COMPARATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN. 263 



sheath becomes thinner, and the connective tissue sheath thicker, until the lymphoid 

 sheath disappears when the latter persists as a thick, almost granular, investment of a 

 thin-walled vessel. It contains several large, round or oval, clear cells, which stain faintly 

 with hematoxylin. The capillaries are large, but not numerous, and therefore do not 

 form a close network. The cells are contained in the meshes of a delicate fibrous 

 reticulum, which seems to be continuous with that of the adjoining pulp. They are 

 mainly of three kinds : — (1) Small, angular, deeply blue-stained lymphoid cells, which fill 

 the greater part of the adenoid tissue ; (2) much less numerous cells, consisting of a large 

 vesicular nucleus, surrounded by granular pink-stained protoplasm, that varies in amount 

 in different cells. Such cells may, however, have a horse-shoe shaped nucleus, or two, 

 three, or more somewhat large nuclei, or sometimes eight to twelve small, deeply blue- 

 stained nuclei, in which case there is only a small amount of intermediate protoplasm ; 

 (3) pink-stained granular cells occurring in clumps, which are situated in the delicate 

 fibrous tissue that surrounds the artery. The characters of the cells, and their grouping, 

 recall similar appearances found in the skate's spleen, but the cells in the skate are much 

 larger. 



The spleen of the Grass Snake shows a cellular cortex and a fibrous medulla. The 

 former is composed of four wedge-shaped masses of lymphoid tissue, the apex of each 

 pointing to the centre. The latter consists of a four-rayed core of fibrous tissue, which 

 contains large ramifying blood-sinuses, each pyramidal ray separating two wedges of 

 adenoid tissue. Thus the base of the wedge is placed upon fibro-muscular tissue 

 containing blood-sinuses, its sides are bounded by fibrous bands that contain blood- 

 sinuses, and its blunt apex is imbedded in fibrous tissue that contains blood-sinuses. 

 About the middle of each wedge there is a comparatively small artery, and running 

 between the lymphoid cells are large, thin- walled, much branched capillaries that anasto- 

 mose with each other to form a network, and open into the venous sinuses that are 

 contained in the surrounding fibrous tissue. The cells of the adenoid tissue resemble those 

 already described in the spleen of the tortoise. The protoplasmic cells are specially 

 noticeable immediately outside the capillary wall. 



The Splenic Follicles make their first appearance in birds, but they do not occur in 

 all ; for instance, they are absent from the spleen of the hawk. 



In the Rook the special artery of the follicle, as well as the capillaries, are of remark- 

 ably large size. The follicles are almost invariably situated on one side of the artery, 

 and not round it. Around the follicles there is a strong belt of muscle, which consists 

 of two or three interlacing layers, and which may very clearly be seen to spring from the 

 hilar sheath. (Plate I. fig. 3.) The cells are of two kinds : — (l) Large protoplasmic 

 corpuscles having a single nucleus — similar cells occur plentifully in the pulp surrounding 

 the follicles ; and (2) ordinary small lymphoid cells. These two kinds are indiscriminately 

 mixed throughout the follicle. There is a delicate adenoid reticulum. 



In the spleen of the Ox the follicles are numerous and conspicuous. Two small 

 arteries may occasionally be seen in one follicle, and fairly large capillaries run in its 



