212 



HISTOLOGY 



ulae, in which the veins continue further. The arteries, however, are still 

 surrounded by a considerable connective tissue layer, the outer portion of 

 which becomes reticular and is rilled with the lymphocytes of the nodules. 

 The nodules occur near where the artery branches. Small arterial twigs 

 ramify in the nodules, in the periphery of which they anastomose before 

 passing into the pulp. When the main stems are about 15 AC in diameter, 

 they lose their surrounding lymphoid layer and pass into the pulp, where 

 they form brush-like groups of -branches (penicilli}. These branches do 

 not anastomose. For a short distance before their termination the walls 

 of the branches possess ellipsoid thickenings, due to a longitudinal ar- 

 rangement of closely applied fibers of reticular tissue. These "sheathed 



arteries" are 6-8 M in dia- 

 meter, and have been sup- 

 posed to regulate the amount 

 of blood which eaters the 

 terminal portion of the artery, 

 beyond them. Some autho- 

 rities state that this distal 

 part connects with the ter- 

 minal veins, meeting them 



FIG. 200. CROSS SECTION (A) AND SURFACE VIEW (B) OF 



TERMINAL VEINS FROM THE HUMAN SPLEEN at an acu t e angle. AcCOrd- 



ing to others such connec- 

 tions are infrequent, and still 

 others believe that the arteries empty only into the reticular tissue. 

 Numerous careful injections have shown the readiness with which the 

 arterial blood mingles with the pulp cells. 



The terminal veins or splenic sinuses begin as dilated structures (some- 

 times unfortunately called "ampullae," the latter term being applied 

 also to the terminal arteries). Their endothelial cells are so long and 

 slender as to suggest smooth muscle fibers, and like certain other endothe- 

 lial cells they are contractile. Their edges are not closely approximated, 

 so that corpuscles may pass between them freely (Fig. 200) . Around them 

 are encircling reticular tissue fibers, and a continuous basement membrane 

 has been described as stretching across the intervals between the endothelial 

 cells. The existence of such a membrane has recently been denied. The 

 endothelial cells project into the lumen of the vessel, and their nuclei are 

 at the summits of the elevations. Frequently the nuclei show one or 

 two longitudinal rod-like markings, said to be due to folds in the nuclear 

 membrane (Fig. 200, B) Several terminal veins unite to form a pulp vein, 

 which enters a trabecula in which it pa'sses toward the hilus. The tra- 

 becular veins join to form the splenic vein. 



For further details regarding the circulation see Weidenreich (Arch. f. mikr. Anat., 

 1901, vol. 58, pp. 247-376) and Mall (Amer. Journ. Anat., 1903, vol. 2, pp. 315-332). 



e., Rod shaped endothelial cells, with projecting nuclei, n ; r., 

 encircling reticular tissue; 1., leucocytes passing between 

 the endothelial cells. (After Weidenreich.) 



