THE THYMUS GLAND The thymus is a light pink gland, consisting of two unequal lobes, which lies in front of the pericardium and great vessels ; it also extends up into the root of the neck to within a short distance of the thyroid gland. It continues to grow until the second year of life, after which it remains stationary until puberty, when it usually atrophies and is represented by a mass of fat. Each lobe is divided up by areolar tissue, and, microscopically, the lobules consist of a cortical and medullary part. The cortex resembles in structure a lymphatic gland (see LYMPHATIC SYSTEM). In the medulla the lymphoid cells are fewer, and nests of epithelial cells are found (concentric corpuscles of Hassall). The vascular supply is derived from all the vessels in the neighbourhood, the lym phatics are very large and numerous, but the nerves, which come from the sympathetic and vagus, are few and small.
Comparative Ana to my.— The thymus is always a paired gland. In most fishes it rises from the dorsal part of all five branchial clefts ; in Lepidosiren (Dipnoi), from all except the first ; in Urodela from third, fourth and fifth, and in Anura from the second only. In all fishes, including the Dipnoi, it is placed dorsally to the gill arches on each side. In Amphibia it is close to the articulation of the mandible. In Reptilia it lies along the carotid artery ; but in young crocodiles it is lobulated and ex tends all along the neck, as it does in birds, lying close to the side of the oesophagus. In mammals the Marsupials are remarkable for - having a well-developed cervical as well as thoracic thymus. In some of the lower mammals the gland does not disappear as early as it does in man.