Final answer:
The part of the hair shaft that is between the scalp and the previously untreated hair is known as the new growth. This new growth is part of the hair shaft that includes the cuticle, cortex, and potentially the medulla, with the cortex being the relevant answer in this context.
Step-by-step explanation:
The part of the hair shaft between the scalp and the hair that has previously not received treatment is known as the new growth. A hair shaft has three zones: the cuticle, the cortex, and the medulla. The cuticle is the outermost layer, the cortex is the middle layer, and the medulla is the innermost layer. As new cells are deposited at the hair bulb, the hair shaft is pushed through the follicle toward the surface, leading to the growth of the hair. The hair matrix contains a layer of basal cells which produce the keratinocytes that form the cortex and the cuticle in a cross-section of the hair bulb. The part of the hair that is within the follicle is known as the hair root, which is living, whereas the hair shaft, the part that is visible above the skin surface, is dead and has no biochemical activity.