3. It is probable that a certain mutual interchange occurs between the gases contained in the alimentary canal, and those present in the blood of the gastric and intestinal blood-vessels.
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.
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.