COMPAKATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN. 275 



2. In the Mammals it is invested either, as also in the rook, by a circumscribed sioell- 

 ing of an ellipsoidal sheath — an ellipsoid, or by a continuous stretch of an ellipsoidal 

 sheath, both containing a few lymphoid cells and some concentrically arranged spindle 

 cells (that are probably, sometimes at least, muscular) imbedded in a homogeneous 

 ground substance, while the former possesses a fibrous network, and the latter longitudi- 

 nally arranged fibres in addition. 



3. The sheath is surrounded by a venous sinus which separates it from the pulp, 

 and which has distinct ivalls in those Mammals alone that have circumscribed 

 ellipsoids. 



4. The ellipsoids have a special nucleated covering, which in some instances is 

 muscular. 



5. The axial vessel of the ellipsoid has, at least sometimes, a circular muscular coat. 



6. The axial vessels, in many animals, give off capillary channels which have no 

 endothelial lining, and which anastomose before opening ultimately into the blood-sinus 

 that surrounds the ellipsoid. 



7. The axial vessels end by leaving the ellipsoid as thin-walled veins which open 

 into the spaces of the pulp. 



8. The peripheral blood-sinus of the ellipsoid communicates directly with adjacent 

 ellipsoidal sinuses and splenic veins, but not with the emergent vessel of another 

 ellipsoid. 



9. The ellipsoid is associated with greater muscularity of the supporting frame- 

 work of the spleen than the ellipsoidal sheath. 



Chapter IV. 

 The Splenic Pulp. 



In the spleen of the Dog the pulp consists of a reticulum formed by the anastomos- 

 ing processes of branching cells, the meshes of which contain several kinds of free 

 cellular elements. 



The supporting cells of the pulp consist of a cell plate, from which numerous radiat- 

 ing plate-like processes pass, and in the middle of which is a round or oval nucleus. 

 The cell plate and processes are transparent, homogeneous or -slightly granular, and stain 

 faintly pink with eosine. When the plate-like processes are cut longitudinally they 

 appear as long thin fibres. (Plate II. fig. 7.) The reticulum seems to be sometimes 

 directly continuous with the terminations of the smallest trabeculse, also with the under 

 surface of the capsule, with the sides of the larger trabeculse and of the hilar sheath, and 

 with the walls of the venous sinuses. But it apparently is not connected with the 

 reticulum of the follicles, and differs from it in nature, inasmuch as the latter seems to 



