HISTOLOGY 



97 



ary arteries may divide forming more than two points of entrance. After 

 entering the organ the arteries divide repeatedly, decreasing gradually in 

 size. During their courses they give rise to numerous capillaries which 

 supply the lymphatic tissue, the white pulp, with blood. Lymphatic tissue 

 is arranged around the arteries in the form of a continuous sheath which 

 thickens at points where branching occurs. Around the small arteries the 

 tunica adventitia is replaced by lymphatic tissue. Lymphatic nodules, or 



Central artery Splenic nodule Red pulp 



White pulp 



Megakaryocyte 



Red pulp 



Capsule 



Trabecula 



Fig. 40. — Spleen. (X50.) 



splenic nodules (Malpighian bodies), are found arranged around central 

 arteries. The outline of these nodules is usually very indistinct. Their 

 central areas sometimes contain many medium sized lymphocytes forming 

 germinal centers, the secondary nodules. As in lymph nodes, these are 

 transitory structures. 



The small arteries of the white pulp, after repeated branching, enter the 

 surrounding tissue, the red pulp, where they divide into many straight 

 arteries, the penicilli. A short distance before their termination, the walls 

 of these vessels are thickened by closely applied fibers of reticular tissue, 

 while the lumina remain narrow. These are the sheathed arteries of the 

 pulp, which after further branching give rise to arterial capillaries. The 

 question of "open" or "closed" circulation, depending on whether 

 the arterial capillaries open into the spaces between the reticular cells or into 

 the venous sinuses, is still under discussion. 



