364 



MANUAL OF PHYSIOLOGY. 



loid substance and peculiar crystals, and the epithelium has dis- 

 appeared from their walls. Although said to be rich in lymphat- 

 ics and to contain follicular tissue, positive proof of the relation 

 of the thyroid body to the lymphatic system is still wanting. 



THYMUS GLAND. 



The functional activity of the thymus is restricted to that pe- 

 riod of life when growth takes place most rapidly. It is well 

 developed in the fcutus, and increases in size for a couple of years 

 after birth ; but it gradually diminishes in bulk and loses its 

 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 

 lympathic gland, and from its great richness 

 in lymphatic vessels, 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. 



FIG. 160. 



SPLEEN. 

 The spleen also resembles a lymphatic 



Elements of Thy- 

 mus (high power). 

 (Cadiat.) (a) Lymph 

 corpuscles. (6) Epi- organ in structure, but differs from it in the 

 theloid nests of Has- 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 



