158 THE LYMPHATIC SYSTEM 



of childhood. After this time it is gradually replaced by adipose 

 tissue, its retrograde metamorphosis becoming complete at about 

 the age of puberty. 



At its maximum the thymus forms a large lymphoid mass, 

 embedded in areolar connective tissue, the trabeculas of which 

 divide the organ into several lobes and innumerable minute Mules. 

 Each lobule is surrounded by a thin fibrous capsule, by which it 

 is loosely united to its neighbors. 



The lobule consists of a mass of lymphoid tissue, which is dense 

 at the periphery but looser in the central portion. It is thus di- 

 visible into a dense cortex and a loose medulla, both composed 

 essentially of lymphoid tissue, but between which, because of the 

 difference in density, there is a sharp line of demarcation. Fre- 

 quently, at some point on its circumference, the medulla reaches 



d 



FIG. 154. A SECTION THROUGH SEVERAL LOBULES OF THE THYMUS OF AN INFANT. 



o, loose fibrous septum between the lobules ; 6, cortex, and c, medulla of the lobule ; 

 d, blood vessels in the connective tissue. Hematein and eosin. Photo, x 60. 



the surface of the lobule, and at such locations a narrow column 

 of medullary lymphoid tissue connects it with the adjoining 

 lobule. 



A close-meshed reticulum, within the narrow meshes of which 

 are closely packed lymphatic corpuscles, composes the lymphatic 

 tissue of the lobule. That of the cortex and the medulla is alike, 



