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216 ANATOMY.

number of branches, though these may vary in size. They are about
8 mm. (^ inch) in calibre. Tliey diiier both in their length and origin,
the left being the longer : it arises directly from the arch of the aorta,
and is more deeply situated than the right.
The Right Common Carotid Artery is the shorter of the two, and at
its origin is more superficially situated than the left. It is one of the
terminal branches of the brachio-cephalic (innominate) artery, the other
branch being the subclavian. The bifurcation of the brachio-cephalic
into the right common carotid and subclavian arteries takes place within
the thorax behind the sterno-clavicular articulation and above the level
of the second dorsal vertebra. In front it is in relation with the right
brachio-cephalic (innominate) vein, and externally with the subclavian
vein and the pneumogastric and phrenic nerves.
The Left Common Carotid Artery arises from the left of the arch of
the aorta, and, with the exeption of the coronary arteries, which supply
the heart, is the second branch given off from that vessel. At its origin
it is situated within the thorax. It then passes upward and a little out-
ward to the left of the sterno-clavicular articulation, from which point
its course is similar to the artery of the right side. It is situated just
b?hind the upper jiortion of the sternum, and is covered by the sterno-
hyoid and sterno-thyroid muscles, and in early childhood by a portion
of the thymus gland. The left brachio-cephalic vein crosses it in front,
and behind it is in relation with the trachea, oesophagus, and thoracic
duct. Externally will be found the pleura, pneumogastric (tenth
cranial), and phrenic nerves.
In the neck the common carotids are generally similar in course and
situation, though they may differ slightly in size, and their termination
may be at a point slightly higher or lower on one side than the other.
They extend from the sterno-clavicular articulation on either side to
their termination opposite the upper border of the thyroid cartilage,
without giving off any branches. At the upper border of the thyroid
cartilage they divide into two large branches, the external and internal
carotids.
The Line of the Common Carotid Artery extends from the sterno-
clavicular articulation to a point midway between the angle of the jaw
and the mastoid process of the temporal bone. At the point where the
carotid arteries emerge from the thorax and enter the neck thev are
closely approximated, being separated only by the anterior semicircum-
ference of the trachea. As they ascend the neck they diverge, and at
their termination are separated by the larynx and the pharvnx, -which
structures are pushed forward between them. They are deeply seated
at their origin, but become quite superficial in the region of the larvnx.
Tlie Sheath of the Common Carotid Artery \ii derived from the deep
cervical fascia, and encloses, together with the artery, the pneumogastric
nerve and internal jugular vein. The artery is situated to the median
side of the nerve, Avhile the vein is external to both artery and nerve.
Each of these structures is separated from the others by a distinct
investment of connective tissue. The descendens noni nerve is occa-
sionally within the sheath of the common carotid, but more frequently
it passes down the neck upon the sheath.