ii4 THE HEAD AND NECK 



of the fascia in the median plane, the pus travels downwards 

 and laterally behind the carotid sheath and reaches the posterior 

 triangle. In this situation the fascia is much weaker, and the 

 abscess points behind the sterno-mastoid. It is best approached 

 by an incision along the posterior border of the muscle (Chiene), 

 which is then retracted forwards together with the carotid sheath. 

 A search is made for the transverse processes of the cervical 

 vertebrae,, which provide the deep landmarks to the site of the 

 abscess (p. 520). Rarely, the pus travels downwards behind 

 the prevertebral fascia and enters the mediastinal space. This 

 variety of abscess must be distinguished from an abscess originat- 

 ing in the lymph glands which occupy the interval between 

 the prevertebral fascia and the bucco-pharyngeal fascia on the 

 outer surface of the constrictor muscles (Fig. 37). They drain 

 the naso-pharynx, and their efferents pass laterally to open 

 into the upper group of the deep cervical lymph glands. An 

 abscess arising in connection with the retro -pharyngeal lymph 

 glands causes a swelling on the posterior wall of the pharynx, 

 usually to one or other side of the median plane. Should it 

 rupture into the pharynx during sleep, it may lead to suffocation. 

 Abscesses in front of the prevertebral fascia should be opened 

 from the mouth with the patient's head inverted (Rose's posi- 

 tion), but this route is avoided in dealing with pus behind the 

 prevertebral fascia which is tuberculous in origin on account 

 of the danger of mixed infection. 



(c) The Pretracheal Fascia is a much thinner sheet than 

 the prevertebral. It lies in front of the trachea and the lower 

 part of the larynx, and forms a fascial sheath for the thyreoid 

 gland (p. 167). It is separated from the prevertebral fascia 

 by the oesophagus and trachea, and from the investing layer by 

 the depressor muscles of the hyoid bone (Fig. 36). The pre- 

 tracheal fascia covers the crico-thyreoid muscle and is firmly 

 adherent to the cricoid and thyreoid cartilages. It does not 

 extend upwards beyond the thyreoid cartilage but, inferiorly, 

 it descends into the superior mediastinum and blends with the 

 outer coat of the arch of the aorta. Laterally, the pretracheal 

 fascia blends with the anterior wall of the carotid sheath. 



(d) The Carotid Sheath. The common and internal carotid 

 arteries, the internal jugular vein, and the vagus nerve, are 

 surrounded by a tubular investment of the deep cervical fascia. 

 The sheath is best seen in relationship to the upper half of the 

 common carotid artery : above and below that area it gradually 



