The Thymus Gland

The thymus is a ductless gland which has a close resemblance to the lymphoid tissues. It is well developed only in late fœtal life and for a few months after birth. After this it undergoes rapid atrophy, fatty infiltration, and amyloid degeneration, so that in the adult it is represented by a thin remnant in the lower part of the anterior mediastinum or has entirely disappeared. In the new-born foal it is of a grayish-pink color, and consists of two lateral lobes (Fig. ). The greater part of the gland is situated in the anterior mediastinum, but the two lobes are continued into the neck by a chain of lobules which lie along the course of the carotid artery, extending sometimes as far as the thyroid gland.

Structure.—The thymus is inclosed in a thin, connective-tissue capsule, and consists of secondary lobules (Lobuli thymi), of varying shape and size, held together by areolar tissue and fat, and all attached to a cord of adenoid tissue termed the tractus centralis. These lobules are subdivided into polyhedral primary lobules, which are composed of lymphoid tissue, and present a dark vascular cortex, and a lighter medulla that contains Hassal’s corpuscles.

Blood-supply.—Internal thoracic and carotid arteries.

Nerve-supply.—Sympathetic and vagus nerve.