So far, then, the law that waste is attendant on action, applies to both dead and living bodies; but beyond this point a remarkable difference between them presents itself. In the physical or inanimate world, what is once lost or worn away is lost for ever. There is no power inherent in the piston of the steam-engine by which it can repair its own loss of particles; and consequently in the course of time it must either be laid aside as useless, or be remodelled by the hand of the workman. But living bodies, whether vegetable or animal, possess the distinguishing characteristic of being able to repair their own waste and add to their own substance. The possession of such a power is in fact essential to their very existence. If the sunflower, which in fine weather exhales thirty ounces of fluid between sunrise and sunset, contained no provision within its own structure for replacing this enormous waste, it would necessarily shrivel and die within a few hours, as it actually does when plucked up by the roots; and, in like manner, if man, whose system throws out every day five or six pounds of substance by the ordinary channels of excretion, possessed no means of repairing the loss, his organization would speedily decay and perish. This very result is frequently witnessed in cases of shipwreck and other disasters, where, owing to the impossibility of obtaining food, death ensues from the body wasting away till it becomes incapable of carrying on the operations of life. In some instances this waste has even proceeded so far that three-fourths of the whole weight of the body have been lost before life became extinct.
It is impossible to reflect on these facts, and others of a similar kind, without having the conviction forced upon our minds, that in every department of nature expenditure of material is inseparable from action, and that, in living bodies, waste goes on so rapidly, and by so many different channels, that life could not be maintained for any length of time without an express provision being made for compensating its occurrence.
In surveying the respective modes of existence of vegetables and of animals, with the view of ascertaining by what means this compensation is effected, the first striking difference between them which we perceive, is the fixity of position of the one, and the free locomotive power of the other. The vegetable grows, flourishes, and dies, fixed to the same spot of earth from which it sprang; and, however much external circumstances may change around it, it must remain and submit to their influence. If it be deprived of moisture and solar heat and light, it cannot go in search of them, but must remain to droop and to perish. If the earth to which its roots are attached be removed, and a richer soil be substituted than that which its nature requires, it still has no option: it must grow up in rank and unhealthy luxuriance, in obedience to an impulse which it cannot resist. At all hours and at all seasons it is at home, and in direct communication with the soil from which its nourishment is extracted. And being thus without ceasing in contact with its food, it requires no storehouse in which to lay up provision, but receives immediately from the earth, and at every moment, all that is necessary for its sustenance.
But it is otherwise with animals. These not only enjoy the privilege of locomotion, but are compelled to use it, and often to go to a distance, in search of food and shelter. Consequently, if their vessels of nutrition were like those of vegetables in direct communication with external substances, they would be torn asunder at every movement, and the animals themselves exposed either to die from starvation, or to forego the exercise of the higher functions for which their nature is adapted. But the necessity for a constant change of place being imposed on them, a different arrangement became indispensable for their nutrition: and the method by which the Creator has remedied the inconvenience is not less admirable than simple. To enable the animal to move about and at the same time to maintain a connexion with its food, He has provided it with a receptacle or stomach, where it is able to store up a supply of materials from which sustenance may be gradually elaborated during a period of time proportioned to its necessities and mode of life. It thus carries along with it nourishment adequate to its wants; and the small nutritive vessels imbibe their food from the internal surface of the stomach and bowels, where the nutriment is stored up, just as the roots or nutritive vessels of vegetables do from the soil in which they grow. The possession of a stomach or receptacle for food is accordingly a characteristic of the animal system as contrasted with that of vegetables; it is found even in the lowest orders of zoophytes, which in other respects are so nearly allied to plants.
The sole objects of nutrition being to repair waste and to admit of growth, Nature has so arranged that within certain limits it is always most vigorous when growth or waste proceeds with the greatest rapidity. Even in vegetables this relation is distinctly observable. In spring and summer, when vegetative life is most active, and when leaves, flowers, and fruit, are to be formed, and growth carried on, nourishment is largely drawn from the soil, and the elaboration and circulation of the sap are proportionally vigorous; whereas in winter, when the leaves and flowers have passed away, and vegetable life is in repose, little nourishment is needed, and the circulation of the sap is proportionally slow. In accordance with these facts, every one will recollect how freely a shrub or a tree bleeds, as it is called, when its bark is cut early in the season, and how dry it becomes on the approach of winter. It is the activity of the circulation in summer which renders its temporary suspension by transplanting so generally fatal at that season; whereas, owing to the comparative sluggishness with which it is carried on in winter, its partial interruption is then attended with much less risk.
In vegetables, the quantity of nourishment taken in entirely depends on, and is regulated by, the circumstances in which they are placed. When they are exposed, as in spring and summer, to the stimulus of heat and light, all their functions are excited, waste and growth are accelerated, and a more abundant supply of nourishment becomes indispensable to their health and existence; and hence, in a dry soil incapable of affording a copious supply of sap, they speedily wither and die. Exposed to cold, on the other hand, and shaded from the light, their vitality is impaired, and the demand for nourishment greatly diminished. This is uniformly the case in winter; and many circumstances shew that the change is really owing to the causes mentioned above, and not to any thing inherent in the constitution of the vegetable itself. In tropical climates, for example, where heat, light, and moisture abound, vegetable life is ever active, and the foliage ever thick and abundant; and even in our own northern region, we are able by artificial heat so far to anticipate the natural order of the seasons, as to obtain the ripened fruit of the vine in the very beginning of spring. The whole system of forcing vegetables and fruit, so generally resorted to for the early supply of our markets, is, in truth, founded on the principle we are now discussing; and by the regulated application of heat, light, and moisture, we are able to hasten or to retard, to a very considerable extent, the ordinary stages of vegetable life. But to ensure success in our operations, we must be careful to proportion the supply of nourishment to the state of the plant at the time. If, by the application of heat, we have stimulated it to premature growth and foliage, we must at the same time provide for it an adequate supply of food, otherwise its activity will exhaust itself, and induce premature decay. Hence the regular watering which greenhouse plants require. But if we have retarded its progress and lowered its vitality by excluding heat and light, the same copious nourishment will not only be unnecessary, but will probably do harm by inducing repletion and disease.
In vegetables, the absorption of food is thus regulated chiefly by the circumstances of heat, moisture, and light, under which the plant is placed, and by the consequent necessity which exists at the time for a larger or smaller supply of nourishment to carry on the various processes of vegetable life. According to this arrangement, nutrition is always most active when the greatest expenditure of material is taking place. When growth is going on rapidly, and the leaves are unfolding themselves, sap is sucked up from the earth in immense quantity; but when these processes are completed as summer advances, and almost no fresh materials are required, except for the consolidation of the new growth and the supply of the loss by exhalation, a much smaller amount of nourishment suffices, and the sap no longer circulates in the same profusion. In autumn, again—when the fruit arrives at maturity, the leaves begin to drop off, and the activity of vegetable life suffers abatement—nutrition is reduced to its lowest ebb; and in this state it continues till the return of spring stimulates every organ to new action, and once more excites a demand for an increased supply.
Nor is the same great principle, of supply requiring to be proportioned to demand, less strikingly apparent in animals. Wherever growth is proceeding rapidly, or the animal is undergoing much exertion and expenditure of material, an increased quantity of food is invariably required; and, on the other hand, where no new substance is forming, and where, from bodily inactivity, little loss is sustained, a comparatively small supply will suffice. But as animals are subjected to much more rapid and violent transitions from activity to inactivity than vegetables are—and thus require to pass more immediately from one kind and quantity of nourishment to another, in order to adapt their nutrition to the ever-varying demands made upon the system—they evidently stand in need of some provision to enforce attention when nourishment is necessary, and to enable them always to proportion the supply to the real wants of the body. Not being, like vegetables, in constant connection with their aliment, they might suffer from neglect if they did not possess some contrivance to warn them in time when to seek and in what quantity to consume it. But in endowing animals with the sense of Appetite, or the sensations of Hunger and Thirst generally included under it, the Creator has guarded effectually against the inconvenience, and given to them a guide in every way sufficient for the purpose.
The very possession of a stomach or receptacle, into which food sufficient for a shorter or longer period can be introduced at one time, and which we have already remarked as characterizing all animals from the lowest to the highest, almost necessarily implies the co-existence of some watchful monitor, such as appetite, to enforce attention to the wants of the system, with an earnestness which it shall not be easy to resist. If this were not the case in man, for example—if he had no motive more imperative than reason to oblige him to take food—he would be constantly liable, from indolence and thoughtlessness, or the pressure of other occupations, to incur the penalty of starvation, without being previously aware of his danger. But the Creator, with that beneficence which distinguishes all His works, has not only provided an effectual safeguard in the sensations of hunger and thirst, but moreover, attached to their regulated indulgence a degree of pleasure which never fails to insure attention to their demands, and which, in highly civilized communities, is apt to lead to excessive gratification. Such being the important charge committed to the appetites of hunger and thirst, it will be proper to submit to the reader, before entering upon the consideration of the more complicated process of digestion, a few remarks on their nature and uses.