1798 



HUMAN ANATOMY. 



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

 ever, becoming thinner and softer. Although later almost completely replaced by 

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



tissue being present even in extreme 



Fig. 1 517. 



Groove for 

 left innominate vein 



Groove for 

 su])erior 

 vena cava 



Aortic, 

 impression 



Pericardial 

 surface 



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-preser\ed 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 

 cortical and looser medullary zone, although these are not sharply defined from each 

 other. 



The coHical substance presents histological characteristics resembling those of 

 dense lymphoid tissue, — closely packed lymphocytes lying within the narrow meshes 

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



Posterior aspect of thymus body hardened in situ. 



Fig. 1518. 



II rib-cartilage 



Sternum 



11 rib-cartilage 



Left lung 



III rib 



Right lung 



Azygos vein 



\\\ rib 



Trachea 



IV rib Head of IV rib Head of IV rib IV rib 



Transverse section of body at level of fourth thoracic vertebra ; from child of about one year 



are derived from the metamorphosed original entoblastic epithelium. The thymic 

 lymphocytes, on the contrary, are descendants of migratory cells, which early invade 

 the reticulum. In addition to the usual elements, eosinophilic cells are found 



