*Divisions of the Subject.*—The body is studied from three standpoints: structure, use of parts, and care or management. This causes the main subject to be considered under three heads, known as anatomy, physiology, and hygiene. _Anatomy_ treats of the construction of the body—the parts which compose it, what they are like, and where located. Its main divisions are known as gross anatomy and histology. _Gross anatomy_ treats of the larger structures of the body, while _histology_ treats of the minute structures of which these are composed—parts too small to be seen with the naked eye and which have to be studied with the aid of the microscope. _Physiology_ treats of the function, or use, of the different parts of the body—the work which the parts do and how they do it—and of their relations to one another and to the body as a whole. _Hygiene_ treats of the proper care or management of the body. In a somewhat narrower sense it treats of the "laws of health." Hygiene is said to be _personal_, when applied by the individual to his own body; _domestic_, when applied to a small group of people, as the family; and public, or _general_, when applied to the community as a whole or to the race. *The General Aim of Hygiene.*—There are many so-called laws of health, and for these laws it is essential in the management of the body to find a common basis. This basic law, suggested by the nature of the body and conditions that affect its well-being, may be termed the _Law of Harmony: The mode of living must harmonize with the plan of the body_. To live properly one must supply the conditions which his body, on account of its nature and plan, requires. On the other hand, he must avoid those things and conditions which are injurious, _i.e._, out of harmony with the body plan. To secure these results, it is necessary to determine what is and what is not in harmony with the plan of the body, and to find the means of applying this knowledge to the everyday problems of living. Such is the general aim of hygiene. Stated in other words: Hygiene has for its general aim the bringing about of an essential harmony between the body and the things and conditions that affect it.(1)