PHYSIOLOGY (Lat. physiologia, from Gk. Ovato")oyla, natural philosophy, from OccrioX6-yos, physiologos, discoursing on nature, natural phi losopher, from 00-LT, physis. nature + X6-yos, logos, word, account, ratio, from Xiyav,legein, to say). Under the term physiology, when used in its widest sense, we include all that is known of the properties of living matter. Following the great line of demarcation of living things into animals and plants, we have an animal physiology and a plant physiology. The two subjects, while closely related. are sufficiently different to be considered as separate sciences and to require specialists of different training for their study. In animal physiology we have to deal especially with the properties of animal protoplasm. and although the fundamental properties of this ma terial are believed to he much the same in all animals, yet for each animal or group of animals there are minor distinctions which are due in part to fundamental differences in structure, and in part to the variety in the physiological mecha nisms developed in different animals. In what follows an attempt will be made to present briefly an outline of the important facts and principles of human physiology, but it should be remem bered that the body of fact. included under this designation have been obtained in many. if not in most cases, by direct study and experiment upon lower forms of life, not only upon the mammals most closely related in structure to man, but also upon the simplest organisms which for one reason or another have offered especial opportunities for study.
It is convenient to consider the physiology of the human body under two general heads. namely. the properties of the tissues and the properties o• functions of the organs and mechanisms. The human body. like that of other animals. starts as a single cell, the fertilized ovum. During the progress of development this cell multiplies to form an infinite number of cell: which gradually become differentiated in structure and segregated into groups that are known as tissues. Each tissue is characterized by its appearance. a. seen by the eye or with the aid of a microscope. and also by known differences in chemical composition and reaction. We distinguish in the human hods the muscular tissue, the nervous tissue. the epi thelial tissue, the gland or secreting tissue, the blood, the connective or supporting tissue, the lymphatic tissue, the tissue forming the red blood corpuscles. and so on. Each tissue is dif ferent not only in structure. but in its properties. Along with the differentiation in form there is a specialization in function. Without attempting to describe their microscopic characteristics, a word or two may be said of the more important physiological properties of some of these tissues.