COMPAKATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN". 259 



is thickened by the addition of an intermediate layer, composed of areolar tissue, which 

 contains many arteries and many relatively large thin-walled veins. The latter feature 

 reminds one of the intracapsular venous sinuses in cartilaginous fishes, in the Amphibia, 

 in the Chelonia, and in the Ophidia. This intermediate layer also contains fat cells and 

 numerous strands of unstriped muscular fibre. The trabecular system is but slightly 

 developed. The trabeculse, which are apparently derived chiefly from the vascular 

 sheaths, are in the form of flattened bands of long muscle fibres, each band so loosely 

 arranged that elongated meshes result, in which numerous lymphoid cells are found. 

 The hilar sheath which surrounds the larger arteries and veins is in the form of a some- 

 what thick investment of fibro-muscular tissue. 



In the Rabbit, Eat, and Guinea-pig the tunica propria consists mainly of white 

 fibrous tissue, containing some connective tissue corpuscles and a few lymphoid cells. 

 There is a small amount of unstriped muscle, chiefly in the deeper portion of the capsule, 

 which does not amount to a layer in the rabbit and the rat, but in the guinea-pig forms 

 about the inner fifth of the capsule ; most of the fibres are transverse, some are longi- 

 tudinal, and these are more numerous near the hilus than elsewhere. The tunica 

 propria contains a considerable number of elastic fibres which are especially numerous 

 in the rat. The trabeculas are somewhat thick but not dense. They are composed of a 

 mixture of white fibrous tissue and of unstriped muscle, and are relatively more muscular 

 than the capsule, which is accounted for by their being derived from its deeper part. In 

 the rat the trabecular framework is more strongly developed, and the trabecules contain a 

 greater proportion of muscle than in the rabbit and guinea-pig. The hilar sheath is, 

 in each case, feebly developed, and contains little muscle. 



In the Hedgehog, as. in the Eodentia, the tunica propria consists chiefly of white 

 fibrous tissue. There is a thin layer of muscle next the parenchyma, and there are a 

 few muscle fibre cells scattered throughout the thickness of the capsule. Contrasting 

 with the scarcity of muscle there is an abundance of elastic tissue. The trabecular 

 system is somewhat more strongly developed, and contains more muscle, than in the 

 Rodentia. The trabecules break up into small secondary ones that resemble the 

 microscopic trabecules found in the spleen of the Ungulata. The hilar sheath is well 

 developed. There is a well-developed muscular outer layer, between which and the 

 outer surface of the artery lies a considerable quantity of areolar tissue containing 

 lymphoid cells, and forming the early stage of an adenoid sheath. This does not appear 

 immediately after the arteries have separated from the veins, but after their second 

 bifurcation. (Plate I. fig. 2.) 



In the Human Spleen the tunica propria is composed almost entirely of white fibrous 

 tissue. There is a small amount of yellow elastic tissue associated with a little muscle. 

 The spindle-shaped muscle fibre cells are practically confined to the deeper portion of 

 the capsule, and are slightly more numerous near the., origin of the trabecules. The 

 trabecular framework is only slightly developed. The trabecules consist chiefly of 

 white fibrous and elastic tissue. They have only a small amount of muscle, but follow 



