LYMPHATIC ORGANS 



183 



separates the tonsils. From the capsule, connective-tissue trabeculce 

 extend into the substance of the organ and branch to form its frame- 

 work. The free surface of the tonsil is covered by a reflection of the 

 stratified squamous epithelium of the pharynx (Fig. 107). This epithe- 

 lium presents the same structure as elsewhere in the pharynx, flat 

 surface cells, beneath which are irregular cells, while the deepest cells 

 are more or less distinctly columnar. The latter rest upon a papillated 

 stroma from which they are separated by thehasement membrane. At 

 several places on the surface of the tonsil deep indentations or pockets 

 occur. These are from ten to twenty in number, are known as the 

 crypts of the tonsil (Fig. 107), and are lined throughout by a continua- 



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Fig. ioS. — Vertical Section through Wall of Crypt in Dog's Tonsil, showing lymphoid 

 infiltration of epithelium. Xiso. (Bohm and von DavidofT.) a, Leucocytes in epithe- 

 lium; b, space in epithelium filled with leucocytes and changed epithelial cells; c, blood- 

 vessel; d, epithelium be\'ond area of infiltration; e, basal layer of cells. 



tion of the surface epithelium which becomes thinner as the deeper 

 part of the crypt is reached. Passing off from the bottoms and sides 

 of the main or primary crypts are frequently several secondary 

 crypts, also lined with the same type of epithelium. 



The stroma consists of diffuse lymphatic tissue in which are found 

 nodules of compact lymphatic tissue similar to those in the lymph 

 node. Each nodule has a germ centre, where active mitosis is going 

 on, and a surrounding zone of more densely packed cells. The nod- 

 ules may have a fairly definite arrangement, forming a single layer 

 beneath the epithelium of the crypts or may be arranged quite irregu- 

 larly, several nodules uniting to form masses of dense lymphatic 

 tissue. At various points on the surface of the tonsil, and especially 

 in the crypts, occurs what is known as lymphoid infiltration of the 

 epithelium (Fig. 108). This consists in an invasion of the epithelium 

 by the underlying lymphoid cells. It varies from the presence of 



