"STRUCTURE OF THE SPLEEN. VENOUS SHEATHS. 353 



From the deeper layers of the splenic capsule fibrous bands are 

 given off at regular distances, which are recognisable with the 

 naked eye, and become continuous with cylindrical cords, the 

 so-called trabeculse of the spleen that penetrate its substance. 

 They communicate with one another by lateral branches, and 

 form a network traversing the entire organ. Their structure 

 is identical with that of the deeper layers of the capsule, except 

 that they for the most part contain bands of smooth muscular 

 fibres. A certain number of these trabeculae extend constantly 

 between the ramifications of the veins, and become attached to 

 their walls either at acute or at right angles. The structure of 

 the latter is thus rendered more complex, as the splenic veins 

 have already at their point of entrance into the organ received 

 an annular investment from the capsule which soon coalesces 

 with the vascular walls. The latter thus acquire remarkable 

 firmness, and from the increased strength afforded by the at- 

 tachment of the numerous trabeculse are prevented from collaps- 

 ing, presenting in consequence, in this respect, a certain simi- 

 larity to the sinuses of the dura mater. This modified venous 

 wall sooner or later becomes incomplete, whilst the connective 

 tissue layers containing muscular fibres split into small bands, 

 between which the lumen of the vessel is only limited by the 

 epithelium layer and by a delicate layer of connective tissue 

 containing numerous cells, and representing the tunica intima. 

 This assumption of a fibrous character by the external vascular 

 layers may even commence in the trunks of the splenic vein, 

 as occurs in the Ruminants ; but more frequently, as in Man, it 

 is first observable in the smaller branches. The slender bands 

 containing muscular fibres, into which the sinus-like venous 

 wall divides, run for a greater or less distance along the 

 branches, ultimately becoming detached and uniting with the 

 trabecular network of the organ (W, Miiller). The object 

 fulfilled by the connection of the trabecular network of the 

 spleen with the walls of the veins is sufficiently obvious. The 

 longitudinal bundles of muscles belonging to the latter tend to 

 shorten the canals, whilst the trabeculse which are laterally at- 

 tached to them widen them, and thus conditions favouring the 

 discharge of fluid from them are established (Tomsa). A coin- 

 cident contraction of the muscles of the capsule and of the 



