Thymus. Light micrograph of a section through a healthy human thymus. At centre are blood vessels (white circles) surrounded by the fibrous capsule (blue). At top left is part of the densely packed cortex (dense, purple). At lower right is tissue of the medulla (purple) containing a Hassal's corpuscle (small oval), an obscure cyst-like structure that may be a degenerated epithelial cell. The thymus is a gland in the lower part of the neck. It is involved in the maturation of T- lymphocytes, which are a type of immune cell. They enter the cortex, where they divide while migrating into the medulla, before re-entering general circulation. Magnification unknown.