The movement of the intestines is peristaltic or vermicular, and is effected by the alternate contractions and dilatations of successive portions of the intestinal coats. The contractions, which may commence at any point of the intestine, extend in a wavelike manner along the tube. This is due to the involuntary longitudinal and circular muscular fibers contracting successively from above downwards and from behind forwards, etc. The movements take place slowly, and in health are commonly unperceived by the mind, but they are perceptible when they are accelerated under the influence of any irritation. CHAPTER XXI. THE ELEMENTARY SUBSTANCES. We have thus far discovered that this terrestrial globe is composed of sixty-four elementary substances; that fifty belong to a class called metals, and the remaining fourteen are non-metallic and are called metalloids. We know with absolute certainty the elementary chemical composition of all the substances known to man; everything within the reach of man has been analyzed, whether of inorganic or of organic origin. We also know the principal elements that enter into the composition of organic substances, animal or vegetable. But a thing that is not generally known is the wonderful role certain elements play in nature, especially in the life of plants and animals. If we examine the extraordinary display of combination or composition of some of the elements--especially those that enter into the composition of organic substances--we shall find how few of these elements are essential for the production of life, and its maintenance; and we shall be surprised to find what force or power, and phenomena, they are capable of producing. We shall be surprised to see how nicely and delicately these elementary compositions are adjusted--with what precision the elements enter into combination with each other--and with what astonishing result. The union of the elements that enter into the composition of living matter, must always be very accurately balanced, to insure a healthy or normal condition of either plant or animal. A very slight deviation or change may prove either injurious or destructive to the living organism. In order to obviate writing the names of the elements, we propose to use symbols. The elementary substances that enter into the composition of living matter being few, it will not be difficult to recognize the meaning of the symbols. The four vital elements mentioned in a previous chapter are Carbon. Hydrogen. Nitrogen. Oxygen. Symbols: C H N O The atmosphere we breathe, for example, is what is called a chemical mixture, and is composed of O22N77, with traces of ammonia, etc. The water we drink is a chemical composition, and is constituted by O H2. The number placed against each element indicates the quantity of each one requisite, or found, in the composition, or chemical combination, of the substance indicated.