Thymus Gland (Gr. Ούμος ), a double vascular or ductless gland, situated, in the human subject, in the upper part of the anterior mediastinum, extending in childhood from the thyroid gland to the anterior surface of the pericardium, but becoming atrophied after the age of puberty. It is divided into two lateral portions, right and left, which are in reality distinct from each other, being connected only by areolar tissue. Each lateral portion is gland-like in structure, being divided into a number of lobules, from a sixth to a third of an inch in diameter, and irregularly rounded and flattened. In its interior is a central cavity, having the form in some cases, according to Kolliker, of a cylindrical canal, communicating with that of the separate lobules, and containing a grayish white or milky-looking fluid, of a slightly acid reaction. Each lobule is invested on its exterior by a thin, nearly homogeneous membrane, which sends partitions a short distance into its substance, dividing it in this way externally into rounded masses or gland follicles, each about 1/50 of an inch in diameter, giving the outer surface of the lobule a granular appearance.

The solid substance of the lobule consists of a soft homogeneous material, containing nucleated cells and an abundance of free nuclei, and penetrated throughout by capillary blood vessels, which radiate from the wall of the central cavity and terminate in vascular loops toward its external portion. Its central cavity has no excretory duct, and its secreted product, if such there be, must be taken up and carried away by the veins or the lymphatics. The thymus gland is highly developed during the latter part of intra-uterine life, and at the time of birth, in man, weighs rather more than half an ounce. It continues to enlarge until the age of two years, at which time its growth ceases. It begins to diminish about the 10th year, but is still usually perceptible, and sometimes well developed, at the age of 20. By the 40th year it has entirely disappeared. It is about the same in the anthropoid apes as in man, and is remarkably developed and may be well studied in the calf, in which, and in the lamb, it is called the sweetbread, and is a delicate article of food; it exists in mammals, birds, and most reptiles, but not in the larvae of batrachians, the peren-nibranchiate amphibians, or fishes. - The precise function of the thymus gland is unknown.

It undoubtedly serves, like the other ductless glands, to accomplish some change in the blood circulating through its tissue, which is essential to the proper nourishment of the body during intra-uterine life, infancy, and childhood.

Transverse Section of an injected Lobule of the Thymus of a Child.

Transverse Section of an injected Lobule of the Thymus of a Child, magnified 30 diameters, a. Membrane of the lobule, b. Membrane of the gland follicles, c. Cavity of the lobule from which the larger vessels branch out into the corpuscles, on the surface of which they terminate, occasionally forming loops.