126 



NORMAL HISTOLOGY. 



FIG. 153. 



pulp readily occur. The venous radicles unite to form .more defi- 

 nite intralobular veins which become tributary to the larger 

 venous trunks occupying the trabeculae. 

 The retarded current within the splenic 

 pulp is favorable to the removal and de- 

 struction of the worn-out red cells and to 

 the acquisition of additional leucocytes. 

 Within the pulp, while passing from the 

 arteries to the veins, the blood is probably 

 confined to channels provided with defi- 

 nite walls, but comes into close relation with 

 the lymphoid tissue. 



The lymphatics of the spleen are limited 

 to the connective-tissue framework of the 

 organ, in which they form a superficial 

 plexus in the deeper layers of the capsule, 

 and a deeper plexus within the trabeculae. 

 The lymphatic clefts within the adventitia 

 of the arteries communicate with the deeper 

 lymphatics of the trabeculae ; regarding the 

 definite relations of the deeper lymphatics 

 our knowledge is incomplete. 



The nerves of the spleen are composed 

 mostly of non-medullated fibres, although 

 a few of the medullated variety are present ; 

 they are distributed to the walls of the blood- 

 vessels ; also ganglion-cells have been ob- 

 served as small groups situated along the nerve-trunks. 



THE THYMUS BODY. 



The thymus body is included among the lymphatic tissues on ac- 

 count of the histological characteristics of the fully-developed organ ; 

 in its early stages, however, the bulk of the organ is epithelial in 

 nature, being derived from the endodermic cells and closely resem- 

 bling many glands in its earliest growth. The rapid invasion of 

 mesodermic tissues, at a later period, so changes the character of the 

 organ that tissues of a lymphoid type predominate, while the original 

 epithelial structures are reduced to mere rudimentary remains. 



The entire organ usually consists of two lateral lobes, more or 

 less intimately united, composed of numbers of lobules, held together 

 by the interlobular areolar tissue and enveloped within the general 

 fibrous capsule of the organ. The irregularly ovoid lobules, 5-10 

 mm. in diameter, are further divided by connective-tissue septa into 

 compartments, each of which includes several smaller secondary 



Diagram of the relations of 

 splenic vessels to the tissue of the 

 pulp : a, -v, small arterial and 

 venous branches of splenic vessels 

 within trabecula (t, ) ; main 

 twig of artery enters the lobule 

 and becomes ensheathed by tis- 

 sue of Malpighian corpuscle, M ; 

 smaller part of artery follows tra- 

 becula for a distance before end- 

 ing in blood-channels (p,p) sur- 

 rounded by lymphoid tissue (/,/); 

 the venous radicles take up the 



within the trabeculae. 



