COMPARATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN. 261 



corpuscles, a few lymphoid cells, and some strands of yellow elastic tissue ; and exter- 

 nally is a layer of unstriped muscle derived from the inner layer of the capsule. 



Near the hilus this sheath surrounds both the artery and the vein, the latter haAang 

 pierced the sheath at some distance internal to the hilus. Shortly after the artery has 

 separated from the vein, there occurs within the hilar sheath a proliferation of lymphoid 

 cells, which seems to begin in the innermost stratum — the layer derived from the sub- 

 serous connective tissue. As the artery becomes smaller, the lymphoid cells increase 

 greatly in number, and the sheath is represented by a small amount of white fibrous 

 tissue together with the external muscular layer, which is itself much thinner. The 

 arterial branch, thus surrounded by loosely arranged lymphoid cells, and externally by 

 the remnant of the hilar sheath, divides in a dichotomous manner. The branches that 

 result are similarly ensheathed, except that the investment of adenoid tissue varies in 

 thickness, so that it resembles in some degree a nodose root, and that the remnant of the 

 hilar sheath is thinner, and consists almost entirely of unstriped muscle. 



The nodosities of the adenoid sheath constitute the splenic follicles or Malpighian 

 bodies, and the extension of the hilar sheath forms for them a capsule-like covering which 

 is contractile, and probably also highly elastic. In a section of a follicle, whether the 

 artery be cut transversely, obliquely or longitudinally, the cells of which the peripheral 

 layer is composed always appear spindle-shaped. The obvious explanation of this is, 

 that as spindle-shaped muscle fibre cells run in all directions over the follicle, adapting 

 themselves to its spherical contour, those fibres alone that are in the plane of the section 

 are seen with characteristic fusiform shape, while those cut obliquely or transversely 

 appear as inconspicuous dots. This peripheral zone consists usually of two, three, or 

 four loose layers of fibres ; and that it is continuous with the outer muscular layer of the 

 hilar sheath is clearly seen when a longitudinal section of the artery and follicle is 

 examined. Springing from the artery thin-walled capillaries run among the lymphoid 

 cells, and they are more numerous, and consequently more conspicuous, towards the 

 periphery of the follicle. 



The substance of the follicle consists of lymphoid cells, most of which are large and 

 uninucleated, contained in the mesjies of an adenoid reticulum. Near the centre of the 

 follicle, a few larger uninucleated or multinucleated lymphoid cells may be seen, and 

 towards the periphery there is a narrow belt of small uninucleated lymphoid cells. 

 Occasionally very large protoplasmic cells, four or five times the size of the largest 

 lymphoid cells, are found near the centre of the follicle. Their protoplasm is coarsely 

 granular and stains very deeply with eosine. Some of these cells appear to be non- 

 nucleated ; in others there is a single nucleus at the margin of the cell, and they often 

 exhibit round vacuoles. 



The intrafollicular reticulum consists of delicate branching threads, that anastomose 

 at irregular intervals. Upon the nodes of the network large, oval, connective tissue cells 

 are placed. The meshes vary in size and the threads in fineness. The meshes are closer 

 and the threads stronger towards the periphery. In addition to this reticulum there is 



VOL. XXXVIII. PART II. (NO. 8). 2 N 



