Integrative Systems 



223 



JYJJL 



Thymus 



Fig. 189. Diagram to show the derivatives of the pharyngeal pouches. (Ie, 

 He, Hie, IVe, Ve) External pharyngeal grooves; (Ii, Hi, Illi, IVi, Vi) internal 

 pharyngeal pouches; (D.th.gl.) ductus thyroglossus; (Ep. Ill, Ep.IV) parathyroid 

 glands; (TA) auditory (Eustachian) tube and tympanic cavity; (Th.) thyroid 

 gland; (Tons.) palatine tonsil; (Ub.) ultimobranchial (postbranchial) body. (After 

 Keibel and Mall. Courtesy, SchaefTer: "Morris' Human Anatomy," Philadelphia, 

 The Blakiston Company.) 



The human thymus glands are of pinkish color in childhood bul 

 in older persons undergo more or less fatty degeneration and become 

 yellowish. They enlarge up to puberty, and thereafter gradually shrink. 

 Contrary to earlier opinion, the gland does not completely disappear 

 in the adult, but persists as a shrunken remnant within the medi- 

 astinum. The endocrinal function of the thymus is doubtful, since 

 removal of the gland is not followed by disturbances of normal func- 

 tions. The enlargement of the gland in early life and its later atrophy 

 suggest that its functional activity may be limited to stages of growth 

 and differentiation. 



There is some evidence that the thymus glands of birds have some 

 relation to the normal formation of the shell, shell-membrane, and 

 albumen of the egg. 



Ultimobranchial Bodies 



Among the pharyngeal derivatives are the ultimobranchial 

 bodies, called also postbranchial and suprapericardial bodies. 



These are paired glands which arise from or near the fifth pair of gill- 

 pouches (Figs. 187, 189). They come to lie near the posterior border 

 of the thyroid and attain a vesicular structure. They occur in all verte- 



