1. The spleen is divided into segmental zones or lobes, corresponding to the branches of the arteria lienalis which enter it. These zones are at once arterial and nervous units. 2. The scheme of subdivision of the splenic nerve corresponds to these segmental zones, and stimulation of a branch of the nerve causes only a section of the spleen to contract. Central to the cæliac ganglion (e.g. in the splanchnic nerve or in the anterior spinal roots) there is no such segmental representation in the nerve branches proceeding to the spleen. 3. The splenic vein is conspicuously larger than the splenic artery, that of the ox having five times the diameter of the artery. When a short section of the vein is electrically stimulated it contracts and remains in strong contraction for an hour or more, the outflow of blood being greatly impeded. 4. When the spleen is perfused with Indian ink the particles may come to rest in four different situations: (1) in the ellipsoids ; (2) in the spleen phagocytes; (3) on the ordinary reticulum; (4) on the walls of the venous sinuses. 5. In the living animal deposition in the ellipsoids is temporary. Within three hours after intravenous injection of the ink, phagocytes have begun to clear the particles from the spaces within the ellipsoids. Twenty‐four hours later the ellipsoids are free. 6. The phagocytes of the pulp retain the particles for weeks on end. At first they clear the ink off the reticulum. After a month they are seen to have undergone a rearrangement, and some have migrated into the Malpighian corpuscles. 7. The ellipsoids act as valves, readily allowing the blood to flow in the normal direction, i.e. from the interior to the exterior of the ellipsoid, but instantly closing against higher pressure applied to their exterior. The existence of these valves accounts for the difficulty of perfusing the spleen backwards, i.e. from vein to artery. 8. Each ellipsoid is surrounded by a number of venous sinuses, which become dilated when the venous pressure is rased.

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