

Iir.MAN ANATOMY, 



ve for 

 left innominate vein 



An nil- 

 impression 



-mg ; this increase is said to continue even after puberty, the organ, how- 

 .iniiiM thinm-r and softer. Although later almost completely replaced by 

 and connective tissue, the thymus never entirely disappears, remains of its 



tissue being present even in extreme 



FIG. 1517. old age (Waldeyer). Until about 



twenty years the organ is usually 

 readily found. In ordinary dissec- 

 tions it is not easily recognized in mid- 

 dle age, although still clearly shown 

 in frozen sections. Occasionally a 

 well-preserved thymus persists in the 

 adult ; on the other hand, it may 

 suffer atrophy very early in child- 

 hood. 



Structure. The histological 

 character of the thymus completely 

 changes during its development, since 

 it begins as an epithelial outgrowth 

 from the third pharyngeal pouch, for 

 a time attains the nature of a tubo- 

 alveolar gland, and later permanently 

 assumes the type of a lymphoid organ. 

 Externally the thymus is invested 

 by a loose fibro-elastic capsule, from 

 which septa, rich in blood-vessels, pass towards the interior and subdivide the organ 

 into a number of indefinite lobes. The latter are broken up into small, almost spheri- 

 cal lobules, which correspond to lymph-nodules, and consist, therefore, of a denser 

 c.iti.al and loo* r medullary /.one. although these are not sharply defined from each 

 other. 



The cortical substance presents histological characteristics resembling those of 

 drn.M- lymphoid tissue, closely packed lymphocytes lying within the narrow meshes 

 of the supporting n-ticulum. The latter consists of the stellate reticulum-cells, which 



Posterior aspect of thymus body hardened in situ. 



uti 



Fio. 1518. 



c 'frnum I' -ih-cartilagc 



Left lung 



III til 



Tracher. 



IV "'' "''" ' IV til. lU-.i.loi IV ,ih IV, ih 



Trmnvre Action of body at level of fourth thoracic vertebra ; in,,,, child of about on, year 



Lerived from th.- m.-t.im..rph,M-.l original entoblastic epithelium. The thvmic 



lymph,,, VI, , on thr contrary, are descendants ,,f migratory cells, which early invade 



In addition to the UMU! elements, eosinophUic cells are found 



