COMPARATIVE HISTOLOGY AND PHYSIOLOGY OP THE SPLEEN. 269 



Chapter III. 

 The Ellipsoidal Sheath of the Splenic Arteries and the Splenic Ellipsoids. 



The smaller arteries in the Kitten's spleen, after leaving the follicles, divide 

 dichotomously into several branches, each ending in a bulbous swelling, termed by 

 W. Muller an ellipsoidal capillary sheath, or simply an ellipsoid. The thick-walled 

 vessel passes for a short distance into the substance of the ellipsoid and is continued 

 either as a single thin-walled vessel running in its long axis and leaving it undivided at 

 the opposite pole, or more usually divided into several such vessels which leave at 

 different points. Each emergent vessel opens into the blood-sinuses of the pulp. 

 As the name implies, they are usually oval in shape ; they are sometimes nearly round, 

 and sometimes pyriform. Each is composed of several rings of nucleated spindle cells, 

 which are probably muscular, arranged concentrically around the axial vessel or vessels, 

 and imbedded in a granular ground substance. In addition to the rings of spindle 

 cells, which are more strongly developed near the periphery of the ellipsoid, there are 

 scattered at irregular intervals throughout its substance a few lymphoid cells. At the 

 margin of each there is a layer of spindle-shaped cells, and the whole is suspended in a 

 blood-sinus by the vessels that enter and leave. 



In the spleen of the Skate the smallest arterial branches are enveloped in a some- 

 what thick fibrous sheath, but this does not show any localised swelling. In the 

 spleen of the cat-fish, Pouchet * has described cylindrical bodies, circularly striated 

 and containing nuclei, which apparently resemble those described in the spleen of the 

 kitten. 



In the spleen of the Cod, after the arteries have become much reduced in size and 

 separated from the veins, they are surrounded by a continuous sheath of almost 

 structureless tissue, which, varying in thickness, has an undulating surface and is oval 

 in section. To distinguish it from the circumscribed ellipsoids it may be termed the 

 ellipsoidal sheath. Imbedded in it are a few large clear cells arranged concentrically, 

 and also here and there smaller lymphoid cells that stain more deeply withhaematoxylin. 

 The ellipsoidal sheath has apparently no limiting membrane, but there is a peripheral 

 blood-sinus in which, together with red blood-corpuscles, are numerous deeply stained 

 lymphoid cells, which are mostly crowded at the edge of the ellipsoidal sheath, and 

 resemble the smaller cells contained in it, and also those forming the adenoid 

 sheath. Separating adjacent blood-sinuses are trabeculse-like strands of fibrous tissue 

 containing two or three layers of spindle cells, which strands are apparently the remnant 

 of the hilar sheath. A reticulum of fine fibres may sometimes be seen to stretch 

 between these strands and the opposite surface of the ellipsoidal sheath. 



In the spleen of the Frog there does not appear to be any ellipsoidal sheath like 



* Pouchet (18), p. 501. 

 VOL. XXXVIII. PART II. (NO. s). 2 



