The Brain the Master Organ of the Body; the Functions of the Brain; Habit and Automatism; the Physiology of the Brain and Nervous System; the Hygiene of Work; the Toxins of Fatigue; Overwork; Signs of Overwork; Nature’s Restoratives; Avocation; the Physiologic Necessity for Laughter; Vacations and Health; Sleep; Insomnia. The Eyes; Eye-strain; Description of the Visual Apparatus; Optical Defects and Their Correction; the Mechanism of Eye-strain; Local Symptoms of Eye-strain; Artificial Lighting; Hygienic Precautions in Reading and Sewing; Injuries to the Eyes; Symptoms and Treatment of Conjunctivitis; Trachoma; Styes. Functional Nervous Disorders, Headache; Neurasthenia. =The Brain the Master Organ of the Body.=—The brain is not only the most important organ in the body, but its essential organ, for the sake of which all the other organs and tissues exist, and it is the master of the whole. It not only receives help from every other organ, but it also largely controls the working of each. By its mental action alone it can hurry the heart’s beat or slow its pace; it can make the skin shrivel or flush; it can quicken or stop digestion; it can stop or change the character of all secretions; it can arrest or improve the general nutrition. Every organ and every vital process is represented in the structure of the brain, by special centers and groups of cells that have a direct relation with such organs and processes, and through which they are controlled. =The Functions of the Brain.=—The brain may be said to have four chief functions; the first is that of motion; it presides over and stimulates all the voluntary muscular movements of the body, regulating their force, and coördinating in their working the different groups of muscles needed to perform them. Mind and muscular movement have the closest possible connection with each other. The second function of the brain is that of feeling and sensation; the third is that of nutrition; through this its own nourishment and that of the rest of the body is regulated. While mind is the fourth and highest form of nerve force, it is not created in the brain, but is absolutely conditioned by that organ. Different groups of brain-cells have different work assigned them; some have motion, some have sensation, some have nutrition, and some have mind. For example, special tracts of brain govern inhibition. While every group does its own work, it is related to and combined with others, influencing them, and being influenced by them.