Final answer:
The medulla is the hair layer that is sometimes absent, particularly in fine hair.
Step-by-step explanation:
The layer of the hair that is sometimes absent is the medulla. The medulla forms the central core of the hair shaft but is not always present, particularly in fine hair. Hair is composed of three layers: the cuticle (outermost layer), cortex (middle layer), and the medulla (innermost layer). The cortex in hair determines the hair's texture, such as curly or straight, while the cuticle is the protective outer layer. The hair's growth and structure originate from the hair matrix, within the hair bulb at the base of the follicle. This interaction between different parts of hair is crucial for its growth and the keratinization process, which results in the external, visible part of the hair that consists of dead keratinocytes.