The exact nature of the function of this organ is somewhat obscure. In order to understand the various changes which it undergoes, it will be necessary to bear in mind certain facts as to its structure. The splenic artery as it enters the organ is accompanied by connective tissue which forms a continuous sheath around its branches. In this sheath there develop at intervals little masses of lymphoid tissue, forming the Malpighian bodies, which are therefore lymphatic follicles closely related to the arterial branches.

The arteries break up into capillaries which are mostly distributed in the Malpighian bodies, where they form a somewhat wide-meshed network. At the periphery of the Malpighian bodies the capillaries open into the splenic pulp, which is, as it were, interposed between the capillaries and the veins. This consists of a hone}'-comb-like structure, with frequent trabecular and meshes (see e, e, Fig. 262). The meshes contain blood, but also large cells and cells containing blood-corpuscles and pigment. The meshwork of the pulp develops occasionally wider channels or sinuses which are virtually the radicles of the veins. The pulp has a brownish colour both from its pigment and from the fact that it is full of blood. The Malpighian bodies being lymphoid in structure, and sparely vascularized, are whitish in colour. It is clear from this structure that the blood passing into the spleen by the arteries will linger long in the organ, and will especially stagnate in the meshes of the pulp. It seems probable that in the pulp the red corpuscles are to some extent destroyed. The existence in the normal spleen of cells containing red corpuscles and pigment seems to indicate this. The spleen also probably gives origin to white corpuscles, but it shares this function with the lymphatic glands and the medulla of bone, as well as probably with the widely-diffused connective tissue throughout the body. Some have supposed that the spleen also forms red corpuscles.

Fig. 262. - Structure of spleen. A, capsule; 6, trabecular; c, c, Malpighian bodies; in that to the right an artery, cut transversely, is seen; d, d, injected arteries, one running into a Malpighian body; e, honeycomb-like pulp. (Kolliker).

From these remarks it will be understood that the amount of blood in the spleen will be subject to great variations according as the pulp is more or less distended. The pulp forms by far the greatest part of the substance of the spleen, and according as its honeycomb structure is more or less full of blood will the size of the organ vary. Accordingly there is no organ in the body which varies so much in size, even under physiological circumstances. During digestion there is an active hyperemia which causes enlargement of the organ. Again, the capsule and trabecular of the spleen are abundantly supplied with smooth-muscle fibre-cells, and its size is influenced by the state of contraction of these. It is well known that by electric stimulation the size of the organ may be diminished, the muscular tissue contracting.