The thymus may appear to be a single organ, but in fact it consists of right and left lobes closely applied to each other for much of their extent. It is usually most prominent in children, where it may be expected to extend from the level of the fourth costal cartilages to the lower poles of the thyroid gland. In front of it lie the pretracheal fascia, the sternohyoid and sterno­thyroid muscles, the manubrium and upper part of the body of the sternum and their adjacent costal cartilages. Behind it are the pericardium, the arch of the aorta with its three large branches (brachiocephalic, left common carotid and left subclavian), the left brachiocephalic vein and the trachea.

The thymus can be distinguished from fat by being slightly different in color (pinker or browner in life), with lobules that are larger, smoother and denser than fat. During surgical exposure for thymectomy through a median sternotomy, care must be taken not to damage adjacent thin-walled veins and, although more laterally placed, the phrenic nerves may be at risk when clearing extrapleural fat. The pretracheal fascia is the guide to the thymus, for this thin layer must be exposed by clearing away the overlying fatty tissue and then incised so that the gland can be dissected away from the posterior surface of the fascia.

Blood supply of thymus:

Small branches enter the thymus from the inferior thyroid and internal thoracic arteries, and there are corresponding veins. Frequently a rather large thymic vein formed by tributaries from both lobes enters the left brachiocephalic vein.

Lymph drainage of thymus:

Channels drain into parasternal, tracheobronchial and brachiocephalic nodes, but the thymus does not receive any afferent lymphatics.

Nerve supply of thymus:

Sympathetic fibers that enter the thymus are presumably vasomotor.

Development of thymus:

The thymus develops from the (endodermal) epithelium of the ventral diverticulum of the third pharyngeal pouch on each side, hence its bilobed structure. Some of the epithelial cells become the thymic (Hassall’s) corpuscles; others form a network of epithelial reticular cells believed to be the source of thymic hormones concerned with the differentiation of T lymphocytes. Connective tissue elements are derived from surrounding mesoderm, but the original colonizing lymphocytes have migrated from the bone marrow. The developing thymus descends from the neck into the mediastinum in front of all the major contents. (The inferior parathyroid comes from the dorsal part of the same pouch.) It regresses after puberty to become largely replaced by fatty tissue.