THE LYMPHATICS. 715 



to their transparency and tenuity. Thanks, however, to the patient and minute 

 researches of Hunter, Cruikshank, Mascagni, Fohmann, Panizza, Cruveilhier, 

 and Sappey, tlie lymphatics were injected by colouring mattera or by mercury, 

 and thus rendered visible to their finest ramifications. 



It is now known that the lymphatics arise from capillaries, which form net- 

 works or terminal cuh-de-snc. 



These terminal cnls-de-sac exist in the intestinal villi ; and it is no longer 

 maintained that the ends of these small appendages have an opening by which 

 the lymphatic vessels receive the chyle that bathes the mucous membrane of the 

 intestine. 



The plexuses are composed of more or less irregular meshes, and their form 

 and volume often vary with the arrangement of the tissues or organs in which 

 they are studied. They may be superficial or deep, and exist together or sepa- 

 rately. In many membranes the two netw^orks are found, but then the superficial 

 is thinner than the deep. They are mixed with, or placed above, the blood-vessel 

 plexuses, but never conmiunicate with them. 



Do these lymphatic plexuses exist in all the tissues, properly speaking ? Here 

 is another question of incontestible importance, the solution of which for a 

 long time occupied the attention of anatomists. Judging by analogies, one is 

 tempted to reply in the atfirmative — why, in fact, should the lymphatics not 

 exist everywhere throughout the organism, when the blood-capillaries are con- 

 stituent parts of the framework of each tissue ? Nevertheless, direct observa- 

 tion has not revealed lymphatic plexuses in all organs ; there are even tissues in 

 which their existence has been alisolutely denied — though prematurely, it is well 

 to say, because we may always ascribe the non-success of a lymphatic injection 

 either to the imperfection of the instruments employed, the insufficiency of the 

 measures adopted, or to certain peculiar conditions as yet unknown and attach- 

 ing to the species of animals selected for the demonstration of the lymphatic 

 networks in a certain region. In support of this last assertion, we may observe 

 that Sappey has not yet been able to inject the pituitary plexuses in Man or the 

 Calf, and that he looks upon their existence as being at least doubtful ; while 

 in the Horse, this lymphatic apparatus is as remarkable for its richness, as for 

 the facility with which it may be filled with mercury. 



It may be asserted that they do not exist in the epidermis, or in epithelial 

 layers. 



The following are the most trustworthy notions available on this subject. 



The lymphatic vessels of the skin are very numerous, and form two networks : 

 one, with extremely fine meshes, occupies the most superficial layer of the dermis ; 

 the other, placed beneath the deep face of the integument, includes vessels more 

 voluminous than the firet, and communicates with it l)y multiple ramuscules. 

 These lymphatic plexuses are far from being equally developed in every region, 

 though it is unanimously agreed that no part is entirely destitute of them. 



In the mucous membranes, an analogous arrangement of these vessels is met 

 with. It is more than probable that they exist throughout the whole extent of 

 these membranes, though their positive demonstration has yet to be made in 

 some regions. In other regions, the injection of these networks is, on the 

 contrary, very easy, and gives the most magnificent results ; we particularly 

 mention the lingual, intestinal, and pituitary mucous membranes. The 

 lymphatics belonging to the latter membrane assume so beautiful an aspect 

 in the Horse, that we would advise anatomists who desire to inject lymphatics 



