132 



THE LYMPHATIC SYSTEM 



surrounds the entire gland and extensions of it divide the gland 

 into small lobules. (Fig. SO.) Under the microscope each lobule 

 appears to be a mass of lymphoid tissue with a darkly staining 

 peripheral cortex of more dense lymphoid cells and a central diffuse 

 medulla. The medullary portion of adjacent lobules connect with 

 each other. The lymphoid cells of the cortex are very similar to 

 small lymphocytes. At the border between cortex and medulla 

 there is an abrupt change to the diffuse distribution of cells in the 

 medulla. Each lobule has a network of reticular cells and fibers 

 continuous with the coarse framework of loose fibroelastic connec- 

 tive tissue. The lymphoid cells are considered by some not to be 



Fig. 80.— Photograph of a section through a portion of a child's thymus gland. 

 Several lobules are shown with cortex and medulla. Note connection of the medullary 

 portions of some adjacent lobules. 



typical lymphocytes, and as derivatives from embryonic endoderm 

 are differentiated as thymocytes. However, their appearance, their 

 ameboid motion, and other reactions are so similar that the usual 

 view is that they are lymphocytes derived from mesenchyme ele- 

 ments that migrated into this region. 



The reticular cells forming a network through the tissue are 

 considered as derivatives of mesoderm into which the e])ithelial 

 thymic invagination extended, and as derivatives of endodermal 

 e])ithelium. From whatever source, these reticular cells of the 

 thymus appear very similar to those foimd in other locations. 

 Fusiform cells, called myoid cells, have striated ffbrils and occur 

 commonly in the thymus of a number of forms, but not in mammals 



