The universal constituents of plant life; of organic existence, which are indispensable to vegetation, are carbon, hydrogen, oxygen, and nitrogen. Every vegetable substance is made up of at least eighty-eight to ninety-nine per cent of these elements. The proper vegetable structure, that is, the tissue itself, consists only of three of these elements, carbon, hydrogen, oxygen; while the fourth, nitrogen, is an essential constituent of the protoplasm, which plays so important a part in the formation of the cell, etc.
Plants prepare or elaborate out of these chemical elements food-substances composed of those elements—starches and sugars—upon which animals subsist. Animals feeding upon these vegetable substances assimilate, elaborate, them into meat substances, flesh, or proteids. These again are composed of carbon, hydrogen, nitrogen, and oxygen.
Nitrogen plays the important role in proteids, being the distinguishing feature, as contrasted with substances of vegetable origin, the carbohydrates.
Thus man is provided with two kinds of food: derived from plants, carbohydrates; derived from animals, proteids, or albumens, besides water and mineral salts.
These foods undergo certain preparations previous to being introduced into the system. In the system the food undergoes farther elaboration, to make it fit to enter into the circulation of the blood, in order to supply suitable material for the master tissues.
We will now examine briefly the organs and their secretions that convert food-substances into blood, and, by the blood, into tissue.
The solvents and diluents of food in the human animal economy are the saliva of the mouth, the gastric juice of the stomach, the pancreative juice of the pancreas, the bile of the liver, and the juices of the intestines—the succus entericus.
The digestive apparatus consists mainly of the alimentary canal together with various glands of which it receives the secretions.
The alimentary canal commences at the mouth and terminates at the anus. The average length is about thirty feet, about five or six times the length of the body.
The part situated in the head and thorax consists of the organs of mastication, insalivation, and deglutition, and comprises the mouth with the teeth, the salivary glands, and the æsophagus or gullet. The parts contained in the abdomen and pelvis consist of the stomach and the small and large intestines.