120 ELEVENTH LECTURE. 



the arteries ; it is gradually changed into a reticular connec- 

 tive tissue containing lymphoid cells, a metamorphosis in 

 which the adventitia soon participates. The sheath struc- 

 ture is extended somewhat further over the venous branch ; 

 at last its fibres begin to separate, and it is also lost in the sep- 

 tum or trabecular system of our organ. 



From this lymphoid metamorphosis of the artery arise the 

 already familiar Malpighian corpuscles of the spleen. They 

 lie in part on the point of ramification of arterial branches, 

 in part laterally on the unramified vascular tube. Finally — 

 and it is a frequent occurrence — the arterial branch passes 

 through the centre of the follicle. If we examine more 

 closely, we find that no line of demarcation can be drawn 

 between the separate follicles and this elongated lymphoid 

 covering of the arterial branch. Every Guinea-pig's spleen 

 teaches us this. 



In the follicle, we never meet with a venous branch, but, 

 rather, a capillary reticulum with rounded meshes, sometimes 

 scantily and poorly developed, sometimes more abundantly. 

 The source of supply varies ; sometimes it is branches of the 

 follicular artery, sometimes it is through the adjacent pulp- 

 tubes. 



We have now to follow the further course of the arterial 

 offshoots, the so called penicilli of the spleen. They enter 

 the pulp-tubes of our organ, to pass through their axis and 

 to become capillaries. The capillary reticulum of the Mal- 

 pighian corpuscle also, at last, sends its offshoots down into 

 the adjacent pulp-tubes (Fig. 113, e). 



Now, these capillaries of the pulp-tubes are quite peculiar. 



We follow them by the greatest attention for a certain dis- 

 tance (on an uninjected spleen or in a good injected prepara- 

 tion), then the capillaries (Fig. 112, d) commence to be 

 uncertain and indistinct [e). Separated cell-demarcations 

 may still be recognized ; but soon even these disappear. We 

 are in the presence of a lacuna — a finest blood current with- 

 out walls (Fig. 113, e). 



Let us recall to mind that the tissue of the pulp-tube pre- 



