COMPARATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN. 271 



vestige of the hilar sheath ; longitudinal fibres are seen to be continuous between the 

 two, and also between both and the peripheral layer of the follicles. The ellipsoids are 

 usually seen to be surrounded by a clear space, probably a venous sinus, but this has 

 not a distinct outer wall. 



In the spleen of the Pigeon, around the terminal portion of the arteries, there is a 

 short length of ellipsoidal sheath, similar to the extended sheath in the spleen of the 

 hawk. It consists of a granular undifferentiated matrix, in which are imbedded a few 

 clear faintly stained nuclei, but no concentric spindle cells. There is a clear space 

 surrounding it, across which the strands of a delicate reticulum stretch, as in the spleen 

 of the hawk ; and there are considerable numbers of lymphoid cells grouped around the 

 sheath, as in the spleen of the tortoise. But there does not appear to be any definite 

 layer at the periphery of the sheath, nor any separating the venous sinus from the pulp. 

 Thus the ellipsoidal sheath in the spleen of the pigeon seems to afford a connecting link 

 between the extended ellipsoidal sheath in the spleens of the hawk, tortoise, and cod, and 

 the circumscribed ellipsoid in the spleen of the rook, and in the spleen of many mammals. 



In the spleen of the Ox, as in the spleen of the pigeon, the arterial terminations are 

 surrounded by short lengths of an ellipsoidal sheath. There is a comparatively clear 

 area surrounding each stretch of ellipsoidal sheath, from which the microscopic trabeculse 

 of the pulp are absent, and in which run numerous anastomosing venous sinuses. (Plate 

 II. fig. 5.) As compared with the ellipsoids of some other ungulate animals, its most 

 striking feature is its length : it is seven or eight times as long as it is broad. In outline 

 it is irregularly undulating, and may be compared to a much gnarled club. An arteriole, 

 having lost its muscular coat, runs in a wavy manner through the long axis of the 

 expansion, but it does not apparently give off any capillaries, nor does it divide, but 

 opens as a thin-walled vessel into the venous sinuses of the surrounding clear area. The 

 substance of the sheath consists of a granular ground substance that stains deeply pink 

 with eosine, in which are imbedded large, round or oval cells having a well marked 

 intranuclear network. These vary in size and also in depth of staining with hsema- 

 toxylin, and, while most are lymphoid cells resembling free nuclei, others have a rim of 

 hyaline protoplasm that stains deeply pink with eosine. Similar cells are sometimes 

 seen lying free near the sheath. There are occasionally large vacuoles in the granular 

 matrix. At the periphery of the sheath is a membrane containing spindle-shaped 

 nuclei, and surrounding that is a venous sinus which apparently communicates with the 

 sinuses of the clear area. 



In the spleen of the Sheep there are numerous ellipsoids,' which are sometimes lobed, 

 but are usually in the form of a long oval. The axial vessel retains its muscular coat 

 for a short distance, and frequently divides into two or three thin- walled vessels before 

 leaving the ellipsoid to open into the venous sinuses of the pulp. Breaks in the con- 

 tinuity of the wall of the axial vessels are frequently seen, but no distinct capillaries 

 were observed. The substance of the ellipsoid consists of a fibrous network, at the nodes 

 of which are faintly stained cells like connective tissue corpuscles. The meshes some- 



