Milady Chemical Texture Services Practice Exam 2025 – The All-in-One Guide to Master Your Certification!

The cortex is the correct answer because it is the middle layer of the hair shaft and plays a critical role in the hair's overall strength and elasticity. Polypeptide chains, which are the primary structure of proteins, are found in the cortex, and these chains are organized into long fibers.

In the cortex, these polypeptide chains are interconnected by various types of side bonds, including hydrogen bonds, salt bonds, and disulfide bonds. These side bonds are essential for maintaining the structure and integrity of the hair, influencing characteristics like texture, curl, and resilience.

The medulla, being the innermost layer of the hair, typically does not contribute to hair's structure in terms of polypeptide chains or side bonds. The cuticle, the outer layer, serves as a protective barrier rather than a structure where polypeptide chains are cross-linked. Lastly, the follicle is not a part of the hair shaft; it is the structure from which the hair grows and doesn't contain the polypeptide chains or side bonds relevant to the question.