Which structure is a small secretory sac or cavity that aids in hair growth?

The correct answer is the follicle. The hair follicle is a small, tubular structure embedded in the skin, which serves as a protective sheath for growing hair. It plays a crucial role in the hair growth cycle by providing the necessary environment for hair cells to proliferate and ultimately form hair strands. The follicle also contains sebaceous glands that secrete oils to help condition the hair and skin, enhancing the overall health of the hair as it grows.

In contrast, glands produce various secretions but may not be directly involved in hair growth specifically like a follicle. Pores refer to the openings on the skin's surface that allow hair and sweat to escape but are not themselves responsible for the growth of hair. The matrix is a part of the hair follicle where cell division occurs, which is essential for producing new hair; however, it is not a structure in itself but rather a component of the follicle.

Thus, the follicle is fundamental to understanding hair physiology and is key to the growth process, which makes it the right answer in this context.