THE SPLEEN. 



The origin of the first lymph-cells, the lymphocytes, is uncertain (page 100), 

 these elements appearing outside the vessels as derivatives of the mesoderm. 

 After the establishment of the lymphoid tissue, new cells are continually 

 being formed within the various lymph-nodes and nodules. The lymph-nodes 

 are formed by two fundamental parts, the lymphoid element, consisting of 

 lymphocytes in a reticulum surrounding the terminal artery and its capilla- 

 ries within the cords and germ-centres respectively, and the sinus-element, 

 represented by channels resulting from multiplication of the lymph-vessels. 

 The vascular factor is constant and present in the simplest nodule; the sinus- 

 element, on the contrary, varies, sometimes (as in the usual node) being 

 developed from closely packed lymph-ducts, and at other times (as in the 

 hemolymph nodes) being venous channels occupied by blood. By the sub- 

 sequent intergrowth of the lymphoid element and the greatly multiplied 

 lymph-capillaries, the intervening bridges of connective tissue are reduced 

 until only a reticulum remains, the lymphoid tissue being brought ultimately 

 into intimate relation with the surrounding sinus. 



THE SPLEEN. 



The spleen lies far back on the left side in the abdominal cavity, between 

 the stomach and the diaphragm, and measures approximately five inches in 

 length and about three inches in width. Its form is variable and greatly in- 



Capsule- 



Primary compartment tiLfe 



^ 



Interlobular trabecu 



Intralobular trabecula 



Interlobular vein 



Splenic nodule 



Capsule 



- Venous space 



Intralobular vein 



-Ampulla 



-Arteriole 



Pulp-cord 



Venous space 



Interlobular trabecula 

 containing vein 



Splenic artery 



FIG. 153. Diagram illustrating architecture of a splenic unit ; splenic pulp is represented in only one 



compartment. (After Mall.) 



fluenced by the surrounding organs, since its substance is soft and yielding. 

 It contains large quantities of blood and, hence, appears of a dark red or 

 purple color. The spleen may be classed as a huge hemolymph node, pos- 

 sessing the functions of producing lymphocytes and destroying erythrocytes. 

 The spleen is enclosed by a distinct capsule, which consists of bundles 

 of dense fibrous tissue, numerous elastic fibres, and sparsely distributed bundles 

 of unstriped muscle. With the exception of the hilum, the area between 

 8 



