This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The operations which are done on the thyroid gland are ligation of its arterial supply and complete or partial removal. These necessitate a knowledge particularly of its blood supply and structure. The thyroid gland consists of an isthmus and two lateral lobes. The isthmus crosses the second, third, and fourth tracheal rings in the adult. In children it may approach nearer to the cricoid cartilage.
The lateral lobes lie under the sternohyoid and the sternothyroid muscles. They rise as high as the oblique line on the sides of the thyroid cartilages which marks the insertion of the sternothyroid muscles. The lobes descend to the level of the sixth ring of the trachea, which is two rings below the isthmus, about two centimetres above the sternum. The inferior constrictor of the pharynx is beneath the gland. The thyroid gland is covered by the pretracheal fascia and possesses a capsule of its own besides. This fascia envelops the gland and its capsule, and from its posterior surface is prolonged down on the trachea and envelopes the vessels coming to and leaving the gland. Therefore we might say that the inferior thyroid veins are in the pretracheal fascia.
Fig. 184. - Excision of the thyroid gland.
As the fascia leaves the gland at the sides one portion of it blends with and helps to form the sheath of the vessels. The other or deeper portion continues around the pharynx and oesophagus, forming the buccopharyngeal fascia. In freeing the gland and its capsule from the overlying pretracheal fascia care must be taken, as pointed out by James Berry (" Diseases of the Thyroid Gland," p. 269), not to be led by this fascia too far posteriorly and therefore wound, as has been done, the pharynx or trachea.
The veins of the gland are more prominent and dangerous than the arteries. They ramify beneath the capsule and as long as the capsule is not torn the bleeding is slight. The arteries of the thyroid gland are the superior and inferior thyroids and sometimes the thyroidea ima. The superior thyroid comes off the external carotid just above the bifurcation. It rises almost to the greater horn of the hyoid bone and then descends to the thyroid gland, which reaches to the level of the oblique line on the thyroid cartilage; it supplies the upper portion of the gland, particularly the anterior portion, but also sends a branch down the posterior surface. The vessels crossing the median line, contrary to what is often the case in the arteries of the lip and even the scalp, are very small. The superior thyroid is superficial and presents no special difficulty in ligation. The vein runs beneath it on its course to the internal jugular. The inferior thyroid artery, a branch of the thyroid axis, crosses behind the common carotid artery about the level of the seventh cervical vertebra, about on a line with the lower edge of the isthmus. It enters the gland from the side and not from below and ramifies on its posterior surface often as a single large trunk beneath the capsule giving off branches to the parenchyma. Usually it is in front of the recurrent laryngeal nerve, but the middle cervical ganglion of the sympathetic lies on it. Sometimes the artery breaks into branches before it enters the gland. In such cases the recurrent laryngeal nerve may run between these branches and so be injured in removing the gland.
The thyroidea ima artery when present enters the gland from below, coming up on the trachea usually from the innominate, in which case the innominate is apt to come off more to the left side and so bring the common carotid closer to the trachea than usual. It may also spring from the aorta or from the right carotid artery.