270 DR A. J. WHITING ON THE 



that in the cod, but only a somewhat thick fibrous sheath around the smaller arteries 

 as in the skate. 



In the spleen of the Tortoise, after the lymphoid sheath of the artery ceases, its 

 fibrous sheath (tunica adventitia) becomes thicker ; the axial vessel becomes smaller, 

 and shortly divides into two or more branches which ultimately open as thin-walled 

 vessels into the venous sinuses of the pulp. The tissue forming the substance of the 

 sheath is so finely fibrillated as to be almost granular. It contains a few large, vesicular 

 lymphoid cells like those of the tunica propria and hilar sheath, which appear to 

 multiply, sometimes, if not always, by transverse fission. The outline of the sheath is 

 irregular, and in some places there is the appearance of a delicate structureless limiting 

 membrane. There is often the appearance of a venous sinus around the sheath, but 

 sometimes its outer wall seems to be connected with a fine reticulum. 



In the spleen of the Grass Snake there does not seem to be any structure homo- 

 logous with the ellipsoidal sheath. 



In the Hawk, as the muscular coat of the splenic artery grows thinner the fibrous 

 coat becomes thicker, until it forms a sponge-like investment containing lymphoid cells. 

 The muscle fibre cells derived from the hilar sheath seem to form a limiting layer /or 

 the ellipsoidal sheath. It is composed of a strong fibrous network, that stretches 

 between the peripheral muscular layer and the vessel wall, in the meshes of which are 

 a few clear, faintly stained lymphoid cells. It is suspended in a capacious venous 

 sinus, within which the cellular elements of the pulp are found, and across which the 

 fine fibres of a delicate reticulum are sometimes seen to stretch. 



The spleen of the Rook contains numerous circumscribed ellipsoids, which resemble 

 in general characters those found in the spleen of the kitten. Into each ellipsoid an 

 arteriole enters, and from each, at one or more points, a thin-walled vessel leaves, while 

 from the axial vessel capillaries destitute of endothelium radiate outwards to the 

 surface. The afferent is distinguished from the emergent vessel by the character of 

 its endothelial lining ; in the former this is composed of spindle-shaped cells occurring 

 at considerable intervals, in the latter of rounded cells placed near together. The 

 endothelium changes in character almost immediately after the entrance of the arteriole, 

 and it again becomes flattened shortly after the vessel has left the ellipsoid. 



In shape the ellipsoids are usually oval, but sometimes trifoliate. Their substance 

 consists of cells, spindle-shaped or round, imbedded in a structureless ground substance. 

 The round cells are either small lymphoid cells like free nuclei, that stain deep blue 

 with hasmatoxylin, or protoplasmic cells, twice, thrice, or four times the size of the 

 former, consisting of a small, round nucleus surrounded by granular protoplasm, and 

 resembling the cells in the follicles. The spindle cells are concentrically arranged 

 around the axial vessel. Running between the round cells, in addition to the capillaries, 

 are highly refractile lines which look like strands of elastic tissue. 



In almost every instance there are indications of an investing membrane of spindle- 

 shaped cells, apparently muscular in nature. This enveloping layer is evidently a 



