毛干也由三层结构组成，依次为毛小皮，皮质和髓质。
The terminal hair shaft is made up of three layers : cuticle , cortex and medulla .
毛干和内根鞘当到达体表时融合在一起：外根鞘仍然保持在原来的位置。
The hair shaft and inner root sheath move together as the hair grows toward the surface : the outer root sheath remains fixed in position .
毛干