Which of the following structures is found in the dermis?

The presence of arrector pili muscles in the dermis is significant because these tiny muscles play a key role in the functioning of hair follicles. They are smooth muscle fibers that are attached to hair follicles and are responsible for causing the hair to stand upright when contracted, a reaction often referred to as "goosebumps." This mechanism is usually a response to cold temperatures or emotional stimuli, enhancing the body's ability to retain heat and providing a protective response in certain situations.

In contrast, the other structures mentioned are not located in the dermis. The stratum corneum is the outermost layer of the epidermis, and the hair shaft extends from the follicle in the dermis but itself is not included within the dermis. Subcutaneous fat, which provides insulation and cushioning, is located beneath the dermis in the subcutaneous layer. Thus, the identification of arrector pili muscles as a dermal structure highlights their functional importance in the integumentary system.