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AREOLAR TISSUE, TEyBONS, AND MUSCLES. 157

way from the neck to its deeper relations. In the median line in front
the fascia is also attached to the hyoid bone.
Below the thyroid body the deep fascia divides into two layers, the
upper and thinner going to the outer and upper portion of the sternum,
to which and the interclavicular ligament it is attached, while the lower
layer is attached to the inner and upper portion of the sternum. Both
layers are superficial to the sterno-hyoid muscles. The space betNveen the
layers of fascia extends laterally until it encloses the sternal heads of the
sterno-cleido-mastoid muscles. The anterior jugular vein passes through
this interfascial space, which contains loose connective tissue and fat,
also sometimes a small lymphatic gland. Thus the upper portion of
the deep fascia of the anterior part of the neck covers in all that por-
tion known as the surgical square of the neck, and externally offers a
barrier to the extension of abscesses and growths from the deeper
parts toward the surface, which causes them to burrow more deeply.
Abscesses forming exterior to this fascia rarely if ever burroAV.
(«) The Deep Portion of the Cervical Fascia.—Near the anterior mar-
gin of the sterno-cleido-mastoid muscle a process is given off from the
superficial layer of the deep fascia which descends behind that muscle
and is associated with the depressors of the hyoid muscular system. It
invests the thyroid body and the front of the trachea, spreads out in
front of the large vessels of the neck, and passes into the thorax as far
as the pericardium. It is supposed to assist in suspending the heart.
(6) The Prevertebral Fascia is a layer of the deep fascia which,
being attached to the base of the skull, descends on the prevertebral
muscles into the thorax, separating them from the pharynx and
oesophagus. Laterally, it becomes continuous with, and forms the back
portion of, the carotid sheath, from which it extends outward and down-
ward over the scaleni muscles, the brachial plexus of nerves and sub-
clavian vessels, which it accompanies beneath the clavicle into the axilla,
where it forms the axillary sheath, and becomes connected with -the
Under surface of the costo-coracoid membrane.
(e) The Carotid Sheath.—The upper portion of this sheath is formed
from the fascia described as a, while the under portion is derived from
that described as b. This sheath forms a complete covering to the caro-
tid artery, the internal jugular vein, and the pneumogastric nerve. A
thin fibrous septum is interposed between the artery and vein, thus form-
ing a separate sheath for each.
(d) The Omo-hyoid Fascia, which encloses the lower belly of the mus-
cle of the same name, is a strong fascia which passes over the muscle
extending down to the first rib. It is from this layer of fascia that the
band binding down the intermediate tendon of the omo-hyoid muscle is
obtained.
(c) The Submaxillary Fascia consists of two triangular layers of the
deep fascia which enclose a space containing the submaxillary salivary
and lymphatic glands ; the fascia is attached below to the intermediate
tendon of the digastric nmscles ; the outer layer passes upward to be
attached to the body of the lower jaw ; the other layer passes inward
to be connected with the fascia covering the raylo-hyoid, the hyo-glossus,
and the stylo-glossus muscles ; surgically speaking, it is attached to the