COMPARATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN. 255 



somewhat thick sheath from the tunica propria, which is therefore composed internally 

 of white fibrous tissue and externally of unstriped muscle. This investment may be 

 called the capsular or hilar sheath. The external layer is almost immediately rein- 

 forced by the junction of the muscular trabeculse. The fibrous tissue next the outer 

 wall of the artery is finer, looser, and contains more lymphoid cells than that near the 

 external muscular layer, and it is apparently derived, in part at least, from the sub- 

 endothelial connective tissue of the tunica serosa, which is more abundant in the 

 neighbourhood of the hilus. 



The splenic vein, while accompanied by the artery and nerves near the hilus and 

 internal to it, is invested by a somewhat thin covering of fibro-muscular tissue derived 

 from the hilar sheath. In the interior of the spleen, when separate from the artery, the 

 vein runs within or at the side of a trabecula. The veins are conducted to the hilar 

 sheath by the trabecula?, where they are surrounded by dense muscular or fibro-muscular 

 tissue without the intervention of a looser fibrous laj 7 er. 



The Tunica Serosa. 



The Tunica serosa in all animals consists of a single layer of cells that individually 

 vary in shape in different animals. The endothelial cells covering the spleen of the Skate 

 are columnar in shape. In the Ling and Cod they are low columnar or cubical. In the 

 Frog and Newt they are somewhat shorter than in the bony fish, and approach a pointed 

 oval shape. In the Tortoise they are a little lower than in the Amphibia. In the Grass 

 Snake they are still more flattened than in the tortoise, but are at the same time 

 somewhat thick. In adult Birds and Mammals they are squamous, but in foetal 

 mammals they are cubical, and during the first few weeks of extra-uterine life they are 

 thick, but flattened like those of the Amphibia and Ophidia. 



Thus the cells of the tunica serosa, as the scale is ascended from fishes to mammals, 

 become gradually more flattened out. 



The Tunica Propria, the Trabecule, and the Hilar Sheath. 



In the Skate the tunica propria is principally composed of white fibrous tissue 

 arranged in large loosely interwoven bundles, between which there are a few lymphoid 

 cells. In the deeper portion of the capsule there are some spindle-shaped muscle fibre 

 cells which do not form a distinct layer. In the substance of the capsule, near the 

 parenchyma, there are a few narrow venous sinuses. There are no true trabeculse. 

 In addition to the capsule, the only representative of a supporting framework is the 

 loose fibrous tissue, derived from the tunica propria, that accompanies the vessels 

 through the hilus. 



In the Ling the tunica propria appears to be composed of three layers. The outermost 

 layer consists of loosely arranged spindle-shaped cells, that stain deeply with hsema- 

 toxylin and closely resemble connective tissue corpuscles. The intermediate layer is 



