The entirety of the visible part of the hair that grows out of the scalp.

Cuticle- The visible part of the hair shaft. It is made up of flat, durable cells that overlap one another and act as scaled armor for the rest of the hair shaft. This protective layer is responsible for hair texture. When the cells of the cuticle are in their proper flattened position they can protect the vulnerable inner layers of the hair from weather and other damaging elements. 

Cortex- The middle layer of the hair made up of tightly coiled strands of keratin, a dense and flexible protein. The keratin itself is composed of polypeptide chains of individual amino acid units. These amino acid units form a chain thanks to peptide bonds. Those chains then form even larger chains through polypeptide bonds. 

• Cortex bonds- There are 3 kinds of polypeptide bonds. Two of those bonds, salt and hydrogen, are physical and are broken and reformed on a regular basis. These bonds can be changed by temperature and moisture, among other external conditions the hair faces, and allow for the structure of the hair to be temporarily altered. Physical bonds are the reason you can straighten curly hair with a flat iron or rollers. The third type of bond is chemical and known as a disulphide bond. This chemical bond is much stronger and, unlike the two physical bonds, it is permanent. Only a chemical process can break and reform the disulphide bond.
  

Medulla- The innermost part of the hair. It is made up of tightly clustered cells that form a shaft through the very center of the hair. In some hair, the medulla is fragmented or absent altogether. Biologists do not yet fully understand the function of the medulla in human hair.