CHAPTER XIV. 

 THE SPLEEN. 



NEARLY the whole surface of the spleen is invested with a cov- 

 ering of peritoneum similar to that which partially covers the 

 liver. Beneath this is the true capsule of the spleen, which com- 

 pletely surrounds it. This capsule is composed of dense fibrous 

 tissue, containing a large number of elastic fibres and a few of 

 smooth muscular tissue. From its inner surface bands of the same 

 tissue, called the " trabeculse," penetrate into the substance of the 

 organ, where they branch, and the branches join each other to form 

 a coarse meshwork occupied by the parenchyma of the organ, the 

 u pulp." 



The bloodvessels of the spleen enter at the hilum and pass into 

 the large trabeculse, which start from the capsule at that point 

 and enclose the vessels until they divide into small branches. The 

 vessels then leave the trabeculse and penetrate the pulp, where they 

 break up into capillaries, which do not anastomose with each other. 

 There is some doubt as to the way in which these capillaries end. 

 According to one view, they unite to form the venous radicles, so 

 that the blood is confined within vessels throughout its course in the 

 spleen. Another view, which is more probably correct, is that the 

 walls of the capillaries become incomplete, clefts appearing between 

 their endothelial cells, which finally change their form and become 

 similar to those of the reticulum of the pulp. The veins, accord- 

 ing to this view, arise in a manner similar to the endings of the 

 arteries. The result of this structure would be that the blood is 

 discharged, from the capillary terminations of the arteries, directly 

 into the meshes of the pulp, after which it is taken up by the 

 capillary origins of the veins (Figs. 151 and 152). 



The pulp consists of a fine reticulum of delicate fibres and cells, 

 with branching and communicating processes, in the meshes of 

 which there are red blood-corpuscles, leucocytes in greater number 

 than normally present in the blood, and free amoeboid cells consid- 

 erably larger than leucocytes, called the " splenic cells." 



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