The thyroid body or gland is a deep red glandular mass consisting of two
lobes which lie one on each side of the upper part of the trachea and
lower part of the larynx; these are joined across the middle line by the
isthmus which lies in front of the second and third rings of the
trachea. Occasionally, from the top of the isthmus, a nearly but not
quite median pyramidal lobe runs up toward the hyoid bone, while in
other cases the isthmus may be absent. The gland is relatively larger in
women and children than in the adult male. It is enclosed in a capsule
of cervical fascia and is supplied by the superior and inferior thyroid
arteries on each side, though occasionally a median thyroidea ima artery
is present. On microscopical examination the gland shows a large number
of closed tubular alveoli, lined by columnar epithelial cells,
unsupported by a basement membrane, and filled with colloid or
jelly-like material. These are supported by fibrous septa growing in
from the true capsule, which is distinct from the capsule of cervical
fascia. The lymphatic vessels are large and numerous, and have been
shown by E.C. Baber (_Phil. Trans._, 1881) to contain the same colloid
material as the alveoli. Accessory thyroids, close to the main gland,
are often found.
_Embryology._--The median part of the gland is developed from a tube
which grows down in the middle line from the junction of the buccal
and pharyngeal parts of the tongue (q.v.), between the first and
second branchial arches. This tube is called the thyro-glossal duct
and is entodermal in origin. The development of the hyoid bone
obliterates the middle part of the duct, leaving its upper part as the
foramen caecum of the tongue, while its lower part bifurcates, and so
the asymmetrical arrangement of the pyramidal lobe is accounted for.
A. Kanthack (_J. Anat. and Phys._ vol. xxv., 1891) has denied the
existence of this duct, but on slender grounds. The lateral parts of
the gland are developed from the entoderm of the fourth visceral
clefts, and, joining the median part, lose their pharyngeal connexion.
Nearly, but not quite, the whole of the lateral lobes probably belong
to this part. (For literature and further details see Quain's
_Anatomy_, London, 1892, and J.P. McMurrich's _Development of the
Human Body_, London, 1906.)
_Comparative Anatomy._--The endostyle or hypobranchial groove of
Tunicata (sea squirts) and Acrania (Amphioxus) is regarded as the
first appearance of the median thyroid; this is a median entodermal
groove in the floor of the pharynx, secreting a glairy fluid in which
food particles become entangled and so pass into the intestine. In the
larval lamprey (Ammocoetes) among the Cyclostomata the connexion with
the pharynx is present, but in the adult lamprey (Petromyzon), as in
all adult vertebrates, this connexion is lost. In the Elasmobranchs
the single median thyroid lies close to the mandibular symphysis, but
in the bony fish (Teleostei) it is paired. In the mud fish (Dipnoi)
there is also an indication of a division into two lobes. In the
Amphibia the thyroid forms numerous vesicles close to the anterior end
of the pericardium. In Reptilia it lies close to the trachea, and in
the Chelonia and Crocodilia is paired. In birds it is also paired and
lies near the origin of the carotid arteries. In Mammalia the lateral
lobes make their first appearance. In the lower orders of this class
the isthmus is often absent. (For further details and literature see
R. Wiedersheim's _Vergleichende Anatomie der Wirbeltiere_, Jena, 1902,
and also for literature, Quain's _Anatomy_, London, 1896.)