The =thyroid gland= (Glandula thyreoidea) is a very vascular ductless
gland, situated on the trachea close to the larynx. It is red brown in
color, and consists of two =lateral lobes= and a connecting =isthmus=
(Fig. 354).
The =lateral lobes= are situated on either side of the first and second
or second and third rings of the trachea, to which they are loosely
attached. Each is about the size of a walnut, and has a convex
superficial face which is related to the parotid gland and the omo-hyoid
muscle, and a slightly concave, deep face, applied to the trachea.
The =isthmus= in the adult horse is usually very rudimentary. It may
occur as a very narrow glandular band which connects the posterior
extremities of the lateral lobes, extending across the ventral surface
of the trachea, but it is frequently only a small strand of connective
tissue, and sometimes is entirely absent. It may be represented by a
small tail-like process of one lobe.
The lateral lobes vary in size and position, and are often dissimilar
on the two sides. The anterior extremity is the larger; it is rounded
and usually lies about a finger’s breadth behind the larynx, but may
be in contact with it. The posterior extremity is often produced to
form a narrow tail-like process, which is continuous with the isthmus
when one is present. In the ass and mule the isthmus is usually well
developed. There is sometimes a narrow band of thyroid tissue which
extends forward from the isthmus for a variable distance; this is
termed the pyramidal lobe. The position of the lateral lobe is
indicated approximately by the junction of the external maxillary and
jugular veins.
=Structure.=—The gland is enveloped by a thin, fibrous =capsule=, from
which =trabeculæ= pass into the gland, dividing it into =lobules=. The
lobules consist of non-communicating =alveoli= or =follicles= of varying
form and size. The alveoli are lined by cubical epithelium, and contain
a viscid colloid substance.
=Blood-supply.=—The arteries are relatively very large; there are
usually two thyroid arteries, which arise from the carotid and enter
each extremity of the gland.
=Nerve-supply.=—The nerves are derived from the sympathetic system.
Nodules of thyroid tissue of variable size, sometimes as large as a pea,
may be found near the anterior extremity of the lateral lobes, on the
course of the anterior thyroid artery or on the trachea, even at a
considerable distance from the thyroid region. They are termed
=accessory thyroids= (Glandulæ thyreoidæ accessoriæ).
The =parathyroids= are small glandular bodies found in the thyroid
region. They are paler and not so dense as the thyroid tissue, but often
cannot be differentiated by their naked-eye appearance. In the horse
there is usually only one on each side in the connective tissue over the
anterior extremity of the lateral lobe of the thyroid. They resemble in
structure embryonic thyroid tissue.