Again, our bodies are continually making heat and giving it out to surrounding objects, the production and the loss of heat being so adjusted that the whole body is warm, that is, of a temperature higher than that of surrounding objects.  Our bodies, also, move themselves, either one part on another, or the whole body from place to place.  The motive power is not from the outside world, but the energy of their movements exists in the bodies themselves, influenced by changes in their surroundings.  Finally, our bodies are continually getting rid of so-called waste matters, which may be considered products of the oxidation of the material used as food, or of the substances which make up the organism.

4.  The Main Problems of Physiology briefly Stated.  We shall learn in a subsequent chapter that the living body is continually losing energy, but by means of food is continually restoring its substance and replenishing its stock of energy.  A great deal of energy thus stored up is utilized as mechanical work, the result of physical movements.  We shall learn later on that much of the energy which at last leaves the body as heat, exists for a time within the organism in other forms than heat, though eventually transformed into heat.  Even a slight change in the surroundings of the living body may rapidly, profoundly, and in special ways affect not only the amount, but the kind of energy set free.  Thus the mere touch of a hair may lead to such a discharge of energy, that a body previously at rest may be suddenly thrown into violent convulsions.  This is especially true in the case of tetanus, or lockjaw.

The main problem we have to solve in the succeeding pages is to ascertain how it is that our bodies can renew their substance and replenish the energy which they are continually losing, and can, according to the nature of their surroundings, vary not only the amount, but the kind of energy which they set free.

5.  Technical Terms Defined.  All living organisms are studied usually from two points of view:  first, as to their form and structure; second, as to the processes which go on within them.  The science which treats of all living organisms is called biology.  It has naturally two divisions,—­morphology, which treats of the form and structure of living beings, and physiology, which investigates their functions, or the special work done in their vital processes.

The word anatomy, however, is usually employed instead of morphology.  It is derived from two Greek words, and means the science of dissection.  Human anatomy then deals with the form and structure of the human body, and describes how the different parts and organs are arranged, as revealed by observation, by dissection, and by the microscope.

Histology is that part of anatomy which treats of the minute structure of any part of the body, as shown by the microscope.