The upper epidermis, or stratum corneum, functions to protect the lower layers of skin and the tissues beneath from both physical and chemical damage. It is composed of plates made from dead skin cells, toughened against physical damage with protein envelopes, protected against chemicals with layers of special lipids, all held together by special protein bridges between the cells. While the protections the upper epidermis provides against direct damage are important, of perhaps even greater importance is its ability to block the entry of pathogenic organisms.
There are five layers of the epidermis, with the stratum corneum being the outermost. The epidermis is thinnest on the eyelids and thickest on the palms of the hands and the soles of the feet. The stratum corneum is exposed to constant wear, and so must shed and replenish itself constantly.
The stratum corneum does not have blood flow to it directly and is exposed to the air, but avoids drying using special compounds which actually absorb moisture from the air. These compounds are water-soluble, so repeated exposure to water actually dissolves them and leaves the outer layer of the skin vulnerable to drying. The thickness of the stratum corneum varies by age, location and ultraviolet radiation exposure.