THE LYMPHATIC VESSELS OF THE SEROUS MEMBRANES. 37 



artery in such a way that the whole system of the capillaries is invagi- 

 nated in a sacciform dilatation of the lymphatic vessel. Not till the 

 root of the artery is reached does the lymphatic as such cease to exist, 

 at which point it loses itself in the lymph-canalicular system ac- 

 companying the artery. Thus we have before us here a case where a 

 vein is invaginated in a lymphatic vessel, while the system of capil- 

 laries belonging to it hangs freely in a lymph-sac, which is a direct 

 continuation of the lymphatic tube. A modification of this form is 

 the following : A lymphatic vessel that simply accompanies a vein 

 possesses a blind sacciform dilatation, in which the system of capillaries 

 belonging to the vein is inserted like the glomerulus of a Malphigian 

 corpuscle of the kidney. We have already mentioned that in the* 

 case where a blood-vessel is invaginated the blood-vessel itself 

 possesses on its outer wall, viz. on that diverted towards the lumen of 

 the lymphatic, an endothelial covering, which exactly resembles that 

 forming the outer wall of the invaginating lymphatic. On the 

 invaginated capillaries the same is to be seen, that is to say, besides 

 the endothelium forming the outer wall of the common lymph-sac, 

 there is an endothelial covering outside the capillaries. From this it 

 is clear that the two kinds enumerated of invaginated capillary net- 

 work are identical. In both cases we have an outer wall of endo- 

 thelium for the lymph-sac, and an endothelial wall around the 

 capillaries. 



The second case of invagination of the capillary system is more 

 frequent than the first, for the reason that the case of a lymphatic 

 vessel simply accompanying a vein is more frequent than the case in 

 which the vein is invaginated. So long as the system of capillary blood- 

 vessels is small there is nothing further in the lymph-sac to be seen, 

 except here and there a migratory cell. Besides these, however, one 

 finds nodular structures which resemble those just mentioned in some 

 respects, but differ from them in other respects. We will direct the 

 attention of the reader to Fig. 50 on Plate X. and Fig. 43 on Plate 

 VIII. 



In Fig. 50 we see that from the endothelial outer wall of a 

 capillary vessel branched nucleated protoplasmic bodies extend to that 

 of a neighbouring capillary ; these occur, therefore, in the lumen of 



