Which layer is NOT one of the three main layers of skin?

The muscle layer is not considered one of the three main layers of skin. The skin is primarily composed of three distinct layers: the epidermis, which is the outermost layer that provides a barrier and is responsible for the skin's pigmentation; the dermis, which lies beneath the epidermis and contains connective tissue, blood vessels, and various structures such as hair follicles and glands; and the subcutaneous layer, which is composed of fat and connective tissue that helps insulate the body and cushion underlying organs.

Understanding the structure of the skin is crucial, as each layer has specific functions that contribute to overall skin health, sensation, and thermoregulation. The muscle layer, while important for movement and support underneath the skin, does not contribute to the skin's primary functions or organization. Therefore, it is outside the classification of the skin layers itself.