126 



NORMAL HISTOLOGY. 



FIG. 153. 



the blood then slowly traverses the partially obstructed channels 

 within the pulp and is collected by the venous spaces and passed on 

 to the larger veins, by which it escapes from 

 the organ. The retarded current within the 

 splenic pulp is favorable to the removal and 

 destruction 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 

 not confined to channels provided with defi- 

 nite walls, but comes into direct 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 along the nerve-trunks. 



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) ; one twig 

 of artery is diverted and becomes 

 ensheathed by tissue of the Mal- 

 pighian corpuscle, M '; the remain- 

 ing part of the artery follows the 

 trabecula and passes directly into 

 the spaces of the pulp in either 

 case the arterial branches termi- 

 nate in the spaces (p, />) within 

 the pulp surrounded by the lym- 

 phoid tissue (/, /) ; the venous 

 radicles take up the blood and 

 carry it from the spaces of the 

 pulp into the larger venous 

 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 



