BLOOD-FORMING GLANDS 



171 



pulp. The pulp is divided into rather indistinct regions, each of which 

 is supplied with a fine arterial branch. 



Both the nodular capillaries and the pulp branches discharge the 

 blood into the pulp, with which it mixes and from which it is afterwards 

 drawn out by the veins which originate in this pulp. The exact degree 

 of direct connection between artery and vein through this pulp is a sub- 

 ject of much doubt and of some controversy. 



The arteries show, near the point at which they terminate in the pulp, 

 a thickened wall which is supposed to regulate the amount of blood that 

 is discharged into the pulp. 

 The veins show at or near 

 their point of origin a basket- 

 work, or open wicker-like ar- 

 rangement, of the circular and 

 longitudinal fibers, through 

 which the blood may enter 

 them from the pulp. These 

 fibers are not muscle fibers, 

 but contractile endothelial 

 elements. 



The pulp itself is com- 

 posed of a reticular connec- 

 tive-t issue framework in 

 whose meshes are to be found 

 the pulp cells. The whole a. 1 



Mai* c. 



FIG. 152. Diagram of a portion of spleen, a., 

 ^^ with branches to the compartment units; 



v< vein with its collecting branches ; M ai. c ., Mai- 



pighian body or lymph node ; sp.c., spleen pulp 



-sp. c. 



****** 



mass is infiltrated with the 



blood Cells which have been 

 tVirnwn i'nrn it hv trio artPnVc 



thrown into it by trie arteries. 



As may be Seen in Figure 153, 



from a salamander, some of 



these red corpuscles swell up 



and break down and are probably ingested by white corpuscles or 



phagocytes. 



The structure of the spleen would possibly permit of the fellowing 

 processes to take place in it: First, the production of lymph cells 

 in the lymph nodules and their passage into the pulp. Second, the 

 passage into the pulp of red blood corpuscles many of which (the 

 broken or diseased or "worn-out" ones) are disintegrated and ab- 

 sorbed by the lymph cells, which then pass out through the terminal 

 veins together with those red corpuscles which have escaped destruc- 

 tion. 



The three (in theory but two) kinds of glands mentioned above 

 operate to produce white blood corpuscles (lymph tissue] and to destroy 



