46 



The Endocrine Organs 



nective tissue conveys blood-vessels, lymphatics, and nerves to and from 

 the organ. 



Each lobule is composed of a cortex which is very dark in stained sections 



FIG. 29. Medulla of thymus of child. Magnified 300 diameters. The small 

 darkly stained cells are lymphocytes. The preparation shows two or three 

 concentric corpuscles, and blood-vessels. 



and a medulla which is lighter (rig. 28). The difference is due to the large 

 number of lymphoid cells in the cortex, which is densely packed with 



them. The medulla is formed of 

 a cell-reticulum as a basis, in the 

 meshes of which are a considerable 

 number of lymphoid cells ; and in 

 addition the characteristic " concen- 

 tric corpuscles of Hassal," which 

 are formed by nests of epithelium 

 cells surrounding one or more cells 

 which occupy the centre of the nest 

 (figs. 29, 30). Some of the concen- 

 tric corpuscles are compound, having 

 more than one centre. Occasionally 

 the medulla is found to contain 

 small ciliated cavities, and rarely 

 striated muscle-fibres are seen with- 



FIG. 30. A concentric corpuscle of the thymus 

 with a part of the adjoining reticulum. 

 (Hammar. ) 



c., a ciliated vesicle. 



in it. 



The thymus is developed in the 

 embryo from sprouts of the epithelium of the third branchial pouch on 

 each side (fig. 2). These unite to form a single median column of cells 

 which becomes hollowed out and tubular, and ultimately branched. Even- 

 tually lymphoid nodules forming the cortex of the lobules appear in the 



