When it's cold, the arrector muscle pulls the hair up. The duct to the sweat glands gets small to conserve heat.
而当很冷的时候，竖毛机会使头发竖起来，汗腺的输送管也会变小来聚集热量。
2
When our ancestors were covered in fur, muscles in their skin called "arrector pili" contracted when they were upset or cold, making their fur stand on end.
Along one side of the follicle, two buds are formed: an upper, which develops into the sebaceous gland, and a lower, which becomes the attachment for the arrector pili muscle.