96 BIOLOGY OF THE LABORATORY MOUSE 



lary part into irregular spaces. In both areas reticular libers and primitive 

 and phagocytic reticular cells form the finer network. 



In the cortex the lymphocytes may form rounded nodules which, how- 

 ever, are not constant structures and may vary in size and position (17), or 

 may be entirely absent, in which case the lymphocytes are arranged diffusely 

 without any definite structure. When a nodule is very active in producing 

 lymphocytes, its central area is lighter staining, and among its cell com- 

 ponents are many medium sized lymphocytes. Such areas contain mitotic 

 figures and have been called secondary nodules or germinal centers. 



In the medulla the lymphatic tissue is arranged in cords, surrounded by 

 wide meshes of the medullary sinuses. In the cords among the lymphocytes 

 free macrophages, eosinophils, plasma cells and occasionally mast cells are 

 also present. The free macrophages originate from the phagocytic reticular 

 cells (fixed macrophages) and are capable of ameboid movement. They are 

 elongated, irregular shaped cells with oval nuclei containing coarse chromatin 

 granules. The plasma cells have eccentric round or oval nuclei, with large, 

 darkly staining chromatin granules distributed in a fairly regular pattern. 

 The cytoplasm is homogeneous and slightly basophilic. The mast cells are 

 large, oval or polyhedral cells, with small round nuclei and cytoplasm contain- 

 ing large granules which stain intensely with hematoxylin and often obscure 

 the nuclei. 



Arteries enter at the hilus. They usually follow the course of the 

 trabeculae and branch repeatedly. The endothelial cells lining the capil- 

 laries are unusually tall, resembling in cross section cuboidal epithelial cells. 

 Several afferent lymph vessels enter through the capsule at the surface of the 

 organ. The lymph circulates throughout the sinuses, and lymphocytes 

 produced here are carried away by this constant flow. At the hilus the 

 lymph is collected into efferent lymph vessels. 



Great variation exists in the size and shape of the lymph nodes, as well 

 as in the relative size and position of the medulla and the cortex. Often the 

 trabecular system is poorly developed, and the nodes contain a diffuse mass 

 of lymphatic tissue (57). 



The spleen. — The spleen is a slightly curved, finger-shaped organ covered 

 with a capsule composed of dense connective tissue containing some smooth 

 muscle fibers. The trabeculae project in from the capsule, and with a net- 

 work of reticular cells and fibers form a framework which is filled by the 

 splenic pulp (Fig. 40). The splenic artery divides into two branches which 

 enter the spleen on its concave dorsal side, thus dividing the organ into three 

 approximately equal parts. Variations exist, and one or both of the second- 



