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220 ANATOMY.

external carotid. It is about 3^ mm. (| inch) in calibre, and arises
close to tlie bifurcation of the common carotid, on a level with or
slightly below the great cornu of the hyoid bone. It is very super-
ficially located within the carotid triangle. It passes slightly upward
and forward at first, after which it passes forward and downward, to
the upper margin of the thyroid cartilage, forming an arch. Here it
passes beneath the omo-hyoid, sterno-hyoid, and sterno-thyroid muscles,
supplying them witli branches, and is finally distributed to the thyroid
gland, breaking up into numerous terminal branches, which anastomose
quite freely with the terminal branches of the inferior thyroid.
The Transverse Artery, which is not universal in its existence, is one
of the terminal branches that pass along the upper border of the isth-
mus of the thyroid body within its capsule. The transverse artery may
be of large size, and give olf branches that overlie the isthmus.
The Superior Thyroid Artery gives off tlie following branches : the
hyoid, superficial descending or sterno-mastoid, superior laryngeal, and
crico-thyroid.
The Hyoid or Inferior Hyoid Artery is a small vessel that passes
inward from the superior thyroid along the under surface of the hyoid
bone beneath the thyro-hyoid muscle. It gives off branches that sup-
ply the muscles attached to the under surface of the hyoid bone, and
anastomoses with similar branches of the opposite side.
The Superficial Descending or Sterno-mastoid Artery passes downward
across the sheath of the common carotid artery. It is of importance to
remember this fact in operations for the ligation of the common carotid
in this region. This artery is distributed to the following muscles : the
omo-hyoid, sterno-hyoid, sterno-thyroid, sterno-cleido-mastoid, and infe-
rior constrictor of the pharynx.
The Superior Laryngeal Artery is the largest of the several branches
of the superior thyroid, being about 2 mm. (yL. inch) in calibre. It
extends inward, accompanied by the superior laryngeal nerve jiasses
;
under the thyro-hyoid muscle to the thyro-hyoid membrane ; pierces
this membrane and enters the larynx, where it separates into two divis-
ions, superior and inferior.
The Superior Dirision supplies the posterior surface of the epiglottis
and its mucous membrane.
The Inferior Division supplies the intrinsic muscles of the larynx and
its mucous membrane.
The Crico-thyroid Branch is a small artery about -^ to 1 mm. (J^- to
^L- inch) in calibre. It passes across the crico-thyroid membrane, and
anastomoses with the corresponding artery of the other side. The
position of this artery gives it importance, on account of the fact that
hemorrhage often occurs from its division in the operation of laryn-
gotomy.
Variations.—The superior thyroid artery may be larger or smaller
than usual. AVhen this difference exists either way, one or more of the
other three thyroid arteries will be found to be increased or diminished
in size. Occasionally it is found to arise in common with the lingual
or facial, or both. It may also be a division of the common carotid.
The branch of this artery which supplies the sterno-cleido-mastoid mus-