408 



MANUAL OP HISTOLOGY. 



lary tube becoming thinner and more freely studded with 

 nuclei ; from some of these nuclei processes are sent out which 

 connect with the fibrillaB of the pulp. At this stage injections 

 into the capillaries pass freely out into the surrounding tis- 

 sue. The exact point where the capillary wall merges into 

 the sustentacular tissue can hardly be determined. 



The venous radicles begin in a somewhat similar way to 

 that in which the capillaries end (Fig. 174). The sustentacu- 

 lar fibrillae (endothelioid plates ?) appear to arrange themselves, 

 first of all, in a circular manner, occasionally interlacing at 

 right angles (Fig. 175). Lying within and upon the fibrillse 



thus arranged are 

 oval cells with promi- 

 nent nuclei. These 

 nuclei are often con- 

 nected with the sus- 

 tentacular fibrillse 

 outside. These cells 

 are not adherent to 

 each other at first, 

 but, as the radicle be- 

 comes more perfect, 

 they unite to form a 

 complete wall ; the 



FIG. 175. From the pnlp of the human spleen, brushed preparation external layer of cir- 

 (combinntion): a, pulp strand with the delicate reticular framework ; _, , , 



transverse flection of the cavernous venous canal ; c, longitudinal sec- CUlar librill8B then 

 tion of such an one ; ti, capillary vessel in a pulp tube, dividing up at 



; /, epithelium of the venous canal ; g, side view of the same ; A, its b 6 C O HI 6 S metamOr- 

 transverse section. 1'rey. 



phosed into a tunica 



intima ; finally, the thick oval cells are replaced by flat endo- 

 thelial cells, and the complete venule is formed. 



Having shown how this vascular channel begins, we turn, 

 for convenience of description, to its other end. The splenic 

 vein enters the hilum, just as the artery does. As it subdi- 

 vides, however, it loses both tunica adventitia and tunica me- 

 dia. The internal tunic remains and becomes firmly united 

 with the fibrous trabeculae of the spleen, so that, on section, 

 the venous wall does not collapse, but appears like a part of 

 the parenchyma. After several subdivisions the veins begin 

 to anastomose, and they finally form a closely reticulated ar- 

 rangement of like-sized vessels having an average diameter 

 of T^f to T f ff mm. These are called the cavernous veins. The 



