SPLEEN 



211 



from the artery to the vein. Surrounding the arterial branches there 

 is a zone of lymphoid tissue, which arises rather late in embryonic life. 

 In reptilian spleens it is so abundantly developed that the organs resemble 

 mammalian haemolymph glands. In the guinea-pig the lymphoid 

 sheath of the arteries is continuous, though narrow; in man it is so inter- 

 rupted as to form a succession of spindle-shaped or spherical masses, 

 called splenic nodules (Malpighian corpuscles). An arterial branch 

 passes through each nodule. Thus, as compared with the haemolymph 



Terminal vein 



[Sheathed artery. Pulpartery. 



Pulp vein. 



Beginning of a 



trabecular vein. 



Capillaries of 

 a nodule. 



Trabecula. 



Penicillus. 



, , Splenic 

 V 1 /obule. 



Hilus. Reticulum. Splenic noudle. 



Capsule. 



FIG. 199. DIAGRAM OF THB BLOOD VESSELS OF THB HUMAN SPLEEN. 



At x is shown the direct connection of terminal arteries with terminal veins (the existence of such a connec- 

 tion has been questioned). At xx and xxx are the free endings of the terminal veins in the pulp and 

 near the nodules respectively. 



gland, the spleen is deficient in lymphoid tissue (Fig. 198). The bulk 

 of the spleen is composed of splenic pulp, which corresponds with the 

 blood sinus of the haemolymph glands. Its framework of reticular tissue 

 is continuous with that of the nodules, and it contains blood corpuscles 

 of all sorts, special phagocytic cells known as splenic cells, and the terminal 

 branches of both arteries and veins. There are no lymphatic vessels 

 within the spleen. The capsule and trabecular framework are highly 

 developed as in the largest lymph glands. The following features of the 

 spleen may be described in turn the blood vessels, the pulp, the nodules, 

 the capsule and trabeculae, and finally the nerves. 



As shown in the diagram, Fig. 199, the splenic artery enters at the 

 hilus and, accompanied by veins, its branches are found in the largest 

 trabeculae. When about 0.2 mm. in diameter the arteries leave the trabec- 



