86 GENERAL ANATOMY. 



arranged longitudinally. The lymphatics are supplied by nutrient vessels, which 

 are distributed to their outer and middle coats ; but no nerves have at present 

 been traced into them. 



The lymphatics are very generally provided with valves, which assist mate- 

 rially in effecting the circulation of the fluid they contain. These valves are 

 formed of a thin layer of fibrous tissue, lined on both surfaces by endothelium. 

 Their form is semilunar; they are attached by their convex edge to the sides of 

 the vessel, the concave edge being free and directed along the course of the con- 

 tained current. Usually two such valves, of equal size, are found opposite one 

 another ; but occasionally exceptions occur, especially at or near the anastomoses 

 of lymphatic vessels. Thus, one valve may be of very rudimentary size and the 

 other increased in proportion. 



The valves in the lymphatic vessels are placed at much shorter intervals than 

 in the veins. They are most numerous near the lymphatic glands, and they are 

 found more frequently in the lymphatics of the neck and upper extremity than in 

 the lower. The wall of the lymphatics immediately above the point of attach- 

 ment of each segment of a valve is expanded into a pouch or sinus, which gives 

 to these vessels, when distended, the knotted or beaded appearance which they 

 present. Valves are wanting in the vessels composing the plexiform network in 

 which the lymphatics usually originate on the surface of the body. 



Origin of Lymphatics. The finest visible lymphatic vessels (lymphatic capil- 

 laries) form a plexiform network in the tissues and organs, and they consist of a 

 single layer of endothelial plates, with more or less sinuous margins. These ves- 

 sels commence in an intercommunicating system of clefts or spaces in the connec- 

 tive tissue of the different organs, which have no complete endothelial lining. They 

 have been named the rootlets of the lymphatics, and are identical with the spaces 

 in which the connective-tissue corpuscles are contained. This then is properly 

 regarded as one method of their commencement, when the lymphatic vessels are 

 apparently continuous with spaces in the connective tissue, and Klein has described 

 and figured a direct communication between these spaces and the lymphatic vessel. 1 

 But the lymphatics have also other modes of origin, for the intestinal lacteals 

 commence by closed extremities, though some observers believe that the closed 

 extremity is continuous with a minute network contained in the substance of the 

 villus, through which the lacteal is connected with the epithelial cells covering it. 

 Again, it seems now to be conclusively proved that the serous membranes present 

 stomata or openings between the epithelial celis (Fig. 62) by which there is an 

 open communication with the lymphatic system, and through which the lymph is 

 thought to be pumped by the ultimate dilatation and contraction of the serous 

 surface, due to the movements of respiration and circulation, 2 so that the serous 

 and synovial sacs may be regarded, in a certain sense, as large lymph-cavities or 

 sinuses. Von Recklinghausen was the first to observe the passage of milk and 

 other colored fluids through these stomata on the peritoneal surface of the 

 central tendon of the diaphragm. Again, in most glandular structures the 

 lymphatic capillaries have a lacunar origin. Here they begin in irregular clefts or 

 spaces in the tissue of the part ; occupying the penetrating connective tissue and 

 surrounding the lacunae or tubules of the gland, and in many places separating the 

 capillary network from the alveolus or tubule, so that the interchange between the 

 blood and the secreting cells of the part must be carried on through this lymph- 

 space or lacuna. Closely allied to this is the mode of origin of lymphatics in 

 perivascular and perineural spaces. Sometimes a minute artery may be seen to be 

 ensheathed for a certain distance by a lymphatic capillary vessel, which is 

 often many times wider than a blood-capillary. These are known as perivascular 

 lymphatics. 



1 AtlaK of Histology, pi. viii. fig. xiv. 



2 The resemblance between lymph and serum led Hewson long ago to regard the serous cavities 

 as sacs into which the lymphatics open. Recent microscopic discoveries confirm this opinion in a 

 very interesting manner. 



