Final answer:
The statement that in the circular layer of smooth muscle, the fibers run around the circumference of the organ is true. This arrangement is critical for peristalsis in the gastrointestinal tract.
Step-by-step explanation:
The statement in the student's question is true. In the gastrointestinal (GI) tract, the muscularis externa layer indeed contains a circular layer of smooth muscle. This layer of muscle fibers runs around the circumference of the organ, effectively encircling it. When the circular muscle fibers contract, they constrict the lumen of the organ, thereby propelling the contents within the organ forward. This is a critical part of the process known as peristalsis, which moves food through the digestive tract.
For example, in the lower GI tract, such as the small intestine which is a long, narrow organ, these circular muscle fibers work alongside the longitudinal muscle fibers, which run the length of the organ, to ensure coordinated contractions that facilitate the movement of digested materials.