SPLEEN. 367 



original structure during the later periods of childhood, so as to 

 become completely degenerated and fatty in the adult. It is com- 

 posed of numerous little follicles of lymphoid tissue collected into 

 groups or lobules connected to a kind of central stalk. The 

 lymphoid follicles of the young thymus have some likeness to 

 those of the intestinal tract, but they differ from these agminate 

 glands not only in arrangement, but also in having small peculiar 

 nests of large cells (corpuscles of Hassall) in the midst of the 

 adenoid tissue of which they are made up. On account of the 

 structure of the lobules being so nearly identical with that of a 

 lymphatic gland, and from its great richness in lymphatic vessels, 



Fin. 157. 



Section of the Thyroid Glaud of a Child, showing two complete sacks and portions of 

 others. The homogeneous colloid substance is represented as occupying the central 

 part of the cavity of the vesicles, which are lined by even cubical epithelium^ (Scliftfer.) 



the thymus is said to be related to the lymphatic system, and is 

 supposed to play an important part in the elaboration of the 

 blood during the earlier stages of animal life. 



SPLEEN. 



The spleen also resembles a lymphatic organ in structure, but 

 differs from it in the relation borne by the blood to the elements 

 of the follicular tissue. It is incased in a strong capsule made 

 of fibrous tissue and unstriated muscle cells. From this many 

 branching prolongations pass into the substance of the organ, so 



