COMPARATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN. 279 



generally stain more deeply with hsernatoxylin than the large nucleus. The protoplasm 

 is frequently vacuolated, the vacuoles being usually about a fourth of the size of a red 

 blood-corpuscle. Occasionally there is an appearance of a particle of protoplasm lying 

 free in a vacuole. In shape the cells are usually oval but frequently irregular and 

 sometimes almost spinous. Their general appearances are much like those of the giant 

 cells found in the dog's spleen. (Plate II. fig. 9.) Within the intracapsular venous 

 sinuses, as in the frog's spleen, are granular bodies that are evidently breaking down 

 coloured blood-corpuscles. 



In the spleen of the Grass Snake there is no true pulp. It is probably represented 

 by the opened out fibrous tissue of the supporting framework, that contains numerous 

 protoplasmic corpuscles, some lymphoid cells, and a few pigment-holding cells. The 

 only reticulum present is that of the adenoid tissue. 



The spleen of the Hawk resembles in many particulars the spleen of the bony fish ; 

 in both, the pulp is rudimentary, and the greater part of the parenchyma consists of the 

 ellipsoidal sheath. There are, however, areas of parenchyma around the veins that 

 appear to represent the pulp. There is in them a feebly developed reticulum consisting 

 of nucleated cells with thread-like processes, in the meshes of which are a large number 

 of protoplasmic corpuscles, a few characteristic lymphoid cells, and very numerous red 

 blood-corpuscles. 



In the spleen of the Pigeon the reticulum of the pulp is much more delicate than that 

 of either the frog or tortoise ; the cells forming it are smaller, the cell plate extends only 

 a little way beyond the nucleus, and the cell processes are longer, more slender, and more 

 numerous. Its meshes are usually oval and comparatively large, but sometimes they are 

 smaller and nearly round. 



The cells of the pulp are in the young pigeon apparently all small lymphoid cells, 

 but in the adult there are in addition a few protoplasmic corpuscles, like the larger cells 

 of the follicles, and a small number of pigment-holding cells. As in the spleen of the 

 tortoise, some of the protoplasmic corpuscles occur in clumps ; they stain more deeply 

 with eosine, and are contained in the meshes of a somewhat stronger reticulum. 



In the spleen of the Rook nearly all the cells of the pulp are large protoplasmic 

 corpuscles like those in the follicles. 



In the spleen of the Pig the reticulum of the pulp consists of nucleated cells that give 

 off processes, which are usually broad and plate-like, but sometimes narrow and fibre-like. 

 In a cell with broad processes they are few in number and comparatively short, and in 

 cells with slender processes the converse as a rule holds true ; the processes in the former 

 case by their anastomosis form nearly round meshes, in the latter case elongated elliptical 

 meshes. 



The reticulum appears to be directly continuous with the numerous microscopic 

 trabecule of the pulp, the two blending without any appreciable line of junction. 

 Often nearly parallel longitudinal ridges pass from a trabecula into an expanded portion 

 of the reticulum, and these seem to be continuous with its muscular fibres. 



