COMPARATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN. 257 



pass from the under surface of the capsule into the substance of the spleen to join a large 

 fibrous core. Both processes and core contain wide venous sinuses which communicate 

 with each other and with those of the capsule. This supporting framework appears to 

 represent a strongly developed hilar sheath rather than a true trabecular system. 



In the Hawk the tunica propria is composed of a loose network of white fibrous 

 tissue, in the meshes of which are a few lymphoid cells, and contains, irregularly 

 interspersed, many spindle-shaped muscle fibre cells. There are no trabecule. There 

 is a somewhat strongly developed hilar sheath, a considerable proportion of which 

 consists of non-striped muscle. 



In the Rook the tunica propria may be divided into three layers ; first, an outer 

 thin layer of white fibrous tissue, containing many elastic fibres ; second, an inter- 

 mediate thin muscular layer, the fibres of which run longitudinally ; and third, a 

 transverse muscular layer, four or five times thicker than either of the other layers. 

 Between the two muscular layers is a row of lymphoid cells. There are no true 

 trabecular. The arteries and veins are invested by a thick hilar sheath, the outer part 

 of which is purely muscular. 



In the spleen of the Pig the tunica propria is composed almost entirely of non- 

 striped muscle. There is externally a thin layer of white fibrous tissue, and the rest is 

 muscle, which is mainly arranged in two layers — an outer layer of transverse fibres 

 and an inner thicker layer of longitudinal bundles ; but there are also many strands 

 arranged obliquely. From the deeper longitudinal layer massive muscular trabecular 

 arise, which, passing inwards, anastomose to form a very coarse network. From the 

 periphery of these large trabecular, and from the under surface of the capsule between 

 their points of attachment, there are given off a few microscopic trabecular, which form a 

 secondary network within the meshes of the larger network. Some of these are attached 

 to the vascular sheaths, others end in the pulp apparently by becoming connected 

 with its reticulum. The venous trunks, before they join the arteries, are intimately 

 connected with the larger or primary trabeculse. The hilar sheath has a very strongly 

 developed muscular layer which unites with the trabecular. A noticeable peculiarity 

 in this spleen is that the arteries divide into smaller branches within the hilar sheath. 



In the Ox the arrangement of the tunica propria resembles closely that in the pig. 

 The layer of loose connective tissue under the tunica serosa is thicker, and contains 

 more connective tissue corpuscles. The deeper muscular portion, as in the pig, is 

 composed mainly of two layers, a superficial layer transverse and relatively thin, and 

 a deep layer longitudinal and very thick ; but oblique fibres are by no means so 

 numerous as in the pig. Springing from the deeper layer, large muscular trabecular 

 pass into the parenchyma, and from them, as well as from the under surface of the 

 capsule, numerous microscopic trabecular arise and scatter themselves throughout the 

 pulp, and cease only around the termination of the arteries. These microscopic 

 trabecular are much more numerous and much stronger than in the pig's spleen. A strong 

 hilar sheath envelopes the blood-vessels and nerves, and its outer portion is composed 



