THE ABDOMEN 815 



enter the spleen pulp to end in clusters of capillaries. Before 

 they terminate in these clusters their external coat undergoes 

 an important modification, which consists in its transformation 

 into lymphoid or adenoid tissue. This Ijonphoid tissue forms at 

 intervals small round or oval enlargements, called the Malpighian 

 corpuscles, which on section appear as minute white specks in the 

 dark red spleen pulp. These corpuscles are simply localized 

 expansions of the Honphoid tissue which forms the external coat of 

 the small arteries. The expansion may be confined to one side of 

 the artery, or it may include the whole of its circumference. Each 

 corpuscle receives minute twigs from the artery on which it is 

 set, and contains large numbers of Ijnnph corpuscles, as well as 

 capillary bloodvessels. 



Veins. — The arterial blood, on leaving the capillary vessels, flows 

 directly into the interstices of the reticular matrix of the spleen 

 pulp, and from these it is taken up by radicle veins which commence 

 in a manner similar to that in which the capillaries end. Endo- 

 thelial cells, continuous with the sustentacular cells of the pulp, 

 come together and cohere, so as to form very delicate tubular 

 vessels, having closed walls. As these radicle veins unite and 

 become larger the usual other coats are superadded to the endo- 

 thelial lining. Ultimately five or six veins leave the spleen at the 

 hUmn, which unite to form the splenic vein, this in turn uniting 

 with the superior mesenteric vein to form the vena portae. 



It is to be noted that, in the circulation through the spleen, the 

 arterial blood leaves tubular vessels and flows through the inter- 

 stices of the reticulum of the spleen pulp, where it bathes the 

 Malpighian corpuscles, after which it enters tubular vessels of the 

 nature of veins. 



The cells of the spleen are of three kinds as follows : (i) the 

 sustentacular cells of the retiform tissue of the spleen pulp ; (2) the 

 splenic cells ; and (3) lymphoid corpuscles. 



Lymphatics. — These are arranged in two groups — trabecular and 

 perivascular. The trabecular lymphatics are contained in the 

 trabeculae, and communicate with a lymphatic network in the 

 tunica propria underneath the peritoneal coat. The 'perivascular 

 lymphatics commence in the lymphoid tissue which forms the external 

 coat of the smaller arteries. At the hilum both sets of lymphatics 

 meet and pass to the splenic glands, and thence to the cceliac glinds. 



Nerves. — These are derived from the splenic plexus, >yhich is an 

 offshoot from the coeliac plexus of the solar plexTis. The fibres, 

 which are mosth' non-medullated, are derived parti v from the svm- 

 pathetic system and partly from the right pneimaogastric nerve. 



Development of the Spleen. — The spleen is usually regarded as being de- 

 veloped from the mesoderm between the two layers of the mesogastrium. It 

 is in close proximity to the developing pancreas, which, howe^^e^, is of ento- 

 dermic origin. Towards the end of the second month the mesenchyme 

 becomes thickened by an accumulation of lymphoid cells. This mass is in- 

 creased by the addition of cells which migrate from the surface-cells of the 

 mesogastrium, and it becomes permeated by bloodvessels. The cells give 

 off processes, and these interlace, thus giving rise to the trabecular network. 



