130 



THE LYMPHATIC SYSTEM 



occur. Cells in the blood stream can pass through the slits in the 

 sinusoid wall into the red pulp proper and cells in the red pulp can 

 pass into the blood stream by way of the sinusoid. In the reticulum 

 and diffuse lymphoid tissue of the red pulp there are the same red 

 and white cells found in the circulating blood. The lymphocytes 

 produced in the nodules enter the reticulum of the red pulp and 

 thus may pass directly into the blood stream. Also in the red pulp 

 are numerous giant cells, macrophages presumably derived from 

 reticular cells in which phagocytic activity is pronounced. Foreign 

 elements and disintegrating blood cells are ingested by these cells. 



Fig. 79.— Photograph of spleen of Necturus, showing the vascular distribution; 

 the vessel on the left is shown with several branches opening out into sinusoids. 

 Mallory stain. 



In certain pathological conditions so many red cells are disinte- 

 grating and so many are being phagocytized that the red pulp 

 acquires a brown color. The endothelial cells also appear capable 

 of phagocytic activity and ameboid movement. 



In embryonic mammals and in lower vertebrates the spleen is 

 hemopoietic, and lymphocyte production is not so clearly localized 

 in nodules. The structure remains fundamentally the same, except 

 that nodules are no longer clearly differentiated, lymphocytes and 

 erythrocytes both being produced in a conunon reticulum, as 

 already observed in the chapter on blood. Although erythrocytes 



