This book provides a full account of the concept of fiber and fiber theory in eighteenth-century British medicine. It explores the pivotal role fiber played as a defining, underlying concept in anatomy, physiology, pathology, therapeutics, psychology, and the life sciences. With the gradual demise of ancient humoralism, the solid fibers appeared on the medical scene both as the basic building unit of the body and as a dynamic agent of life. As such, fiber stands at the heart of eighteenth-century medicine, both iatromechanism and iatro-vitalism. Touching on the cultural aspects of fiber, the Baroque, and the culture of sensibility, this book also challenges the widely held assumption that the eighteenth century was the age of the nerve and instead offers an alternative model of fiber.