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 inferior thyroid artery, unlike the superior, lies deep from the surface, and it is a far more difficult vessel to reach. It is a branch of the thyroid axis, the other branches being the transverse cervical and suprascapular. The thyroid axis comes from the first part of the subclavian just a little to the inner side of the edge of the scalenus anticus muscle. The inferior thyroid artery ascends on the longus colli muscle, just to the inner side of the scalenus anticus and almost in front of the vertebral artery. When it reaches about the level of the seventh cervical vertebra it bends inward and behind the carotid artery to reach the lower posterior edge of the thyroid gland. The transverse process of the sixth cervical vertebra, called the carotid tubercle of Chassaignac, is above it. As it bends to go inward it gives off the ascending cervical artery. In front of the artery are the internal jugular vein, common carotid artery, pneumogastric nerve, and the middle ganglion of the sympathetic. The recurrent laryngeal nerve usually passes upward behind the branches of the artery just before they enter the thyroid gland. The thoracic duct on the left side passes over the front of the artery low down.
An incision 7.5 cm. long is made along the anterior border of the sternomastoid muscle, extending upward from the clavicle. This will bring the upper extremity up to, or even above, the cricoid cartilage. The anterior jugular vein will have to be ligated and the muscle displaced outward. The common carotid artery should then be isolated and it, together with the pneumogastric nerve and internal jugular vein, drawn outward. The omohyoid muscle may appear at the upper edge of the incision. Feel for the carotid tubercle on the sixth transverse cervical process: the artery lies below the omohyoid muscle and cricoid cartilage and below the tubercle and beneath the sheath of the carotid. If the trunk of the sympathetic or its middle cervical ganglion, which lies on the artery, is encountered, it should be pushed to the inner side, the artery isolated outwardly and ligature applied. Do not go too far out or the scalenus anticus will be reached and the phrenic nerve may be injured, nor too far in, to avoid wounding the recurrent laryngeal.
Fig. 176. - Collateral circulation after ligation of the third portion of the subclavian artery.
The thyroidea ima (inferior thyroid) veins do not cross outward nor accompany the artery, but proceed downward on the trachea to empty into the innominate veins.