Final answer:
The hollow tube in which the hair grows is the hair follicle, with the visible part above the skin known as the hair shaft.
Step-by-step explanation:
The hollow tube in which the hair grows is called the hair follicle. This is an epidermal penetration of the dermis where hair originates and is the only living part of a hair. The external part of the hair that is not anchored to the follicle, and much of which is visible above the skin's surface, is called the hair shaft. The root ends deep in the dermis at the hair bulb, which is surrounded by the hair papilla. The hair bulb includes a layer of mitotically active basal cells known as the hair matrix, which differentiates into cells that form the inner root sheath. The hair shaft itself has three zones: the cuticle, the cortex, which is the middle layer, and the innermost zone called the medulla.