From Medieval Magic to Modern Medicine page 2

It's in the skin

All frogs and toads have two types of glands in their skin. The mucous glands, which occur all over the body, continuously secrete a clear, viscous - or slimy - coating that keeps the animal moist and enables it to absorb both water and oxygen through its permeable skin. The granular, or poison, glands are distributed across the body, often with the heaviest concentration around the head or neck, where a frog or toad is most likely to be grabbed by a predator.

The granular glands are among frogs' and toads' most important defense mechanisms, along with escape and camouflage. When activated by stress or injury, the glands secrete a milky substance that varies by species from slightly noxious to extraordinarily toxic. The ooze also has antimicrobial properties that combat bacteria, fungi and parasites that might find moist skin an inviting environment.

The granular-gland secretions make frog skin a veritable pharmacopoeia, with some of the best pharmaceutical promise found among the most potently toxic frogs. Batrachotoxin, unique to Phyllobates frogs - among the most toxic creatures in the animal kingdom - is a particularly powerful steroid-like alkaloid that behaves in a unique, lightning-fast way. Batrachotoxin introduced into an animal's bloodstream via a poisoned blowdart immediately halts all nerve and muscle function, causing cardiac arrest.

Research into the way this toxin works clarified to scientists how electrical messages are transmitted across the nervous system. Discovery of the sodium channels may lead to insights into severe neurological disorders such as Lou Gehrig's disease.