Substitution of lost anatomical structure or physiological function by various devices was one of the earliest technologies in medicine. The wooden leg, the spectacle lens, and the ear trumpet were antecedents of today's powered limb prostheses, pacemakers, valves, vessels, and hearts. Transplantation of tissues or organs from humans and animals represents a further step along the same path. The replacement of deficient organic chemicals such as insulin and coagulation factors might be considered as part of the same continuum. Such substitutes may be wholly external. These include the extension of extremities foreshortened by congenital or acquired abnormality and various communication aids that allow the transmission or reception of information. Replacing the chemical activity of the kidney by dialysis is another external device.