324 THE LYMPHATIC SYSTEM, BY F. v. RECKLINGHAUSEN. 



coalesce, and form larger cavities, which then become invested 

 with an epithelium : of this the serous cavities may be taken 

 as an example in a physiological point of view, and the so- 

 called serous cysts in a pathological. Where spaces of this 

 kind form in or upon the tunica adventitia of the blood- 

 vessels, we obtain sheath-like investments resembling the 

 lymph sheaths of the tubuli seminiferi. To these belong 

 the perivascular lymphatics described by His as existing 

 partly in the membrane, and partly in the substance of the 

 brain and spinal cord; these are really interstitial spaces bet ween 

 the bloodvessels and the substance of the brain, continuous 

 with a wide " epicerebral cavity " situated beneath the pia 

 mater. That this last does not constitute a mere interstitial 

 space may be maintained on the ground that it can be filled 

 from the true lymphatic vessels of the pia mater. His has 

 demonstrated the existence of an epithelium in the larger of 

 these perivascular canals and sheaths, and they therefore repre- 

 sent the same grade of organization as the lymphatic capillaries. 

 Macgillavry also found, in injected preparations of the liver, 

 lymphatic sheaths around the bloodvessels, but it has not, been 

 satisfactorily ascertained whether they are or are not lined with 

 an epithelium. Strieker has, moreover, described a similar ar- 

 rangement of sheaths around the blood capillaries of the lower 

 eyelid of the Frog ; whilst Langer has shown that in this region 

 only two lateral lymph tubes are present, which lie close to the 

 bloodvessel, and occasionally unite by transverse anastomoses 

 which cross the vessel like a bridge. It further appears from 

 Langer's careful investigations in the Frog, where the large 

 bloodvessels are ensheathed by lymph sacs, or by processes of 

 the lymph sacs, that from the point of their entrance into the 

 different organs an " invagination of the bloodvessels by the lym- 

 phatic tubes is no longer to be distinguished " ; in the serous and 

 mucous membranes two lymph vessels, but in the interior of 

 the parenchymatous structures only a single lymphatic vessel 

 accompanies each artery. These investigations afford an im- 

 portant caution against too hastily admitting the existence of 

 lymphatic sheaths around the bloodvessels. Many authors 

 were formerly inclined to ascribe a perivascular system of 

 canals to the bloodvessels of other organs, or at least to seek for 



