Which layer of the hair is responsible for strength and elasticity?

The cortex is the layer of the hair responsible for its strength and elasticity. This middle layer comprises long, coiled protein chains called keratin, which provide structural support and resilience to the hair. The cortex is thick and makes up the bulk of the hair's mass, allowing it to withstand various forms of stress, such as pulling or styling. Additionally, the properties of the cortex contribute to the hair's ability to hold shape when styled, as its elasticity allows it to return to its original form after being manipulated.

In contrast, the medulla, which is the innermost layer, often has no significant role in the hair's strength or elasticity, and it is not always present in all hair types. The cuticle, being the outermost protective layer, primarily serves to shield the inner layers from damage and external elements; while it plays a role in the overall health of the hair, it does not directly contribute to strength and elasticity. The follicle, which is the part of the skin that produces hair, is not a layer of the hair itself but rather the structure that surrounds the hair. Thus, the cortex is key in defining the physical characteristics of hair that relate to its resilience and flexibility.