378 LYMPHATIC SYSTEM. 



lymphatic spaces is also met with between the anastomosing muscular fibres of the 

 heart. 



What may be regarded as a third mode of origin of lymphatics is to be found in 

 the open communications which subsist between the serous cavities and the lym- 

 phatic vessels in their walls. These orifices or stomata, which will be described with 

 the serous membranes, allow of the passage of lymph from the serous cavities into 

 the lymphatics, so that those cavities may, in a certain sense, be looked upon as 

 large lymph lacunae. Owing to this communication fluid is not, under normal cir- 

 cumstances, suffered to accumulate in them. 



In some of the lower animals the lacunar condition of lymphatics has been long known. 

 Rusconi found that the aorta and mesenteric arteries of amphibia are inclosed in large lym- 

 phatic spaces. Johannes Miiller recognised the spaces which so extensively separate the frog's 

 skin from the subjacent muscles as belonging to the lymphatic system, and Recklinghausen 

 showed that the subcutaneous lymph-spaces of the frog's leg communicate with lymphatic 

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

 finds its way into the blood. The lymphatic system, in being thus constituted by lacunas or 

 interstitial receptacles, so far agrees with the sanguiferous system of crustaceans and insects. 



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

 except that 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 lacunas, they are found to have three coats. The 

 internal coat is covered with an epithelial lining (endothelium), 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. 436, a). Outside the endothelial layer 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. Over the dilatations which occur in the vessels beyond each of the 

 valves, the circular disposition of the muscular fibres gives place to a more irregular 

 disposition, taking the form of an intricate interlacement of fibres. The external 

 coat is composed mainly of white connective tissue with a sparing intermixture of 

 longitudinal elastic fibres, and some longitudinal and oblique bundles of plain 

 muscular tissue. In the thoracic duct there is a sub-epithelial 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 muscular 

 fibres of the middle coat, although for the most part transverse in direction, are 

 nevertheless many of them oblique or even longitudinal. 



The largest lymphatics have blood-vessels ramifying in their outer coat. 



The commencing lymphatics or lymphatic capillaries, whether in plexuses or 

 single (as in the villi), have a much simpler structure, their wall being entirely 

 formed of a layer of endothelium 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 (fig. 437). 



Graskell has described an attachment of elastic fibres to the walls of smaller lymphatics in 

 some parts, and infers that the patency of the lumen of these vessels may by this means be 

 restored after it has been temporarily obliterated by pressure (or by contraction of the muscular 

 coat). 



Valves. The lymphatic and lacteal vessels are furnished with valves serving 

 the same office as those of the veins, and for the most part constructed after the same 

 fashion. They generally consist of two semilunar folds arranged in the same way as 

 in the valves of veins already described, but deviations from the usual structure here 



