282 



ABSORPTION LYMPH AND CHYLE. 



oblique. The fibres of connective tissue are very abundant and unite the 

 vessels to the surrounding parts. The internal and the middle coats are 

 closely adherent to each other ; but the external coat may readily be separated 

 from the others. Blood-vessels have been found in the walls of the lym- 

 phatics, and the existence of vaso-motor nerves is probable. 



The walls of the lymphatic vessels are very closely adherent to the sur- 

 rounding tissues ; so closely, indeed, that even a small portion of a vessel is 

 detached Avith great difficulty, and the vessels, even those of large size, can 

 not be followed out and isolated for any considerable distance. 



In all the lymphatic vessels, beginning a short distance from their plexus 

 of origin, are semilunar valves, generally arranged in pairs, with their con- 

 cavities looking toward the larger trunks. These folds are formed of the 

 middle and inner coats ; but the fold formed from the lining membrane is by 

 far the wider, so that the free edges of the valves are considerably thinner 

 than that portion which is attached directly to the vessel. The valves are 

 most abundant in the superficial vessels. The distance between the valves is 

 one-twelfth to one-eighth of an inch (2 to 3 mm.), near the origin of the ves- 

 sels, and one-quarter to one-third of an inch (6 to 8 mm.), in their course. 

 In the lymphatics situated between the muscles the valves are less abun- 

 dant. They are always relatively few in the vessels of the 

 head and neck and in all that have a direction from above 

 downward. Although there are a number of valves in the 

 thoracic duct, they are not so abundant here as in the 

 smaller vessels. 



In their anatomy and general properties, the lymphatics 

 bear a close resemblance to the veins. Although much 

 thinner and more transparent, their coats have nearly the 

 same arrangement. The arrangement of valves is entirely 

 the same ; and in both systems, the folds prevent the reflux 

 of fluids when the vessels are subjected to pressure. 



The lymphatics are very elastic ; and it is generally 

 admitted that the larger vessels and those of medium size 

 are contractile, although the action of their muscular fibres, 

 like that of all fibres of the non-striated variety, is slow 

 and gradual. 



One of the most important points in connection with 

 the physiological anatomy of the lymphatic vessels is the 

 question of the existence of orifices in their walls, which 

 might allow the passage of solid particles or of emulsions. 

 Anatomical observations have indicated the existence of 

 stomata, of variable size and irregular shape, in the small- 

 est vessels ; and a strong argument in favor of the existence of these orifices 

 has been the fact of the actual passage, through the walls of the vessels, of 

 fatty particles, the entrance of which can not be explained by the well known 

 laws of endosmosis. The anatomical evidence of the existence of openings 

 is derived mainly from preparations stained with silver nitrate, It is assumed 



FIG. 91. Valves of 

 the lymphatics 

 (Sappey). 



