Page 280 - Clinical Anatomy

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The thyroid gland 265

Fig. 189◊The thyroid and its blood vessels.

upper pole lies the external branch of the superior laryngeal nerve passing to
the cricothyroid muscle.

Blood supply
Three arteries supply and three veins drain the thyroid gland (Fig. 189):
•◊◊the superior thyroid artery—arises from the external carotid and passes to
the upper pole;
•◊◊the inferior thyroid artery—arises from the thyrocervical trunk of the 1st
part of the subclavian artery and passes behind the carotid sheath to the
back of the gland;
•◊◊the thyroidea ima artery— is inconstant; when present, it arises from the
aortic arch or the brachiocephalic artery;
•◊◊the superior thyroid vein — drains the upper pole to the internal jugular
vein;
•◊◊the middle thyroid vein— drains from the lateral side of the gland to the
internal jugular;
•◊◊the inferior thyroid veins—often several—drain the lower pole to the bra-
chiocephalic veins.
As well as these named branches, numerous small vessels pass to the
thyroid from the pharynx and trachea so that even when all the main
vessels are tied, the gland still bleeds when cut across during a partial thy-
roidectomy.

Development

The thyroid develops from a bud which pushes out from the floor of the