90 THE LYMPHATIC SYSTEM. 



thelioid cells, remains of the original epithelial structure of 

 the gland. The thyraus is abundantly supplied with blood 

 and lymphatic vessels. 



The tonsils and solitary and agminate glands of the intes- 

 tines are lymphoid glands, and are described in connection 

 with the alimentary system. 



The functions of lymphoid tissue and glands are not entirely 

 understood, but some important purposes which they serve 

 are known. They are the source of the lymph-corpuscles 

 and leukocytes of the blood. The lymphoid cells of adenoid 

 tissues, after undergoing enlargement and certain changes, 

 enter the efferent lymph-stream as lymph-corpuscles, and 

 after passing into the blood they form leukocytes. Lymphatic 

 glands also serve to arrest toxic and foreign bodies which 

 enter the lymph-stream ; their tissue acts as a sort of filter in 

 this respect. Thus, in cases of vaccination, carcinoma, 

 chancre, and many similar conditions, the virus from the 

 affected spot is taken up by the lymph-vessels, but is arrested 

 in its passage at the next group of lymphatic glands, which 

 swell up and become inflamed, thus for a time retarding the 

 progress of the disease. The interchange between the blood 

 and lymph circulations in the lymphatic glands may also be 

 associated with some sort of internal secretory process. The 

 special purpose of the thymus is obscure. 



