What is the most inner part of the hair, containing pigment granules and air spaces?

The most inner part of the hair is known as the medulla. It is composed of loosely packed cells and contains spaces filled with air, which can contribute to the hair's overall structure and appearance. The medulla can also contain pigment granules, which can affect the color of the hair. This layer is surrounded by the cortex, which is the thicker, middle layer that provides strength and elasticity to the hair. The outermost layer is the cuticle, which consists of overlapping scale-like cells that protect the inner layers. The root refers to the part of the hair embedded in the follicle and does not describe a specific layer of the hair structure in the same way the medulla, cortex, and cuticle do. Therefore, identifying the medulla as the correct answer highlights its unique composition and its role within the hair's structure.