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Oxygen plays a most important role on this terrestrial globe. Life, health, and food depend on it. This element penetrates, pervades, everything and everywhere, unites and disunites with all other elements, preserves and destroys. While its absence from a living being, whether plant or animal, is death.
When a liquid such as water is exposed to an atmosphere containing a gas such as oxygen, some of the oxygen will be dissolved in the water, that is to say will be absorbed from the atmosphere. The quantity which is so absorbed will depend on the quantity of oxygen which is in the atmosphere above; that is to say, on the pressure of the oxygen; the greater the pressure of the oxygen, the larger the amount which will be absorbed. If, on the other hand, water containing a good deal of oxygen dissolved in it be exposed to an atmosphere containing little or no oxygen, the oxygen will escape from the water into the atmosphere.
In plant life the permanent fabric consists of only three elements—carbon, hydrogen, oxygen. We know that plants alone convert inorganic or mineral substances into organic matter, and that plants as a necessary result assimilate their inorganic food, decompose carbonic acid, and restore its oxygen to the atmosphere.
Vegetation is constructed of cells or vesicles, and has a cellular tissue. A cell is a living organism. It is that which makes up the tissue of plants. For the whole life of the plant is that of the cells which compose it; in them and by them its products are elaborated, and all its vital processes are carried on. Cell multiplication by division, cell growth, cell modification, exist in plants. Fluids are transferred from cell to cell by a process called endosmose. Absorption takes place by the roots, and the substance absorbed is carried up into the leaves, even to the topmost bough of a tree, passing in its course many millions of apparently water-tight partitions. Plants exchange gases, taking in carbonic acid and giving off oxygen. They evolve heat, have organs of reproduction, and elaborate the material for the final evolution of the seed. This seed, whether of grain, of vegetables, or of fruits, is composed of carbon, hydrogen, and oxygen. And these constitute the starches and sugars which we find have been evolved by the vegetable or plant, and which form the food for animals. Plants, then, convert the elementary substances, the crude material, into food. In doing so, they pass through the processes known as the essentials of life; these are, birth, growth, development, decline, and death.
All organic compounds are transitory. They are constantly appearing and disappearing, composing and decomposing, organizing and disorganizing; and they are always dependent upon a certain degree of heat and moisture for their existence or non-existence.
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.
The glands which are most immediately connected with digestion are very numerous small organs, situated in the mucous membrane of the alimentary canal, and the larger glands, such as the salivary glands, pancreas, and liver, whose ducts open on its inner surface.
The mouth is included between the lips and the throat, bounded by the lips, cheeks, tongue, and hard and soft palate. It communicates behind with the pharynx, and through the pharynx with the æsophagus. It is lined throughout with mucous membrane.
The mouth contains 32 teeth, 16 in the upper jaw and 16 in the lower jaw. The inferior maxillary bone, or lower jaw, is the only movable bone about the head. The teeth have for their functions biting, grinding, chewing, or triturating any hard food substance that may be introduced into the mouth.
The tongue is a muscular organ covered with mucous membrane. By its muscular structure it takes part in the process of mastication and deglutition, and in the articulation of speech, while its mucous membrane, with common and tactile sensibility, is the seat of the sense of taste.
The tonsils are two prominent bodies which occupy the recesses formed, one on each side of the fauces, between the anterior and posterior palatine arches and the pillars of the fauces.
The saliva, which is poured into the mouth and there mixed with the food during mastication, is secreted by three pairs of glands named from their respective situation parotid, submaxillary, and sublingual.
The parotid is the largest of three salivary glands. It lies on the side of the face, in front of the ear, and extends deeply into the space behind the ramus of the lower jaw. Its weight varies from 5 to 8 drachms. It has a duct called the parotid or Stenson’s duct. It is about 2½ inches long, and about a line and a half in thickness. Its orifice is opposite the crown of the second molar tooth of the upper jaw.
The submaxillary gland weighs about 2 to 2½ drachms, and is situated on the inner surface of the inferior maxillary. The duct is named Wharton’s, and is about 2 inches in length. Its orifice is found under the tongue.
The sublingual gland weighs about a drachm. It is situated on the floor of the mouth. The ducts are called the ducti Rivintiani. They are from 8 to 20 in number. They may be seen when the tip of the tongue is lifted up.
Saliva. Mixed saliva, as it appears in the mouth, is a thick, glairy, generally frothy, turbid fluid.
The quantity of saliva secreted in 24 hours varies. The average amount is probably from two to three pints in 24 hours.
The composition of saliva is:
The solids are:
The specific gravity varies from 1.004 to 1.008.
The rate at which saliva is secreted is subject to considerable variation. When the tongue and muscles concerned in mastication are at rest, and the nerves of the mouth are subject to no unusual stimulus, the quantity secreted is not more than sufficient, with the mucus, to keep the mouth moist.
The purposes served by saliva are of several kinds:
1. Acting mechanically in conjunction with mucus, it keeps the mouth in a due condition of moisture, and facilitates the movements of the tongue in speaking, and the mastication of the food.
2. It serves also in dissolving sapid substances and rendering them capable of exciting the nerves of taste.
3. By mixing with the food during mastication, it makes it a soft pulpy mass, such as may easily be swallowed.
4. Saliva performs a chemical part in the digestion of food. It transforms starchy substances into dextrine and grape sugar.
Starch is a carbohydrate—carbon 18, hydrogen 30, oxygen 15.
Ptyaline is the salient feature of saliva. It is known as a ferment—acting upon starch and converting it into dextrine and grape sugar.
The action of saliva varies in intensity in different animals.
The food after having been acted upon and prepared is propelled, by the act of deglutition, through the æsophagus into the stomach, by way of the pharynx.
The pharynx is that part of the alimentary canal which unites the cavities of the mouth and nose to the æsophagus. It extends from the base of the skull to the lower border of the cricoid cartilage, and forms a sac open at the lower end, and imperfect in front, where it presents apertures leading into the nose, mouth, and larynx. The pharynx is about four and a half inches in length, and is considerably wider across than it is deep from before backwards.
The æsophagus or gullet, the passage leading from the pharynx into the stomach, commences at the cricoid cartilage opposite the lower border of the fifth cervical vertebra, descends in front of the spine, passes through the diaphragm opposite the ninth dorsal vertebra, and ends by an opening at the cardiac orifice of the stomach. It is from nine to ten inches in length.
The stomach is situated in the abdominal cavity. It lies in part against the anterior wall of the abdomen, and in part beneath the liver and diaphragm, and above the transverse colon. It is somewhat conical or pyriform in shape. The left part is the larger, and is named the cardiac, or splenic, the right is named the pyloric, extremity. The upper border is about three or four inches in length, is concave, and is named the lesser curvature, while the lower border is much longer, is convex, and forms the greater curvature. The dimensions vary greatly in different subjects, and also according to the state of distension of the organ. When moderately filled, its length is about ten to twelve inches, and its diameter at its widest part from four to five inches. It weighs when freed from other parts about four and a half ounces in the male and somewhat less in the female.
The structure of the stomach consists of four coats—a serous, a muscular, an areolar, and a mucous coat. The external or serous coat is derived from the peritoneum. There are three kinds of muscular fibers—longitudinal, circular, and oblique, and the internal mucous lining is a rather thicker, soft, smooth, pulpy membrane, lying
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