THE THYEOID AXIS. 387 



-cervical, or one of the branches into which that artery, when present, 

 divides, viz., the superficial cervical. 



Varieties. — The thyroid axis has been known to arise beyond the scalenus 

 anticus muscle. It may be associated at its origin with another branch ; thus, it 

 sometimes give origin to the internal mammary, and has been known to give 

 origin to the vertebral, superior intercostal, or deep cervical arteries, 



A. The inferior thyroid artery passes directly upwards, resting on 

 the longus colli muscle, and after a short course bends inwards and 

 downwards behind the sheath of the large cervical vessels, and also 

 behind the sympathetic nerve (the middle cervical ganglion of which, 

 when present, often rests upon this vessel). The artery then makes 

 another curve in the opposite direction, and is distributed to the under 

 part of the thyroid body. Its branches communicate fi-eely with those 

 of the superior thyroid artery, and with the corresponding artery of the 

 other side. 



Branches. — (a) The ascending cervical branch arises at the point where the 

 inferior thyroid turns inwards behind the carotid artery ; it proceeds upwards, 

 close to the phrenic nei-ve, on the liiie of separation between the scalenus anticus 

 and rectus anticus major, giving muscular branches to both, and a few which 

 pass transversely outwards across the neck. These muscular branches commu- 

 nicate with others sent outwards from the vertebral artery. To the spinal canal 

 the ascending cervical artery sends one or two branches {sjjinal branches), which 

 enter the intei-vertebral foramina along the cervical nerves, and assist in sup- 

 plying the bodies of the vertebras, and the spinal cord and its membranes. 



(Jj) A larijnf/cal branch of irregular size is usually supplied by the inferior 

 thyi-oid artery ; it ascends on the trachea and the back of the larynx, and is dis- 

 tributed to the muscles and mucous membrane in that situation. 



(<0 Tracheal branches ramify over the trachea, and anastomose below with the 

 bronchial arteries. 



(rZ) (Esophageal branches are given off, and one or more descend upon the 

 trachea into the chest. 



Varieties. — Origin. — The inferior thyroid artery occasionally arises as an 

 independent branch from the subclavian artery, and rarely from the common 

 carotid or the vertebral. Instances have occurred — very rarely, however — of the 

 presence of two inferior thyroid arteries, one passing over the common carotid 

 artery. 



The ascending cervical artery is occasionally derived from the subclavian or 

 from one of the branches of that vessel, as from the transverse cervical or the 

 suprascapular, or from a trunk common to those two arteries. It is sometimes 

 much larger than usual, and takes the place of the occipital artery. A branch 

 from it not unfrequentlj' compensates for the small size of the deep cervical 

 artery. 



B. The suprascapular artery (transverse scapular, or transverse 

 humeral), a smaller vessel than the transverse cervical arises almost 

 constantly from the thyroid axis, and runs from within outwards deeply 

 at the root of the neck. At first it descends obliquely towards the 

 clavicle, resting upon the scalenus anticus, and covered by the sterno- 

 mastoid muscle ; it then crosses the subclavian artery, and continues 

 transversely outwards behind and parallel with the clavicle and sub- 

 clavius muscle, and below the posterior half of the omo-hyoid 

 muscle. Approaching the upper margin of the scapula, under 

 cover of the trapezius muscle, it inclines downwards with the supra- 

 scapular nerve towards the suprascapular notch. At this point the 



nerve usually passes beneath the ligament stretched across the notch, 



c c 2 



