Fig 178. Deep cervical fascia. The pharynx and larynx have been cut away, exposing the prevertebral and pretracheal layers.

About 3 cm. (1 1/4 in.) above the sternum the deep fascia splits into two layers, one to be attached to the anterior and the other to the posterior edge of the sternum in front of the sternohyoid and sternothyroid muscles. Between these two layers is the space of Burns; it contains the lower ends of the anterior jugular veins with the branch that joins them, some fatty tissue and lymphatic nodes, and the sternal origin of the sternomastoid muscle. Sometimes a vein comes up from the surface of the chest below to open into the anterior jugular vein.

The prevertebral layer passes from side to side directly on the bodies of the vertebras. It covers the muscles attached to the spine, as the scalene, longus colli, rectus capitis anticus, and also the nerves, as those of the brachial plexus, coming from the spine. On reaching the carotid artery and jugular vein it helps to form their sheath. Its upper edge is attached to the base of the skull at the jugular foramen and carotid canal and thence across the basilar process to the opposite side. Inferiorly it passes down on the surface of the bodies of the vertebras into the posterior mediastinum.

From the sheath of the vessels outward, beyond the posterior edge of the sterno-mastoid muscle, the prevertebral fascia covers the scalene muscles, the brachial plexus of nerves, and the subclavian artery. On reaching the clavicle the fascia is attached to its upper surface, blending with the superficial layer; it is then continued down over the subclavian muscle, forming its sheath, and ends as the costocoracoid membrane. The part over the subclavian artery and vein is continued over them and the brachial plexus and follows them into the axilla. This fascia forms the floor of the posterior cervical triangle; the roof is formed by the superficial layer of the deep fascia. It is between these layers that the suprascapular artery and veins run. The descending branches of the cervical plexus, the spinal accessory nerve, omohyoid muscle, and some fat and lymph-nodes are also found there.

The pretracheal layer passes from side to side in front of the trachea. Laterally it too blends with the sheath of the vessels and is continued posteriorly behind the pharynx and oesophagus as the buccopharyngeal fascia. In front it blends in the median line with the superficial layer and is attached to the hyoid bone and cricoid cartilage. It splits to enclose and form a capsule for the thyroid gland, and below encloses in its meshes the inferior thyroid veins, and thence passes to the arch of the aorta to be continuous with the pericardium. Laterally it passes under the sternohyoid, omohyoid, and sternothyroid muscles to blend with the sheath of the vessels and the layer on the posterior surface of the sternomastoid muscle. This is its lateral limit. Underneath the sternomastoid muscle a loop of fascia proceeds downward from the omohyoid muscle to the first rib. This is derived from the sheath of the vessels beneath and the layer on the under surface of the sternomastoid superficially.

The sheath of the vessels envelops the carotid artery, jugular vein, and pneumo-gastric nerve. Thin layers of fascia pass between these structures, separating one from the other. The sheath is formed by the union of the outer edge of the pretracheal fascia and the prevertebral fascia, with the fascia lining the under surface of the sternomastoid muscle. This sheath follows the vessels down into the chest and out into the axilla.

The capsule of the parotid gland is formed by the splitting of the superficial layer of the deep cervical fascia as it passes from the mastoid process to the angle of the jaw. Its superficial portion is attached to the zygomatic process. Its deep portion passes from the styloid process to the angle of the jaw and is known as the stylomandibular ligament.

The capsule of the submaxillary gland is formed by a splitting of the superficial layer at the hyoid bone. It forms the covering of the gland and from the hyoid bone sends a process upward which lies on the digastric and mylohyoid muscles and follows the latter up to be attached along the mylohyoid ridge of the mandible. It proceeds with the submaxillary gland around the posterior edge of the mylohyoid muscle to cover its upper surface. The stylomandibular ligament alluded to above separates the parotid from the submaxillary gland.

The capsule of the thyroid gland is not very thick and the gland is readily separated from it, as is also the case with the submaxillary gland. It is continued downward in front of the trachea as the pretracheal layer and laterally it blends with the sheath of the vessels. It follows the vessels downward into the chest and is continuous with the pericardium. The veins of the gland, which are at times very large, run beneath the capsule and bleed freely if wounded. The Buccopharyngeal Fascia. - Between the pharynx in front and the vertebral column behind is the retropharyngeal space. The fascia forming the posterior wall of this space is the prevertebral fascia already described. Forming its anterior wall is a thin layer of connective tissue called the buccopharyngeal fascia. It invests the superior constrictor of the pharynx and is continued forward on the buccinator muscle. It is continued downward behind the pharynx and oesophagus into the posterior mediastinum: laterally it blends with the sheath of the vessels and is continuous with the pretracheal fascia around the larynx, trachea, and thyroid gland (Fig. 179).