COMPARATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN. 265 



Their protoplasm stains very deeply with eosine, and they usually possess a single, oval, 

 somewhat small nucleus. In the outer zone the cells are smaller, more nearly uniform 

 in size ; they stain more deeply with hsematoxylin and closely resemble free nuclei. 

 They are arranged with considerable regularity in circular lines, the outermost circles 

 being separated by long connective tissue strands derived apparently from the hilar 

 sheath. At the periphery of the follicle is a zone of spindle-shaped muscle fibre cells, 

 similar to the zone already described in the kitten, only not quite so distinct. 



In the spleen of the Dog the follicles are much like those in the cat's spleen. The 

 special artery is immediately surrounded by a thick fibrous sheath, which seems to 

 represent the original tunica adventitia of the artery. The intrafollicular reticulum is 

 feebly developed. The follicular cells are divisible into three zones : the germinal 

 central cells and those of the peripheral zone resemble the cells described in the cat's 

 spleen ; the intermediate zone is composed of cells like those of the peripheral zone, 

 but slightly smaller. There is a limiting belt of spindle-shaped muscle fibre cells, which 

 is more prominent in the spleen, of the puppy than in the adult dog. In the spleen of 

 the Puppy nearly all the cells of the follicle are like the central germinal cells ; smaller 

 cells are found in a narrow ring at the periphery of the follicle, and plentifully in the 

 pulp outside it. It seems to be the rule that the follicular cells are divisible into three 

 zones in the adult alone. 



In the spleen of the Porpoise there are numerous accumulations of lymphoid cells, 

 but they are not sharply defined. The intrafollicular capillaries are somewhat large 

 and very conspicuous. They may occasionally be clearly seen to open into the spaces 

 of the pulp. The lymphoid cells do not show any distinct division into zones. They 

 resemble the cells in the germinal area of other adult, or those of the entire follicles 

 of the young mammal. In many of the cells the nucleus is surrounded by a layer of 

 protoplasm that stains deeply pink with eosine ; and very, large cells similar in character 

 may sometimes be seen near the middle of the follicle. The follicles have apparently 

 no fibrous limiting layer. Large venous trunks form a kind of boundary for them. 

 In several points of structure this spleen resembles the reptilian spleen ; in the large 

 venous spaces of the capsule, in the diffuse adenoid tissue partially surrounded by 

 venous channels, and in the large size of the capillaries within the adenoid tissue. 

 The macroscopic appearances of the spleens of the tortoise and porpoise are similar, and 

 both are developed in the mesentery of the small intestine. 



In the spleen of the Narwhal the follicles are numerous but small, and the cells 

 that form them, being of large size, are in comparatively small number. The artery 

 divides simultaneously into a number of arterioles which run a nearly parallel course. 

 There may be as many as eleven branches, and together they produce a characteristic 

 brush-like appearance. A similar, although less pronounced, arrangement obtains in 

 the pig's spleen. This bundle of vessels is contained in a matrix of fibrous tissue, that 

 contains a few lymphoid cells, and forms a rudimentary adenoid sheath. When the 

 arterioles separate from each other, lymphoid cells accumulate in the fibrous tissue 



