

 intima, composed of a considerable layer 

 of subendothelial connective tissue con- 

 taining a net-work of longitudinally dis- 

 posed elastic fibres. The muscular tissue 

 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 



Perivascular lymphatic 

 (6) enclosing a small ar- 

 tery (a), from the silvered 

 mesentery of frog : c, 

 branching lymphatic cap- 

 illary. 



FIG. 140. 



Transverse section of human tho- 

 racic duct : i, m, and o, respectively 

 the inner, middle, and outer tunics; 

 x, 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. 



