It divides into six or more branches, which enter the hilum of the spleen and ramify throughout its substance, receiving sheaths from an involution of the external fibrous tissue.

These ultimately leave the trabecular sheaths, and terminate in the proper substance of the spleen in small tufts or pencils of minute arterioles, which open into the interstices of the reticulum formed by the branched sustentacular cells.

The arterioles, supported by the minute trabeculae, traverse the pulp in all directions in bundles (penicilli) of straight vessels.

The altered coat of the arterioles, consisting of adenoid tissue, presents here and there thickenings of a spheroidal shape, the white pulp.

In this manner the vessels end, and the blood flowing through them finds its way into the interstices of the reticulated tissue of the splenic pulp.