2O4 BLOOD AND BLOOD-FORMING ORGANS. 



arterial capillaries into the channels of the pulp. The lymphoid 

 sheaths and nodules derive their blood supply from arteries which 

 arise from the lateral branches of the splenic vessels, and which 

 divide into capillaries inside of the lymph sheaths or nodules, and 

 only assume a venous character outside of the lymphoid substance. 

 These vessels constitute the nutritive vascular system of the spleen. 



The small arterial branches above mentioned break up into very 

 fine arterioles which gradually lose their lymphoid sheath, so that 

 branches with a diameter of 0.02 mm. no longer possess a lymphoid 

 sheath, but again assume an adventitia of the usual type. The 

 smallest arterioles now pass over into capillaries which are for a 

 time accompanied by the adventitia (capillary sheath), while the 

 terminal branches have the usual structure of the capillary wall and 

 are gradually lost in the meshes of the pulp. (See below.) On the 

 other hand, the beginnings of the venous capillaries may be dis- 

 tinctly seen in the pulp spaces. Groups of these capillaries com- 

 bine to form larger vessels, which, however, still retain a capillary 

 structure, and these again form small veins which unite to form the 

 larger veins. 



F. P. Mall, whose recent contributions on the structure of the spleen 

 have greatly extended our knowledge of the microscopic anatomy of 

 this organ, states that the trabecular and vascular systems together 

 outline masses of spleen pulp about i mm. in diameter, which he has 

 named spleen lobules. Each lobule is bounded by three main in- 

 terlobular trabeculae, each of which sends three intralobular trabe- 

 culae into the lobule which communicate with each other in such a 

 manner as to divide the lobule into about ten smaller compartments. 

 An artery enters at one end of the lobule and, passing up 

 through its center, gives off a branch to the spleen pulp found in 

 each of the ten compartments formed by the intralobular trabeculae. . 

 The spleen pulp in these compartments is arranged in the form of 

 anastomosing columns, or cords, to which Mall has given the name 

 of pulp cords. The branches of the main intralobular artery, going 

 to each compartment, divide repeatedly ; the terminal branches 

 course in the spleen-pulp cords, and in their path give off numerous 

 small side branches which end in small expansions known as the 

 ampulla of Thoma. An ampulla of Thoma may be divided into 

 three parts. The first part, which is the ampulla proper, is lined by 

 spindle-shaped cells, directly continuous with the endothelial cells of 

 the artery. The second third, which often communicates with neigh- 

 boring ampullae, contains large side -openings. The remaining third, 

 which is the intermediary segment of Thoma (Thoma 's Ziviscken- 

 stuck], is difficult to demonstrate. It is bridged over by fibrils of 

 reticulum, and its communication with the vein is not wide. The 

 circulation through the spleen is therefore not a closed one, through 

 a system of capillaries completely closed, but rather through spaces 

 in the spleen -pulp, certain of which are more direct, leading from 

 the terminal arteries to the veins. According to this view, then, 



