The horse's largest and most visible organ is his skin. Its job is to protect the internal organs from the outside environment; to help maintain constant temperature, water, and mineral balance; and to provide the sensations of pain and touch. The skin is composed of the epidermis--a dried cellular layer--and the dermis--the live portion of the skin. The dermis holds the nerve endings, hair follicles, blood vessels, and sweat glands. Most skin problems originate in the dermal layer.