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218 ANATOMV.

fascia, and by the anterior border of the sterno-cleido-mastoid muscle.
On dissection, however, because of the slirinkage which takes place in
the muscle after death, it is usually found that the anterior border
of the sterno-cleido-mastoid does not cover tlie artery in this position,
but it is found in the carotid triangle, which is bounded above by
the posterior belly of the digastric muscle, in front by the anterior
belly of the omo-hyoid muscle, and behind by tlie sterno-cleido-mas-
toid muscle, the sterno-mastoid branches of the superior thyroid artery,
and by the facial, lingual, and superior thyroid veins, crossing it in this
triangle. Occasionally it is partially covered by the thyroid gland.
Deep Relcdions.—The common carotid artery lies directly in front of
the cervical vertebme, separated from them by the longus colli and the
rectus capitis anticus major muscles. The interval between the artery
and the transverse processes of the vertebrae being small, compression
backward in this situation to a great extent controls the flow of blood
through the vessels. In the median line the vessel is related, as it
passes from below upward, with the trachea, thyroid gland (the gland
at times overlapping the artery), larynx, oesophagus, and pharynx.
Variations.—Normally, the common carotid artery gives off no
branches, and it is of the same calibre from its commencement to its
bifurcation. At other times either the superior or inferior thyroid
artery may arise from it, the artery being reduced in size above the
branches. The division of the common carotid into the external and
internal carotids may take place as high or higher than the hyoid bone,
or it may bifurcate lower down than its normal position ; the common
carotid may be entirely absent, when the external and internal caro-
tids will generally arise directly from the aorta. At times, however,
the right common carotid may arise directly from the aorta alone or
in conjunction with the left—a condition common in some of the lower
animals.
The origin of the left common carotid is more varied than that of the
right. It may arise in either of the following ways : from or in con-
junction with the brachio-cephalic or subclavian, or by a common trunk
with the right common carotid, when the subclavian will arise directly
from the aorta.
Collateral Circulation.—If the common carotid artery be ligated, the
blood for the parts usually supplied by the internal carotid will be car-
ried by the vertebral arteries and the internal carotid of the opposite side.
These arteries, together with the other internal carotid, freely communi-
cate with each other through the circle of Willis at the base of the
brain. The blood for the parts supplied by the external carotid is car-
ried by the superior and inferior thyroids, which freely anastomose ; by
the occipital and deep cervical,- which also anastomose ; and by the
external carotid of the opposite side through its communication with
the two superior thyroids, the lingual, facial, temporal, internal maxil-
lary, and occipital arteries.
The External Carotid Artery is so called because it supplies the
external portion of the head with blood. It is about 6 mm. {\ inch)
in calibre, and arises from and is one of the terminal branches of the
common carotid. This origin is within the carotid triangle opposite