What is the name of the middle layer of the hair?

The middle layer of the hair is known as the cortex. This layer is crucial because it contains the majority of the hair's mass and contributes significantly to the hair's strength, texture, and color. The cortex houses long chains of protein and melanin, which is responsible for the hair's pigment. Being the central part of the hair structure, the cortex is also vital in determining the overall characteristics of the hair, including its elasticity and resilience.

In contrast, the cuticle is the outermost layer that protects the inner layers and the medulla is the innermost layer, which is not present in all hair types. The outer layer as a term does not specifically refer to any established classification in hair anatomy and lacks the specificity found in the other terms. Therefore, the cortex is the correct answer as it specifically identifies the middle layer with a key role in the hair's properties.