The principal subject of the author's research has been the comparative histology of the spleen in twenty-two different kinds of animals, embracing all the typical vertebrates from the fish to Man.

The capsule of a typical mammalian spleen, such as that of the cat, consists of two layers, the outer composed of ordinary fibrous tissue, the inner of nonstriped muscle. The splenic artery and vein, as they enter the organ at the hilum, are enclosed in a sheath formed by an inflexion of the capsule, and, like it, consisting of a fibrous and a muscular layer, the fibrous layer being next the vessels, and the muscular layer external to it, adjoining the pulp.