Final answer:
The circular muscles of the small intestine mix the contents by contracting and constricting the lumen, whereas the longitudinal muscles aid in peristalsis by shortening segments of the intestine, propelling the contents forward. The correct answer to the student's question is Option A.
Step-by-step explanation:
The small intestine is composed of two primary layers of smooth muscle: the inner circular layer and the outer longitudinal layer. These muscle layers work in concert to move material through the small intestine efficiently. Circular layers primarily control the diameter of the intestine. When they contract, they constrict the intestinal lumen and mix the contents within. This action helps in mechanical digestion and ensures that the food makes good contact with digestive enzymes. Segmentation, a rhythmic contraction of the circular muscles, further aids in this mixing process. On the other hand, the longitudinal layers run along the length of the small intestine. When these muscles contract, they shorten segments of the intestine, aiding in the process known as peristalsis. This sequential, wave-like movement is responsible for propelling the digested material further along the gastrointestinal tract.
Option A in the student's question states: 'Circular layers contract to mix and propel content, while longitudinal layers shorten to aid in peristalsis.' This description correctly identifies the roles of the muscle layers. Thus, the answer to the question 'How do the circular and longitudinal layers contribute to moving material through the small intestine?' is A.