ORIGINS OF THE LYMPHATIC VESSELS. 83 



percolate through these tissues, which are not supplied by 

 blood-capillaries, and afford pabulum for their nutrition which 

 would otherwise be unprovided. 



Cell-spaces are also especially observable in the fibrous 

 tissue of the central tendon of the diaphragm, irregular stellate 

 lymph-spaces corresponding to connective-tissue cells, with 

 branches communicating with one another and perhaps with 

 lymph-vessels. 



The spaces in areolar tisane also serve as lymph-spaces, 

 communicating freely with the lymph-vessels, and from their 

 looseness are capable of holding large accumulations of lymph 

 and fluid, as in oedematous conditions and subcutaneous injec- 

 tions. These spaces are simply fissures in the substance of 

 the connective tissue, without any specialized walls. 



In certain regions, notably the nervous centres, occur well- 

 marked perivascular and perineurial lymphatics, consisting of 

 longitudinal passages or clefts in the tunica adventitia of the 

 bloodvessels and fibrous tissue surrounding the nerves ; these 

 passages when well developed may be lined with endo- 

 thelium. 



The open meshes of lymphoid tissue also constitute lymph- 

 spaces, partially lined with flat connective-tissue or endo- 

 thelioid cells. 



The serous sacs, as the pleura, pericardium, peritoneum, 

 sy no vial sacs, bursse, etc., may be regarded as representing 

 highly developed lymph-spaces. They are large cavities 

 completely and definitely lined by serous membranes, which 

 have been already described as fibrous membranes lined on 

 the free surface by endothelium. The cavity of the serous 

 sacs communicates with the lymph- vessels by stomata. The 

 function of serous membranes is, however, apparently more 

 to obviate friction and facilitate motion between opposing 

 surfaces than to serve as components of the lymphatic system. 

 Ordinarily the serous sacs contain only a small amount of 

 fluid, which acts as a lubricant ; in the synovial sacs and 

 bursse this fluid is much different from ordinary lymph, being 

 glairy and viscid and having the character of a special secre- 

 tion. 



Origins of the lymphatic vessels: The lymph-spaces just 

 considered constitute the chief portion of the ultimate origins 



