This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Pus which tends to point into the pharynx may come from disease of the vertebrae, in which case it is posterior to the prevertebral fascia; or it may originate from the lymphatic nodes in the retropharyngeal space.
When coming from caries of the vertebrae. it may point either in the pharynx or, pushing its way outward, pass behind the great vessels and show itself behind the outer edge of the sternomastoid muscle. I have seen it point in both these places in the same case. When originating in the retropharyngeal space it lies in front of the prevertebral fascia and behind the buccopharyngeal fascia. It either points forward into the pharynx or, going down, follows the posterior surface of the oesophagus into the posterior mediastinum. It may also perforate the oesophagus and enter its lumen.
If above the prevertebral layer this bulges directly forward and tends to open through the skin. Its progress downward is obstructed by the attachment of the superficial layer to the top of the clavicle as it blends with the prevertebral layer. If pus is beneath the prevertebral layer it may then follow the brachial plexus and subclavian artery down beneath the clavicle and appear in the axilla. The attachments of the costocoracoid membrane tend to direct the pus laterally under the pectoralis minor muscle into the axilla rather than to allow it to come forward on the anterior portion of the chest.