नाखूनों में अनुप्रस्थ गड्ढे (transverse depressions) जो नाखून के विकास में अस्थायी गड़बड़ी का संकेत देते हैं, उन्हें क्या कहा जाता है?

Which primary layer of the skin acts as the outermost protective barrier against bacterial entry, chemical exposure, and mechanical injury?

Which structure within the skin is responsible for the formation of 'goose bumps' by contracting?