physiology (fĭzēŏlˈəjē) [key], study of the normal functioning of animals and plants during life and of the activities by which life is maintained and transmitted. It is based fundamentally on the activities of protoplasm. The study of function is usually undertaken along with a study of structure (see anatomy), the two being intimately related. Since the discovery of the cell structure of tissues, the science of physiology has undergone rapid development. It includes the study of vital activities in cells, tissues, and organs—of processes such as contractility of muscle tissue, coordination through the nervous system, feeding, digestion, excretion, respiration, circulation, reproduction, and secretion. Virtually every specialized field in the biological sciences (e.g., embryology, pathology, botany, zoology) involves a consideration of the physiological aspects of its subject. The study of human physiology was stimulated by the development of medicine, and it embraces many chemical and physical principles. Plant physiology includes also the study of photosynthesis and transpiration. A separate and specialized branch, plant physiology arose from attempts to apply the findings of animal physiology to plants and in its turn contributed to the development of general physiology, especially in the study of cells.