RELATION OF THE SEROUS CANALS TO THE BLOODVESSELS. 325 



lymphatic sheaths generally within the tunica adventitia of the 

 bloodvessels. But this only is certain, that in the latter situa- 

 tion the serous canal system presents an extraordinary expansion, 

 and is on this account predisposed to cellular infiltration. 



The fluid contents of the serous canals, as well as of the 

 lymphatic vessels, that is to say, the lymph itself, primarily 

 comes from the blood ; it is therefore a question of peculiar im- 

 portance to determine what relation the serous canal system bears 

 to the bloodvessels, and especially to the blood capillaries. At 

 first sight it appears most natural to consider that the serous 

 canals stand in the same communication with them as with the 

 lymphatic capillaries. This was the relation which the authors 

 of the last century understood by their vasa serosa, vessels 

 which, on account of their small calibre, only permitted the pas- 

 sage of the colourless serum, and arrested that of the corpuscles. 

 Leydig has translated this view into modern language, in 

 stating that the connective tissue corpuscles are continuous not 

 only with the lymphatic vessels but also with the bloodvessels. 

 Fuhrer, and before him Lessing, had already maintained the 

 view that "the vasa serosa formed a plasmatic system con- 

 necting together the blood and lymphatic capillaries," in the 

 interior of which the cells were situated. I formerly held 

 it to be improbable that the serous canals were continuous 

 with the bloodvessels, since I had not then given up the 

 old view that the wall of the bloodvessels consists of a homo- 

 geneous substance. Since, however, it has been demonstrated 

 by Aeby, Auerbach, and Eberth, by means of solutions of nitrate 

 of silver, that the walls of the capillaries were composed of an 

 epithelium, at all events in such organs as they had examined ; 

 since, moreover, the permeability of the vascular wall for the 

 red blood corpuscles (Virchow, Strieker), and also for the 

 colourless corpuscles (Cohnheim), has been noted under circum- 

 tances which, though certainly not normal, yet can never- 

 theless be so rapidly brought about that it is impossible to 

 admit the occurrence of a qualitative change in the nature of 

 the capillary wall, I consider it to be very possible that the serous 

 canals may stand in the same open continuity with the blood- 

 vessels as with the lymphatics. That such communications do 

 actually exist under normal conditions is also rendered highly 



