This source provides an overview of hair as an accessory organ of the integumentary system. It explains that hair, similar to nails and skin's outer layer, is primarily composed of dead, hardened keratinized cells, but differs in the bonding of these cells. The text describes the anatomy of hair, dividing it into the bulb, root, and shaft, and notes that only the bulb and root contain living cells responsible for growth. It discusses the distribution and density of hair follicles, highlighting the reasons for differences in hairiness between humans and primates, and among different ethnicities and sexes, attributing these differences partly to hormonal sensitivity and the activation of dormant follicles. Finally, the source categorizes three types of human hair – terminal, lanugo, and vellus – and briefly mentions the structure and composition within a cross-section of hair, explaining how these contribute to hair texture and color, before touching upon hair growth cycles, hair loss (alopecia), unwanted hair growth (hirsutism), and the vestigial nature of human hair compared to its functional importance in animals.