HISTOLOGY. 69 



lymphoid tissue. The thymus is the only lymphoid 

 gland that has no germ center, but cell division can be 

 demonstrated in the gland. Hassal's corpuscles are 

 the dense bodies found in the thymus. They are re- 

 garded as epithelial remains of the gland in early life. 



THE TONSILS. 



The tonsils are situated in the mouth at the base of 

 the tongue. They are surrounded by a dense mem- 

 brane of white fibrous tissue on the under side, and by 

 the mucous membrane, which is made up of pavement 

 epithelial cells of the buccal cavity, on the outer side. 

 The membrane dips down sometimes into the sub- 

 stance of the tonsil, and is called the crypts. The ton- 

 sils are supplied with blood vessels, lymphatics . and 

 nerves, which enter the organ through the capsule. 

 They are lymphoid bodies. 



SEROUS CAVITIES AND MEMBRANES. 



The serous membranes may be considered as ex- 

 panded lymphatics. These membranes are composed 

 of a thin sheet of fibrous tissue, supporting a layer of 

 thin, irregular, polyhedral cells, placed edge to edge 

 and united by a cement substance. At very short dis- 

 tances there are little openings in the mucous mem- 

 brane, called stomata, which are the mouths of lym- 



