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. 358). 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.[132]
=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.