STRUCTURE OF LYMPHATICS. clxxxv 



marked character has been shown to exist on the walls of the interstitial 

 lymph spaces or lacunae of origin. 



The lymphatics receive va?a vasorum, which ramify in their outer and 

 middle coats : nerves distributed to them have not yet been discovered, 

 although their probable existence has been inferred on physiological grounds. 



Vital properties. That the lymphatics are endowed with vital contractility is 

 shown by the effect of mechanical irritation 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. 



Vahes. The lymphatic and lacteal vessels are furnished with valves 

 serving the same office as those of the veins, and for the most part con- 

 structed 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 and there occur. Thus Mr. 

 Lane has observed some valves in which the planes of the semilunar flaps 

 were directed not obliquely but transversely across the vessel, an arrange- 

 ment calculated to impede the flow of fluid in both directions, but not com- 

 pletely to intercept it in either. In others, described by the same authority, 

 the two folds, placed transversely as before, coalesced at one end, so as to 

 represent a transverse septum with an incomplete transverse slit. In a 

 third variety, he found the valve formed of a circular fold corresponding 

 with a constriction outside, and probably containing circular contractile 

 fibres capable of completely closing the tube. 



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 much distended, a beaded or jointed appear- 

 ance. Valves are placed at the entrance of the lymphatic trunks into the 

 great veins of the neck. They are wanting in the reticularly arranged 

 vessels which compose the plexuses of origin already spoken of ; so that 

 mercury injected into one of these vessels runs in all dirsctions so as to fill 

 a greater or less extent of the plexus, and passes along the separate vessels 

 which issue from it. 



The lymphatics of fish and naked amphibia are, generally speaking, desti- 

 tute of valves, and may therefore be injected from the trunks ; in the turtle 

 a few valves are seen on the larger lacteals which pass along the mesentery, 

 but none on those upon the coats of the intestine ; and valves are much 

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

 mammiferous animals. 



Orifices. It was at one time a prevalent opinion among anatomists that the 

 lymphatic and lacteal vessels begin on various surfaces by open mouths, through 

 which extraneous matters are absorbed. This was especially insisted on as regards 

 the commencing lacteals in the intestinal villi. That opinion has been since given 

 up ; but quite recently von Recklinghausen has obtained what he considers satis- 

 factory evidence of openings in the lymphatics on the surface of the peritoneum. He 

 stretched the tendinous centre of the diaphragm, excised from a rabbit, over a ring of 

 cork, covered it with a film of milk, and then, watching it with the microscope, saw 

 the milk-globules at various points drawn down as if in a vortex, and disappearing. 

 He then found they had passed into the lymphatics of the peritoneal covering of the 

 diaphragm, by small openings, not more than twice the diameter of a blood-corpuscle, 

 over which the peritoneal epithelium was similarly perforated. Observations in con- 

 firmation of these have since been made in the Physiological Institute of Leipsic, 

 under the direction of Professor Ludwig, by Dr. Dybkowski, who has found epithe- 

 lial apertures (answering very nearly to those described by von Recklinghausen) on 



