Final answer:
The fibers of the cricopharyngeus muscle are primarily circular, functioning as part of the upper esophageal sphincter to regulate the passage of food from the pharynx into the esophagus during swallowing.
Step-by-step explanation:
The fibers of the cricopharyngeus muscle are predominantly circular. This muscle functions as part of the upper esophageal sphincter and plays a critical role in the process of swallowing, also known as deglutition. The cricopharyngeus muscle relaxes to allow food to pass from the pharynx into the esophagus. Following this, the muscle contracts to prevent reflux of food back into the pharynx. These actions are part of a larger coordination of muscles during the pharyngeal phase of swallowing, assisted by the pharyngeal constrictor muscles.
Circular muscles throughout the body, also known as sphincters, have the general function of regulating the flow of contents through an orifice or a hollow organ. They typically do so by contracting to close the opening and relaxing to open it. Similar to other circular muscles such as the orbicularis oris around the mouth, the cricopharyngeus muscle's fibers are arranged concentrically.