Intracellular Receptors: New Instruments for a Symphony of Signals In the late eighteenth century, it was proposed on theoretical grounds that each of the body's organs, beginning with the brain, must be "a factory and laboratory of a specific humor which it returns to the blood", and that these circulating signals "are indispensable for the life of the whole" (Bordeu 1775). During the nineteenth cen- tury, some remarkable physiological experiments...