Physiology is the term used to describe the study of the function of living organisms, or the science that examines the processes and mechanisms by which living organisms (plants and animals; for our discussion in this chapter, we will concentrate on animals) function under various conditions. The term “physiology” was first used by the Greeks as early as 600 B.C.E. to describe a philosophical inquiry into the nature of things. It was not until the sixteenth century that the term was used in reference to vital activities of humans. During the nineteenth century, its usage was expanded to include the study of all living organisms using chemical, physical, and anatomical experimental methods (for more about the anatomy, or physical attributes, of animals, see the chapter “Anatomy: Animals Inside”).