COMPARATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN. 267 



lymph follicular tissue, which exists in the form of a continuous adenoid sheath around 

 the artery. The intrafollicular capillaries are much less evident than in the follicles of 

 other Eodents. The arrangement of the cellular elements is similar. The peripheral 

 fibrous layer contains little muscle, resembling in this particular the hilar sheath and the 

 tunica propria. 



In the spleen of the Hedgehog the follicles are few. The intrafollicular reticulum is 

 very delicate, and the walls of the capillaries are very thin. The cells of many of the 

 follicles show a germinal character near the middle of the follicle with the daughter cells 

 outside. There is a well-marked limiting fibrous zone, composed principally of muscle, 

 which is arranged in two or three layers. In this spleen the continuity of the peripheral 

 muscular layer with the hilar sheath is very clearly seen. (Plate I, fig. 4.) 



In the spleen of the Child the follicles are neither numerous nor sharply defined from 

 the surrounding pulp. The special arterial branch is large in relation to the size of the 

 follicle. Between its muscular coat and the lymphoid cells there is a somewhat thick 

 layer of loose fibrous tissue, which contains a few elliptical nuclei. The follicular capil- 

 laries are neither conspicuous nor numerous ; their walls are very delicate, and contain 

 few nuclei. They are most numerous near the middle of the follicle, and are often sur- 

 rounded by a fibrous sheath derived from that of the artery. The intrafollicular 

 reticulum is very strongly developed, and its outer portion seems to consist of the 

 separated fibres of the inner layer of the hilar sheath. The cellular elements of the 

 follicle are of two kinds, lymphoid cells and large protoplasmic corpuscles. In many 

 follicles the latter are grouped around the artery so as to form a core that is surrounded 

 by lymphoid cells. Such a core may occupy about a fourth of the entire follicle. The 

 cells that form it are round, oval, or polygonal in shape, and consist of a round 

 nucleus staining faintly blue with hematoxylin, surrounded by coarsely granular 

 protoplasm, staining deeply pink with eosine. Although these cells are found princi- 

 pally collected together in a central area, isolated examples also occur among the lym- 

 phoid cells at all distances from the centre, and even at the periphery of the follicle. 



The cells vary much in size ; the smaller are usually near the middle of the core, and 

 these are much less deeply stained than the larger ones, both as regards their protoplasm 

 and nucleus. The nuclei are sometimes large, round, and vesicular, sometimes horse-shoe 

 shaped, and occasionally two, situated at a considerable distance from each other, are 

 connected by a slender filament of nuclear substance. There is evidence of division by 

 karyokinesis as well as by simple transverse fission. Sometimes a large cell may be seen 

 to be filled with deeply stained nuclei, a mere rim of pink-stained protoplasm remaining. 

 Many of the cells, and especially the larger ones, are vacuolated. 



The origin of these cells can as yet only be conjectured. Frequently they appear to 

 spring from the fibrous sheath of the artery and capillaries of the follicle. But such 

 cells occur also in the arterial lumen, both intrafollicular, and while surrounded by the 

 hilar sheath alone. Stilling* in 1886 described " bright centres" in the follicles of the 



* Stilling (21), p. 18. 



