THE NECK. 153 



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 mem- 

 brane. 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 pretrachcal 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 en- 

 closes 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 sterno- 

 hyoid, 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 down- 

 ward from the omohyoid muscle to the first rib. This is derived from the sheath 

 of the vessels beneath and the layer or* 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 pre- 

 tracheal 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 por- 

 tion 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 ver- 

 tebral column behind is the retropharyngeal space. The fascia forming the pos- 

 terior 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). 



Abscesses of the Neck. Abscesses of the neck usually arise in connection 

 with the lymphatic nodes. They may also start from infected wounds, carious 



