Skin and senses

The border and the input channels.

Skin

Skin is the largest organ, with layers, nerves, glands and blood vessels of its own. It keeps water in, keeps infection out and regulates temperature through sweat and blood flow.

The outer layer is dead cells that shed and renew roughly every month.
Sweating cools by evaporation, which is why humidity makes heat harder to bear.

The senses

Each sense turns something physical into nerve signals: light at the retina, pressure waves in the ear, chemicals on the tongue and in the nose. Balance comes from fluid in the inner ear, not from the eyes.