The Whirlpool of Life
Life is manifested as a process of release of energy, involving continuous death or destruction, since it is only by the breakdown of cells and tissues in which energy from food has been stored that the phenomenon of life can appear or continue. Thus, the paradox that we live by dying. There is, however, more than one kind of death. The kind referred to in this connection is the local death of cells and tissues, which is usually accompanied by regeneration and recovery, while what may be called general death is that in which the correlation of functions depending upon the brain, heart, and lungs is interrupted so that it cannot again be resumed. Even in this latter case the component tissues may live on for some time after correlation is no longer possible, as shown, for example, by the excitability of the muscles of a frog’s leg under electrical stimulation after the frog has been irrevocably killed by the complete removal of its brain and heart.
Huxley likened an organism to the whirlpool below Niagara Falls. At no two moments of time is it made up of the same mass of water, yet its identity remains, and if photographed on succeeding days from the same point, the pictures would appear alike. In a similar manner all living things may be conceived as whirlpools of living matter and energy, which nevertheless maintain a continuous individuality throughout the duration of life.
The digestive system is the mechanism that makes good the constant losses which are inevitable in the mortal expense of living. It is with the intake aspects of the organic whirlpool that this chapter is concerned.
Rate of Living
The rate at which the metabolic waters of life flow through the organic whirlpool varies greatly with the age of the individual. During the first part of life while growth is taking place, the intake, like a spring freshet, is greatly in excess of the outgo, but later there follows a prolonged period of balance during which losses of energy are simply made good, then the stream of life flows more slowly and becomes less and less in volume, and eventually ceases entirely as the head waters gradually dry up.
It is not at all easy to realize the abounding life of animals during the onset of growth. A human baby normally doubles its weight in 200 days. A new-born mouse quadruples its weight in twenty-four hours, and a silkworm increases its size 500 times during the first day’s intake of mulberry leaves. Dr. Keen says: “Were the same rule to hold, a baby weighing seven pounds at birth would weigh thirty-five hundred pounds the very next day, and when a month old would weigh one hundred and five thousand pounds, or over fifty ‘short tons,’ which, however, could hardly be called ‘short weight’.”
Hunger and Thirst
Food, water, and oxygen are the necessary materials of subsistence taken into a going organism. Food carries energy to be stored up in the tissues for later use. Water is the universal solvent and fluid necessary for manipulating and shifting about materials within the organism, while oxygen effects the breakdown of tissues and the liberation of imprisoned energy.
The essential concern of every animal is the securing of these three primary prerequisites for continued activity. This fact is so obvious that it escapes our attention. Anyone who has tried to follow the incessant activities of a wild bird, for example, during the daylight hours will realize in part the imperious demands of hunger and thirst. It may be observed that most animals rarely succeed in overtaking their appetites.
Even in the highly specialized routine of human society, the daily program of business, pleasure, education, religious activities, politics, philanthropy, and all the rest, is secondarily tucked in between meals around which the day’s activities are arranged, and any serious deviation from the periodic exercise of the sacred rites of intake are likely to border on the disastrous.
The Intake Mechanism of Animals and Plants Contrasted
Most plants are restricted to a diet. The food they use is monotonous in the extreme, yet there is no complaint. It is made up in the synthetic laboratories of the green cells of leaves or stem out of uniformly distributed raw materials, such as carbon dioxide and oxygen from the air, and water impregnated with dissolved salts of the soil.
Liquid intake from soil-water is soaked up by osmosis through the delicate walls of root hairs (Fig. 209), which would quickly collapse if exposed to dry air. This does not ordinarily happen, however, as root hairs remain constantly protected in damp soil, since the plant is not forced to travel about seeking water and what it may devour.
Animals, on the other hand, do not have the power of synthesizing foods out of air, water, and inorganic salts of the soil, so ordinarily they cannot remain anchored in one spot, manufacturing their foods out of raw materials at hand, but are obliged to forage for food already made.
Like plants, animals depend upon osmotic intake through thin cell membranes, cellular middlemen between indispensable food and the animal body, which cannot remain without harm on the outside of bodies of adventurous locomotor organisms. The intake cells of animals, as well as of plants, must be protected from mechanical injury and from drying up, while their possessors are seeking food. This explains the evolution in locomotor animals of the digestive tube, an enclosed passage-way arranged for one-way traffic and paved with thin-walled absorbing cells that correspond to the osmotic root hairs of plants. In one sense the digestive tube is simply an infolding of the integument, making a protected subway where food admitted at the entrance is exposed to intake cells, which proceed to do their osmotic duty in security without drying up while being transported to fresh fields of food supply. Thus, in a way, an animal may be regarded as a plant turned outside-in.
The Mission of the Food Tube
In the process of living, while energy is being released by the oxidation of the tissues, it becomes imperative that replacements be made from outside sources, or in other words, that food be obtained. It is not enough, however, simply to get food, since energy-containing substances cannot be utilized until they are so liquefied and transformed that they may be taken into the blood, to be forwarded to the needy tissues where the actual feeding, or incorporation of food materials, occurs. To accomplish these transformations is the mission of the digestive tube with its accompanying contributory devices.
The everyday miracle of a cat taking a captured mouse and changing it over into more cat, or of human flesh and blood, endowed with personal idiosyncrasies, made out of the hodge-podge of materials that appear on daily bills of fare, is so ordinary and familiar that these marvels have ceased to excite wonder.
Kinds of Feeders
Animals may be classified according to the prevailing character of their intake into herbivores, carnivores, omnivores, parasites, symbionts, and saprozoans.
Herbivores are direct plant feeders. Carnivores feed upon animals, but in reality are plant feeders at least once removed, since the ultimate food of all animals is plants. Omnivores feed directly upon both animals and plants. Parasites feed at the expense of living organisms which “entertain” them as “hosts” without necessarily fatal results. Symbionts, such as green hydras and certain green worms, live vicariously at the expense of micro-scopic green plants embedded in their bodies, which have the ability common to green plants of synthesizing food on the spot; while saprozoa, like certain flagellates and infusorians, are scavengers, specializing upon dead organisms in the last stages of their reduction into inorganic materials. Most vertebrates belong in the first three groups.
Animals with a wide range of foods have a better chance in the struggle for existence than those that have become specialized for a single source of nutrition, such as the pronuba moth, which feeds only on the pollen of the yucca flower; termites with a diet of woody cellulose; boll-weevils that spurn everything except cotton “squares” before they bloom; and coprophagous beetles that revel only in feces.
There are a few curious carnivorous plants, like the Venus fly-trap, bladderwort, pitcher plants, and sundews, that have deviated far from the self-reliance of most green plants, which manufacture their own food. These isolated plants by various cunning devices have augmented the usual source of food of green plants by capturing small animals which they devour. A greater variety of plant forms, including bacteria and fungi that have no chlorophyll, live saprophytically on the dead organic remains of other plants.
Animals in satisfying their demands of hunger from all sorts of sources are quite unaware that the chemist finds only three fundamental kinds of food in the world, with certain necessary additional inorganic trimmings in the form of water and salts. These three basic food substances, which occur in an infinite number of guises in the bill of fare of animals and plants, are proteins, that furnish building materials for growth, maintenance, and reproduction, and fats, and carbohydrates, which supply the immediate energy indispensable to the business of living.