As you may recall from chapter 1, Robert Hooke had observed only the empty cell walls of cork when he first named the cell in 1663. Later, he studied thin slices of fresh wood and saw cells "filled with juices"—a fluid later named protoplasm.2 Two centuries later, Theodor Schwann studied a wide range of animal tissues and concluded that all animals are made of cells. Schwann and other biologists originally believed that cells came from nonliving body fluid that somehow congealed and acquired a membrane and nucleus. This idea of spontaneous generation—that living things arise from nonliving matter—was rooted in the scientific thought of the times. For centuries, it was considered simple common sense that decaying meat turns into maggots, stored grain into rodents, and mud into frogs. Schwann and his contemporaries merely extended this idea to cells. The idea of spontaneous generation wasn't discredited until some classic experiments by French microbiologist Louis Pasteur in 1859. By the end of the nineteenth century, it was established beyond all reasonable doubt that cells arise only from other cells. The development of biochemistry from the late nineteenth to the twentieth century made it further apparent that all physiological processes of the body are based on cellular activity and that the cells of all species exhibit remarkable biochemical unity. Thus emerged the generalizations that constitute the modern cell theory:
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.