352 THE SPLEEN, BY WILHELM MULLER. 



animals. No observations have hitherto been made on the 

 lymphatics or on the nerves of the spleen in Reptiles. 



THE SPLEEN OF FISHES, AMPHIBIA, CHELONIANS, BIRDS, 

 AND MAMMALS. However various may be the structural 

 arrangements of the spleen in these several divisions of the 

 animal kingdom, the essential features of construction are the 

 same in all. The organ is constantly invested by a capsule 

 which sends off processes into the interior. These either hold 

 some determinate relation to the venous system of the organ, 

 forming venous sheaths, septa, and trabeculse, or to the arterial 

 system in the form of arterial sheaths. The interspaces of 

 these tissues are filled with the peculiar parenchyma termed 

 the splenic pulp. 



THE CAPSULE OF THE SPLEEN. The thickness of the 

 splenic capsule appears to bear a direct proportion to the 

 whole volume of the organ. In the embryo it is invested by 

 a short form of cylinder epithelium, resembling the ordinary 

 epithelium of the peritoneum. As the organ grows this be- 

 comes flattened, and in adults forms delicate, partly square, 

 partly rhomboidal plates. In all Vertebrata fibrillar connective 

 tissue, with which elastic fibres are abundantly intermixed, 

 enters into the composition of the capsule. In Fishes and 

 Amphibia, so far as observation has at present extended, these 

 elements form the entire capsule. In the higher Vertebrata, 

 from the Chelonians upwards, a variable proportion of smooth 

 muscular fibres, which are always situated in the deeper layers 

 of the capsule, is likewise present. In Carnivora, in the Rumi- 

 nants, and in the Pig, these are so largely developed, that the 

 physiological experiment of merely dipping the spleen into 

 warm water furnishes evidence of their presence, whilst in 

 the Rodentia and Cheiroptera they are much less abundant. 

 Muscular fibres, even if they are constantly present, are only 

 sparingly distributed in the splenic capsule of Man. 



SEPTA AND SHEATHS OF THE VEINS. 



The association of these two constituents is justified by the 

 constancy of the relation which they bear to one another. 



