The act of swallowing depends upon adequate and timely PES opening . Opening of the PES depends upon muscular relaxation of the CPM , elevation of the larynx, and pharyngeal contraction. Jacob et al. (1989) described five phases of PES opening (Table 5.1) . In the first phase there is an inhibition of tonic PES contraction (Fig. 5.4). This depends on muscular relaxation of the tonically active CPM. Phase I of PES opening is followed by elevation of the hyoid and larynx. Hyolaryngeal excursion (Phase II) provides opening of the PES (Fig. 5.5) through active distraction of the larynx and cricoid cartilages away from the cervical spine. The anterior–superior elevation also helps bring the larynx forward underneath the base of tongue and helps direct the bolus posteriorly toward the hypopharynx . The cartilages do not actually distend off of the spine to open the PES in Phase II, but the region is primed to accept the bolus in preparation for definitive opening in Phase III. The priming provided by elevation appears to be more important than muscular inhibition in Phase I. Elevation is necessary to allow a bolus to enter the PES. Muscular relaxation of the CPM without elevation will not result in PES opening. This has significant clinical implications, as swallowing in individuals with good hyolaryngeal elevation but poor CPM relaxation is possible and often encountered (CPM bar) . Safe and effective swallowing in individuals who can relax their CPM but cannot elevate their larynx off of the cervical spine has not been observed. The advancing bolus will reach a closed PES and follow the path of least resistance into the airway. The third phase of PES opening involves distension of the PES through bolus size and weight (Fig. 5.6). This phase relies upon pharyngeal and lingual peristalsis to propel the bolus past the spacious hypopharynx , through the closed but primed PES, behind the elevating hyolaryngeal complex, and into the cervical esophagus. The elasticity of the elevating PES allows it to be opened by the increasing pressure exerted by the passing bolus. If there is inadequate lingual and pharyngeal contraction, the bolus will not exert enough pressure and the PES will not open. The bolus will again follow the path of least resistance and threaten the airway. The PES cannot adequately be evaluated if lingual and pharyngeal contractility are ineffectual. This is a major limitation of the VFSS , as PES pathology (e.g., CPM bar, stricture, web) cannot be assessed in persons with a significantly weakened tongue or pharynx. In Phase IV of PES opening, the elasticity of the PES causes passive collapse and closure as the bolus passes (Fig. 5.7). The final phase of PES opening, Phase V, involves PES closure through active contraction of the CPM (Fig. 5.8). A disease process that affects any one of the five phases of PES opening can cause the symptom of dysphagia and objective evidence of swallowing dysfunction .