THYROID AXIS. 371 



The posterior cerebral artery is occasionally given off on one side from the internal 

 carotid artery. 



II. THYROID AXIS. 



The thyroid axis springs from the fore part of the subclavian artery, 

 close to the inner side of the anterior scalenus muscle. It is a short thick 

 trunk, and receives the name of " axis," because, at a line or two from its 

 origin, it divides into branches, which diverge in different directions, viz., 

 the inferior or ascending thyroid, the suprascapular, and a third branch, 

 which is either the transverse cervical, or one of the branches into which 

 that artery, when present, divides, viz., the superficial cervical. 



PECULIARITIES. 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 gives origin to the internal mammary, and has been known to give origin 

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



1. THE INFERIOR THYROID ARTERY passes directly upwards, resting on 

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

 wards 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 carve in the 

 opposite direction, and is distributed to the under part of the thyroid 

 body. Its branches communicate freely 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 nerve, on the line of separation between the scalenus anticus and 

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

 versely outwards across the neck. These muscular branches communicate with others 

 sent outwards from the vertebral artery. To the spinal canal the ascending cervical 

 artery sends one or two branches (spinal branches), which enter the inter vertebral 

 foramina along the cervical nerves, and assist in supplying the bodies of the vertebrae, 

 and the spinal cord and its membranes. 



(b) A laryngeal branch of irregular size is usually supplied by the inferior thyroid 

 artery ; it ascends on the trachea and the back of the larynx, and is distributed to 

 the muscles and mucous membrane in that situation. 



(c) Traclieal branches ramify over the trachea, and anastomose below with the 

 bronchial arteries. 



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

 into the chest. 



PECULIARITIES. 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 unfrequently 

 compensates for the small size of the deep cervical artery. 



2. 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 



