, and at this level the common
carotid artery may be compressed against the _carotid tubercle_ on the
transverse process of the sixth cervical vertebra. The cricoid also
marks the junction of the larynx with the trachea, and of the pharynx
with the oesophagus; at this point there is a constriction in the food
passage, and foreign bodies are frequently impacted here. At the level
of the cricoid cartilage the omo-hyoid crosses the carotid artery--a
point of importance in connection with ligation of that vessel. The
middle cervical ganglion of the sympathetic lies opposite the level of
the cricoid. (6) Seven or eight rings of the _trachea_ lie above the
level of the sternum, but they cannot be palpated individually. The
_isthmus_ of the thyreoid gland covers the second, third, and fourth
tracheal rings. As the trachea passes down the neck, it gradually
recedes from the surface, till at the level of the sternum it lies
about an inch and a half from the skin. The _thyreoidea ima_
artery--an inconstant branch of the anonyma (innominate) or of the
aorta--runs in front of the trachea as far up as the thyreoid isthmus.
The inferior thyreoid plexus of veins also lies in front of the
trachea. In the superficial fascia, cross branches between the
anterior jugular veins cross the middle line.

In children under two years of age the _thymus gland_ may extend for
some distance into the neck in front of the trachea and carotid
vessels, under cover of the depressors of the hyoid bone.

_Cervical Fascia._--This fascia completely envelops the neck, and from
its deep aspect two strong processes--the prevertebral and pretracheal
layers--pass transversely across the neck, dividing it into three main
compartments. The posterior or _vertebral compartment_ contains the
muscles of the back of the neck, the vertebral column and its
contents, and the prevertebral muscles. This compartment is limited
above by the base of the skull, and below is continued into the
posterior mediastinum. The middle or _visceral compartment_ contains
the pharynx and oesophagus, the larynx and trachea with the thyreoid
gland, and the carotid sheath and its contents. These different
structures derive their special fascial coverings from the processes
that bound this compartment. The middle compartment extends to the
base of the skull and passes into the anterior mediastinum as far as
the pericardium. The connective tissue space around the subclavian
vessels is continu