The oily substance that lubricates the mammalian hair to keep it flexible and water repellent is secreted by the?

Step 1: Analyze the question The question asks which structure secretes the oily substance that keeps mammalian hair flexible and water repellent.

Step 2: Explain the concept In mammalian skin, each hair grows from a hair follicle. Attached to the follicle is a small gland called the sebaceous gland. It secretes sebum, an oily substance. Sebum:

• lubricates the hair, keeping it soft and flexible
• makes the hair and skin water repellent
• helps prevent the skin from drying and cracking
• has mild antibacterial properties

Step 3: Evaluate each option

• A. Sweat glands: These secrete sweat (water, salts, urea) for cooling and excretion. They do not produce oil.
• B. Sebaceous glands: These secrete sebum, which lubricates hair. This fits the question.
• C. Fatty cells: These store fat in the subcutaneous layer for insulation and energy. They do not secrete onto hair.
• D. Granular layer: This is a layer of the epidermis (stratum granulosum) involved in keratinization. It is not a gland.

Step 4: Answer The correct answer is **B.

Exam tip: Remember "Sebaceous = Sebum = oil." Sweat glands give sweat, and sebaceous glands give oil. If a question mentions oil or lubrication of hair, think sebaceous glands.