The nerves operate in harmony with each other and with the organs to maintain health. The nervous system comprises the brain, the spinal cord, the nerves, and the ganglia. Aside from presiding over the special senses, this system controls and directs the processes of defense and repair. In doing so, the force and frequency of the heart beat, the calibre of the vessels and the chemistry and composition of the blood are all altered. These phenomena are the ones which concern us in our present subject, acting as they do upon the blood and the organs which contain it. They will be considered more fully in the following: +THE CIRCULATORY SYSTEM+ In health and disease the force and frequency of the heart beat and the calibre of the arteries undergo momentary alterations to meet changes surrounding us. Dilatation and constriction of the arteries, and arterioles through the activity of the vasomotor nerves, permit of increasing and diminishing amounts of blood reaching the various organs and regions of the body. In this way the peripheral circulation, through the activity of the heat centre in the brain, acting upon the superficial capillaries, maintains the normal temperature of the body during the changes of season. In winter, heat conservation is effected by vaso-constriction of the superficial capillaries, while in summer, heat radiation is accomplished by vaso-dilatation together with the evaporation of moisture so abundantly supplied by the active sweat glands. Other phenomena demonstrating the vasomotor function are blushing, going pale, and the redness and swelling following injury or infection. Of the latter we will treat under the heading “Inflammation.” Certain changes also occur in the blood in order that it may perform its functions. These changes are found both in the chemistry of its fluid content and in the number and kind of its solid elements: the corpuscles. The blood is a tissue; its corpuscles are the functioning cells and its fluid content the basement substance. In the fluid content of the blood or _plasma_, as it is called, certain chemical changes occur in its fibrin-forming capacity. Clot formation, an effort on the part of the economy to arrest hemorrhage, is thus facilitated when there is active bleeding, also during labor and certain diseases. The number and kind of white blood cells also undergo changes, as we shall see, under circumstances in which the defences of the organism are called into operation, for it is the function of the white blood corpuscles to combat bacterial invasion. It is the preponderance of these white cells which imparts the peculiar milky color to pus. Nourishment and oxygen are carried to the tissues by the red blood corpuscles. Under circumstances which we shall see later, these are also altered both in number and in size, and also in their arrangement in the vessels. (See Chapter IV, “_Inflammation_.”) +THE LYMPHATIC SYSTEM+