RELATION OF LYMPHATICS AND CONNECTIVE TISSUE. 



379 



and there occur. A difference is found in the epithelium upon the two surfaces of 

 the valves similar to that which has been noticed in the valves of the veins. 



Valves are not present in all lymphatics, but where they exist they follow one 

 another at much shorter intervals than those of the veins, and give to the lymphatics 

 when distended, a beaded or jointed appearance. Valves are placed at the entrance 



Fig. 437. PART OF A LYMPHATIC 



VESSEL IN THE PLEURAL COVERING 

 OP THE DIAPHRAGM. ^. (Rail- 



vier. ) 



L, L, the lymphatic vessel with 

 characteristic epithelium ; c, cell- 

 spaces of the connective tissue. 



of the lymphatic trunks into 

 the great veins of the neck. 

 They are generally wanting 

 in the reticularly arranged 

 vessels which compose the 

 plexuses of origin already 

 spoken of ; so that fluid in- 

 jected into one of these vessels 

 runs in all directions, so as to 

 fill a greater or a less extent 

 of the plexus, and passes along 

 the separate vessels which 

 issue from it. 



The lymphatics of fishes and amphibia are, generally speaking, destitute of 

 valves, and may therefore be injected from the trunks ; and valves are much less 

 numerous in the lymphatics and lacteals of reptiles and birds than in those of 

 mammiferous animals. 



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

 connective tissue. It has been already stated (p. 233) that the cells of the con- 

 nective tissue lie in spaces in the ground-substance which they more or less com- 

 pletely fill. These cells and cell-spaces form in many parts an intercommunicating 

 network of varying fineness extending throughout the substance of the tissue (fig. 

 437, c, fig. 438, d, d, fig. 439, c, c), whilst in other parts the cells acquire a broad 

 flattened form, and joining edge to edge with other similar cells may in this way 

 form an epithelioid patch in the ground substance. Not unfrequently the cells in 

 such a patch take on the wavy border described above as met with in the lymphatics 

 of origin (see the isolated patches in fig. 436). Further, the flattened cells which 

 form the walls of the lymphatics are connected here and there both with the more 

 ramified cells of the tissue (fig. 438, e) and with those which form the epithelioid 

 patches, and in silvered preparations they appear to be continuous with one another. 

 The epithelioid patches look in fact like a part of the lymphatic vessels, and are 

 often regarded as such ; it must be understood, however, that the spaces here 

 spoken of, whether containing single cells or groups, are not true vessels, but merely 

 vacuities in the ground-substance of the tissue containing flattened cells, which do 

 not form a continuous vascular wall. And although the spaces present a very close 

 relation to the lymphatic vessels, they can hardly be considered as actually opening 

 into them by patent orifices, for the lymphatics proper have a complete wall of 

 flattened cells united by a small amount of intercellular substance : at the same time 

 this thin film can offer but a very slight resistance to the passage of fluid from the 

 tissue into the vessel, or even to the passage of leucocytes or migrating cells, which, 

 as is well known, penetrate the at least equally closed wall of the blood-vessels. 



VOL I. 



c c 



