clxxxiv 



LYMPHATIC SYSTEM. 



Fig. CI. 



leads to the conclusion that the injected material does not find its way from the ducts 

 into the lymphatics by any naturally existing communication, but by accidental 

 rupture of contiguous branches of the two classes of vessels. It seems probable, 

 also, that the communications often held to exist between the commencing lympha- 

 tics, both superficial and deep, and capillary blood-vessels, have no better foundation, 

 and that the passage of injection, here also relied on as evidence, is to be accounted 

 for in the same way. A fact mentioned by Kolliker throws light on these alleged 

 communications with sanguiferous capillaries. In investigating the lymphatics of 

 the tadpole's tail with the microscope, that observer not unfrequently noticed that 

 blood-corpuscles got into the lymphatics from the small blood-vessels, and he was 

 able to recognise in the living animal the communications by which they passed. 

 At first he looked on these communications as natural, but, after repeated and careful 

 investigations, he satisfied himself that they were produced accidentally by contusion, 

 or some other injury inflicted on the parts. 



Structure. In structure the lymphatic vessels much resemble the veins, 

 only their coats are thinner, so thin and transparent indeed that the con- 

 tained fluid can be readily seen through them. When lymphatics have 

 passed out from the commencing plexuses and lacunse, they are found to have 

 three coats. The internal coat is covered with a lining of epithelium, con- 

 sisting of a single layer of flattened nucleated cells, which in the larger 

 lymphatics have mostly an oblong figure, but in small or commenciug 

 vessels are more rounded, with an indented, bluntly serrated, or wavy 

 border, by which the adjacent cells fit to each other, like the epidermic 

 cells of grasses and some other plants (fig. ci.). Beneath the epithelium 



the inner 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 reticulating elastic fibres taking the 

 same direction. The external coat is 

 composed mainly of white connective 

 tissue with a sparing intermixture of 

 longitudinal elastic fibres, and some longi- 

 tudinal and oblique bundles of plain 

 muscular tissue. In the thoracic duct 

 there are striated white layers (as in the 

 aorta) beneath the epithelium, between 

 it and the elastic layers of the inner coat ; 

 and in the middle coat there is a longi- 

 tudinal layer of white connective tissue 

 with elastic fibres, immediately within the 

 muscular layer. 



The commencing lymphatics, whether in plexuses or single (as in the 

 villi), for the most part look like mere channels excavated in the surround- 

 ing tissue, without independent coats, and they were regarded as such by 

 various eminent authorities. It has now, however, been ascertained that 

 they invariably have a lining of epithelium formed of the characteristic 

 indented scales (fig. ci.), as in other small lymphatics. Tins is made appa- 

 rent by injection of solution of nitrate of silver, which blackens and brings 

 into view the serrated lines of juncture of the flattened cells, whilst the 

 nuclei may be made to appear by means of acetic acid or carmine. But it 

 is not clearly determined whether there is any other coat outside the epi- 

 thelium, even in cases where the vessels are separable from the adjoining 

 tissue. By the same method of preparation an epithelial lining of similarly 



Fig. CI. PORTION OF A LYMPHATIC 

 VESSEL SHOWING ITS PECULIAR EPI- 

 THELIUM. TREATED WITH NITKATE 

 OP SILVER. FROM THE INTER- 

 MUSCULAR LAYER OF THE INTES- 

 TINE OF THE GUINEA PIG (after 

 Auerbach). MAGNIFIED 240 DIA- 

 METERS. 



