778 THE VASCULAR SYSTEM. 
which is separated from it by the layer of deep cervical fascia which binds the posterior — 
belly of the omo-hyoid to the posterior border of the subclavian groove. More superficially 
the third part of the artery is covered by the superficial layer of the deep fascia, the 
descending clavicular branches of the cervical nerves, the platysma, and the skin. 
BRANCHES OF THE SUBCLAVIAN ARTERY. 
(1) The vertebral artery is distributed almost entirely to the head and neck, 
and its chief function is to supply the posterior part of the brain. Its description 
has therefore been given with that of the other cerebral arteries (see p. 773). 
(2) Thyroid Axis (truncus thyreo-cervicalis, Figs. 552 and 554).—This branch | 
arises close to the inner border of the scalenus anticus, and directly above the origin — 
of the internal mammary artery, from the upper and front part of the subclavian 
artery. After a short upward course of about two lines (4 mm.), 1t ends under 
cover of the internal jugular vein by dividing into three branches—viz. the inferior 
thyroid, the transverse cervical, and the suprascapular. 
(A) The inferior thyroid artery (a. thyreoidea inferior, Fig. 552) ascends 
along the anterior border of the scalenus anticus, and turns inwards opposite the 
cricoid cartilage to the middle of the posterior border of the lateral lobe of the 
thyroid body; it then curves inwards and downwards, and descends to the lower 
end of the lobe, where it divides into ascending and inferior terminal branches. 
x 
, 
Relations. —2ehind it is the vertebral artery externally and the longus colli muscle 
internally ; the recurrent laryngeal nerve passes either in front of or behind the vessel, 
opposite the lower border of the thyroid body. It is covered im front by the carotid 
sheath, which contains the common carotid artery, the internal jugular vein, and the 
vagus nerve; the middle cervical ganglion of the sympathetic lies in front of the artery 
as it bends inwards, and on the left side the thoracic duct also passes in front of it. 
Branches.—It vives off the following branches :— 
(a) Muscular. Numerous small branches pass to the scalenus anticus, the longus 
colli, the infra-hyoid muscles, and the inferior constrictor of the pharynx. 
(>) The ascending cervical branch (a. cervicalis ascendens) usually springs from the 
inferior thyroid near its origin, though not uncommonly it rises separately from the thyroid 
axis. It ascends parallel with and internal to the phrenic nerve, in the angle between the 
rectus capitis anticus major and the scalenus anticus, to both of which it gives branches. 
It also gives off spinal branches which pass through the intervertebral foramina to the — 
spinal canal, and it anastomoses with branches of the vertebral, occipital, ascending — 
pharyngeal, and deep cervical arteries. ; 
(c) Gsophageal (rami cesophagei) are small branches given to the walls of the 
cesophagus, which anastomose with the csophageal branches of the thoracic aorta. 
(d) Tracheal branches (rami tracheales) are distributed to the trachea ; they anasto- 
mose with branches of the superior thyroid and with the bronchial arteries. 
(ec) An inferior laryngeal branch (a. laryngea inferior) accompanies the recurrent 
laryngeal nerve to the lower part of the larynx. It enters the larynx, beneath the lower 
border of the inferior constrictor, gives branches to its muscles and mucous membrane, 
and anastomoses with the laryngeal branch of the superior thyroid. 
(7) The ascending terminal branch supplies the posterior and lower part of the 
thyroid body, and anastomoses with branches of the superior thyroid artery. 
(7) The inferior terminal branch is distributed to the lower and inner part of the 
© thyroid body. It anastomoses with its fellow of the opposite side and with branches of 
the superior thyroid artery. 
(B) The transverse cervical artery (a. transversa colli, Figs. 552 and 554) 
runs upwards, outwards, and backwards from the thyroid axis across the posterior 
triangle of the neck to the anterior border of the trapezius, where it divides into 
superticial cervical (ramus ascendens) and posterior scapular (ramus descendens) 
branches. It is very variable in size, and not infrequently the posterior scapular 
arises separately from the third part of the subclavian. 
Tinmediately after its origin, under cover of the internal jugular vein, it crosses- 
the scalenus anticus, lying superficial to the phrenic nerve and under cover of the 
sterno-mastoid muscle ; on the left side it is also crossed superficially by the terminal 
part of the thoracic duct. Passing from beneath the sterno-mastoid, it enters the 
