GENERAL CONSIDERATIONS. 629 



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

 maintained that the ends of these small appendices have an opening by 

 which the lymphatic receives 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 disposition of the tissues or organs in 

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

 or separately. In many membranes the two networks are found, but then 

 the superficial is thinner than the deep. They are mixed with, or placed 

 above, the blood-vessel plexuses, but never communicate with them. 



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

 is another question of incontestible importance, and whose solution is even 

 now occupying the attention of anatomists. Judging by analogies, one is 

 tempted to reply in the affirmative : why, in fact, should the lymphatics not 

 be spread everywhere throughout the organism, when the sanguine capil- 

 laries are constituent parts in the framework of each tissue ? It is true we 

 may ask if lymphatic absorption is a necessary act in the vital movement ; and 

 although science is far from being satisfied on this point, we know some 

 facts which at least authorize the doubt. On the other hand, direct ob- 

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

 tissues in which their existence has been denied : though prematurely, it is 

 well to say, because we may always attribute the non-success of a lymphatic 

 injection either to the imperfection of the instruments employed, the 

 insufficiency of the measures adopted, or certain peculiar conditions as yet 

 unknown attaching to the species of animals selected for the demonstration 

 of the lymphatic networks in a certain region. As bearing out this last 

 assertion, we may observe that M. Sappey has not yet been ahle to inject 

 the pituitary plexuses in Man or the Calf, and that he loolis 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. 



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



The lymphatic vessels of the sMn are very numerous, and form two net- 

 works : one, with extremely fine meshes, occupies the most superficial 

 layer of the dermis ; the other, placed beneath the deep face of the integu- 

 ment, includes vessels more voluminous than the first, and communicates 

 with it by multiplied 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 internal tegument, or mucous membranes, an analogous disposition 

 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 always to choose that animal. 

 The operation is simple and constantly performed, and we are astonished 

 that in the hands of some individuals it should fail. Not only can the. two 

 networks of the membrane be filled, but also the trunks arising from them, 

 and which are directed towards the entrance of the nasal cavities, collect in 

 several thick branches around the nostril, and bend up towards the ffce 



