The thymus is surrounded by a capsule (not easily seen in HA9) with trabeculae (white lines) extending inward to define thymic lobules. These lobules are all continuous at the medulla. Note that the outer portion of the lobule - the cortex - contains densely packed lymphocytes. The inner portion of the lobule - the medulla - contains more lightly staining tissue (HA9, c = cortex, m = medulla).