In hair, which layer is the middle layer that contains pigment in dark hair but air bubbles in gray hair?

The middle layer of hair that contains pigment in dark hair and air bubbles in gray hair is the cortex. The cortex is responsible for providing strength and flexibility to the hair while also influencing its color. In individuals with dark hair, the cortex is rich in melanin, the pigment that gives hair its color. As hair grays, the melanin is replaced by air bubbles in the cortex, resulting in a lighter appearance.

The cuticle is the outermost layer of the hair shaft and serves a protective role, helping to prevent damage to the inner structures of the hair. While important for overall hair health and appearance, it does not contain pigment or air bubbles.

The medulla is the innermost layer of the hair, which may not even be present in all types of hair. It can be hollow or filled with cells, but like the cuticle, it does not contribute to the color or the presence of air bubbles.

The bulb is the part of the hair follicle where new hair cells are produced. It is responsible for hair growth but does not make up the structure of the hair shaft itself. Thus, understanding the function and characteristics of each layer highlights why the cortex is the correct answer in this context.