124 



THE ESSENTIALS OF HISTOLOGY. 



or crypts in the substance of the organ (fig. 148). These recesses are 

 all lined by a prolongation of the stratified epithelium, and into them 

 the ducts of numerous small mucous glands open. The tonsils are 

 composed almost entirely of lymph oid tissue, which, besides being 

 diffused over the whole organ, is at intervals aggregated into small 

 nodules, in which the lymph-cells are more closely arranged than else- 

 where. In these nodules active multiplication of the lymph-cells by 

 karyomitosis is constantly proceeding. Even the epithelium is in- 

 filtrated with lymph-corpuscles (Stohr), and they may also wander out 

 on to the free surface, and become mingled with the saliva as salivary 

 corpuscles (see Lesson VI., 1). 



FIG. 148. SECTION THROUGH ONE OF THE CRYPTS OF THE TONSIL. (Stohr.) 



e, e, stratified epithelium of surface of mucous membrane, continued into crypt ; /, /, follicles 

 or nodules of the lymphoid tissue, which is elsewhere diffuse ; opposite each nodule 

 numbers of lymph-cells are passing through the epithelium ; s, masses of cells which 

 have thus escaped from the organs to mix with the saliva as salivary corpuscles. 



The mucous membrane of the neighbouring part of the pharynx and 

 of the back of the tongue is similar in structure to the tonsils. 



The thymus gland is a lymphoid organ which is found only in the 

 embryo and during infancy. It is composed of a number of larger and 

 smaller lobules (fig. 149), which are separated from one another by 



