THE SPLEEN 



129 



The adventitia of such arteries has loose fil»roelastic connective 

 tissue in which Iymi)hocytes become aggref^ated into rod-like or 

 oval masses, the splenic nodules, or Malpighian corpuscles. The 

 branches of these small arteries emerge into the red ])ulp, 

 where they are known as ])ul]) arteries. Each pulp artery di\ides 

 into a group of short vessels, the penicilli, which have a pulp, a 

 sheathed, and a terminal region. The tunica media of the first 

 division is rich in smooth nuiscle and is embedded in red i)uli); the 

 sheathed portion is devoir! of muscle and has a narrow lumen 

 surrounded by connective tissue infiltrated with the lymphocytes 

 and connected with the reticulum of the red pulp; the terminal 

 portion is an endothelial tube which unites with the peculiar sinu- 

 soids in the red pul]). 



Endothelial 

 cells 



Nucleus 

 Membrane 



Reticular fibers 



Fig. 78. — Diagram of part of a splenic sinusoid which consists of long endo- 

 thelial cells, a non-cellular membrane, and encircling a network of reticular fibers. 

 Openings are indicated from the lumen of the sinusoid through the membrane. 

 (Redrawn from Braus, in Bailey's Histology, Williams & Wilkins C'ompany.j 



The sinusoids (Fig. 7S) have a wider lumen than ordinary capil- 

 laries, and their elongated endothelial cells are separated from each 

 other at intervals and are associated with the reticular cells which 

 form a network through the red pulp. At intervals, fusiform cells 

 and reticular fibers encircle the endothelial cells. The many slit- 

 like spaces in the walls of the sinusoids communicate with the loosely 

 organized lymphoid tissue of the red pulp. The sinusoids join 

 complete endothelial-walled venous capillaries, which continue into 

 venules and veins carrying the blood back into the trabecular and 

 out of the spleen. (Fig. 79.) 



The sheathed portions of the penicilli where the lumen is 

 small slow down the blood issuing from the small arteries of 

 the splenic sinusoids. The sheathed penicilli may act as valves, 

 pre\enting backward flow of blood. In the red pulp of the spleen 

 and of hemal nodes, definite openings in the blood-vascular system 

 9 



