1S6 LYMPHATIC SYSTEM. 



long-er known. Rusconi found that the aorta and mesenteric arteries of amphiljian 

 reptiles are inclosed in large Ijinphatic spaces. Johannes MilUer recognised the 

 spaces which so extensively separate the frog"s skin from the subjacent muscles 

 as belonging to the lymphatic system, and Von Recklinghausen has shown that 

 the subcutaneous lymph-spaces of the frog's leg communicate with Ijauphatic 

 vessels which envelope the blood-vessels of the foot ; also that milk injected into 

 these spaces finds its way into the blood. The lpni)hatic sj-stem, in being thus 

 partly constituted by lacunas or insterstitial receptacles, so far agrees with the 

 sanguiferous system of crustaceans and insects. 



It has been sometimes maintained that the lymphatics of glandular organs 

 communicate at their origm with the ducts ; but, althovigh it is no uncommon 

 thing for matters artificially injected into the ducts of glands, as, for instance, 

 those of the liver and testicle, to pass into the lymphatics, a careful examination 

 of such cases leads to the conclusion that the injected material does not find its 

 way from the ducts into the IjTuphatics by any natui'ally existing communication, 

 but by accidental rupture of contiguous walls of the two classes of vessels. 



Structure. — In structure the larger lymphatic vessels much resemble 

 the veins, only their coats are thinner, so thin and transparent indeed 

 that the contained fluid can be readily seen through them. When 

 lymphatics have passed out from the commencing plexuses and lacunre, 

 they are found to have three coats. The internal coat is covered with 

 an epithelioid lining, consisting of a single layer of flattened nucleated 

 cells, which have mostly an oblong or lanceolate figure, with an indented 

 or bluntly serrated border, by which the adjacent cells fit to each other 

 (fig. 124, a). Beneath the epithelioid layer .the innei coat is formed of 

 a layer or layers of longitudinal elastic fibres. The middle coat consists 

 of plain muscular tissue disposed circularly, mixed with finely reticu- 

 lating elastic fibres taking the same direction. The external coat is 

 composed mainly of w^hite connective tissue with a sparing intermix- 

 ture of longitudinal elastic fibres, and some longitudinal and oblique 

 bundles of plain muscular tissue. In the thoracic duct there is a 

 subepithelial layer (as in the arteries) ; and in the middle coat there 

 is a longitudinal layer of white connective tissue with elastic fibres, 

 immediately within the muscular layer. 



The larger lymphatics receive vasa vasorum, which ramify in their 

 outer and middle coats : nerves distributed to them have not yet been 

 discovered, although their existence is inferred on physiological grounds. 



That the lymphatics are endowed with vital contractility is shown by 

 the effect of mechanical iri'itation applied to the thoracic duct, as well 

 as by the general shrinking and emptying of the lacteal and lymphatic 

 vessels on their exposure to the contact of cold air, in the bodies of 

 animals opened immediately after death. 



The commencing lymphatics or " lymphatics of origin," whether in 

 plexuses or single (as in the villi), have a much simpler structure, their 

 wall being entirely formed of a layer of flattened epithelioid cells either 

 similar in form to those lining the larger vessels or (more frequently) 

 presenting a characteristic waved border like the epidermic cells of 

 grasses and some other plants (figs. 124, h, 126, a). The outlines of 

 the cells are brought into view by staining with nitrate of silver, after 

 Avhich the nuclei may be made to appear by means of carmine or 

 haematoxylin. 



Helation of the lymphatics of origin to the cells and cell-spaces 

 of the connective tissue. — It has been already stated (p. 57) that 

 the colls of the connective tissue lie in spaces in the ground-substance 



