Final answer:
Bolus formation occurs during the voluntary phase of swallowing, which is controlled by the individual and marks the beginning of the swallowing process.
Step-by-step explanation:
Bolus formation occurs during the voluntary phase of deglutition, which is the first stage of swallowing. As the name suggests, this phase is under voluntary control and involves the action of chewing being completed, signaling readiness for swallowing. The tongue plays a crucial role by moving upward and backward against the palate, pushing the processed food, now called a bolus, toward the back of the oral cavity into the oropharynx. This action prepares the bolus for the subsequent involuntary phases of swallowing: the pharyngeal phase and the esophageal phase. The latter two phases are controlled by the autonomic nervous system and include various protective mechanisms to ensure the bolus is directed toward the stomach and not the respiratory pathways.