766 . HUMAN ANATOMY. 



and terminates by dividing into three branches; (i) the inferior thyroid, (2) the 

 superjicial cervical, and ( 3 ) the suprascapular. 



{a) The inferior thyroid artery (a, thyreoidea inferior; (Fig. 692) is the 

 largest of the branches which arise from the thyroid axis. It passes at first verti- 

 cally upward to about the level of the transverse process of the sixth cer\'ical 

 vertebra, and then bends medially. It passes behind the common carotid artery, 

 the internal jugular vein, and the pneumogastric nerve, either behind or in front of 

 the recurrent laryngeal nerve and in front of the vertebral artery, and finally breaks 

 up into branches which supply the lower part of the thyroid gland and anastomose 

 with their fellows of the opposite side and with the superior thyroid artery. 



Branches. — In addition to these terminal branches, the inferior thyroid gives origin to the 

 following arteries : 



{aa) Muscular branches to the scalenus anticus and the inferior constrictor of the pharynx. 



{bb) The ascending cervical artery (a cervicalis ascendens) frequently arises directly from 

 the thyroid axis and passes vertically upward, parallel to the phrenic nerve, in the interval be- 

 tween the scalenus anticus and the rectus capitis anticus major. It supplies the deep muscles 

 of the neck, sends branches through the spinal foramina which accompany the spinal branches 

 of the vertebral artery, and anastomoses witft the vertebral, the occipital, the ascending pharyn- 

 geal, and the deep cervical arteries. 



{cc) The inferior laryngeal artery (a. laryngea inferior) passes upward in the groove be- 

 tween the trachea and oesophagus in company with the recurrent laryngeal nerve. It passes 

 beneath the lower border of the inferior constrictor of the pharynx and enters the larynx, to 

 whose mucous membrane and muscles it is distributed. It anastomoses with the superior 

 laryngeal branch of the superior thyroid. 



Finally, it gives off small branches to the pharynx, oesophagus, and trachea, one of those 

 to the last-named structure extending down upon its lateral surface to anastomose below with 

 the bronchial arteries. 



The anastomoses which the inferior thyroid makes by its thyroid branches with the supe- 

 rior thyroid and by its ascending cervical branch with the occipital constitute important connec- 

 tions between the subclavian and carotid systems and play an important part in the establish- 

 ment of the collateral circulation after ligation of the common carotid artery. 



Variations. — The thyroid axis occasionally arises under cover of or even lateral to the 

 scalenus anticus, and it may be entirely wanting, its branches arising directly from the subcla- 

 vian. The inferior thyroid maybe absent on one side or on both, and its size varies inversely 

 to the development of its fellow of the opposite side or to that of the superior thyroid arteries. 



Practical Considerations.— The inferior thyroid may be tied for a wound or 

 during the operation of thyroidectomy. It has been frequently tied, in conjunction 

 with the superior thyroid, in various forms of goitre, but the procedure has been 

 abandoned. It may be reached through the incision for tying the carotid below the 

 omo-hyoid (page 732). The sterno-mastoid and the carotid sheath and its contents 

 are drawn outward. The carotid tubercle being found, the inferior thyroid should 

 be sought for at a slightly lower level, — opposite the body of the sixth cervical ver- 

 tebra or about the level of the omo-hyoid crossing, — coming out from behind the 

 sheath of the great vessels and running in front of the vertebral artery obliquely 

 upward and inward towards the gland. It should be remembered that before enter- 

 ing the gland it lies for a short distance close to its posterior surface, and that the 

 recurrent laryngeal nerve is in intimate relation to this part of the vessel or to its 

 terminal branches. It should therefore be tied in the fissure between the oesophagus 

 and the great vessels, as close to the carotid sheath — i.e., as far from the inferior 

 angle of the gland — as possible, to avoid inclusion of this nerve. The middle cer- 

 vical ganglion cf the sympathetic, the phrenic and the descendens hypoglossi nerves, 

 and, on the left side, the thoracic duct should be carefully avoided. 



(b) The superficial cervical artery (a. cervicalis superficialis) (Fig. 705) 

 passes almost directly laterally from the thyroid axis, passing in front of the scalenus 

 anticus and then across the lower part of the posterior triangle of the neck at a level 

 of about 25 cm. above the clavicle. Arrived at the anterior border of the trapezius, 

 it passes beneath that muscle and breaks up into ascending and descending branches 

 which supply the trapezius, the levator anguh scapulae, the rhomboidei, and the 



