Popular Education. Ira Mayhew
person will dispute the universality of its operation. But when we observe shrubs and trees waving in the wind, and animals undergoing violent exertion, year after year, and continuing to increase in size, we may be inclined, on a superficial view, to regard living bodies as constituting an exception to this rule. On more careful examination, however, it will appear that waste goes on in living bodies not only without intermission, but with a rapidity immeasurably beyond that which occurs in inanimate objects.
In the vegetable world, for instance, every leaf of a tree is incessantly pouring out some of its fluids, and every flower forming its own fruit and seed, speedily to be separated from, and lost to its parent stem; thus causing in a few months an extent of waste many hundred times greater than what occurs in the same lapse of time after the tree is cut down, and all its living operations are at a close.
The same thing holds true in the animal kingdom: so long as life continues, a copious exhalation from the skin, the lungs, the bowels, and the kidneys goes on without a moment's intermission, and not a movement can be performed which does not in some degree increase the circulation, and add to the general waste. In this way, during violent exertion, several ounces of the fluids of the body are sometimes thrown out by perspiration in a very few minutes; whereas, after life is extinguished, all the excretions cease, and waste is limited to that which results from ordinary chemical decomposition.[6]
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 forever; 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 essential to their existence. But there is a wide difference between them in other respects. In surveying the respective modes of existence of vegetables and of animals, we perceive 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. However much external circumstances change around it, it must remain and submit to their influence. At all hours and at all seasons, it is at home, and in direct communication with the soil from which its nourishment is extracted. But it is otherwise with animals: these not only enjoy the privilege of locomotion, but are compelled to use it, and often to go a distance in search of food and shelter. The necessity for a constant change of place being imposed on them, a different arrangement became indispensable for their nutrition. The method which the Creator has provided is not less admirable than simple. To enable animals to move about, and at the same time to maintain a connection with their food, they are provided with a stomach. In this receptacle they can store up a supply of materials from which sustenance may be gradually elaborated during a period of time proportioned to their necessities and mode of life. Animals thus carry with them nourishment adequate to their wants; and the small nutritive vessels imbibe their food from the internal surface of the stomach and bowels, where it 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 distinguishing characteristic of the animal system.
The sole objects of nutrition being to repair waste and to admit of growth, the Creator 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 provision is distinctly observable. It is also strikingly apparent in animals. Whenever growth is proceeding rapidly, or the animal is undergoing much exertion and expenditure of material, an increased quantity of food is invariably required. 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. In endowing animals with the sense of appetite, including the sensation of hunger and thirst, the Creator has effectually provided against any inconvenience which might otherwise exist, and given to them a guide in relation to both the quality and quantity of food needful for them, and the times of partaking of it, with that beneficence which distinguishes all his works. He has not only provided an effectual safeguard in the sensations of hunger and thirst, but he has 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. Their end is manifestly to proclaim that nourishment is required for the support of the system. When the body is very actively exercised, and a good deal of waste is effected by perspiration and exhalation from the lungs, the appetite becomes keener, and more urgent for immediate gratification; and if it is indulged, we eat with a relish unknown on other occasions, and afterward experience a sensation of internal comfort pervading the frame, as if every individual part of the body were imbued with a feeling of contentment and satisfaction; the very opposite of the restless discomfort and depression which come upon us, and extend over the whole system, when appetite is disappointed. There is, in short, an obvious and active sympathy between the condition and bearing of the stomach, and those of every part of the animal frame; in virtue of which, hunger is felt very keenly when the general system stands in urgent need of repair, and very moderately when no waste has been suffered.
We have seen that waste is every where attendant upon action, and that the object of nutrition is to repair waste and admit of growth. We come now to consider the Process of Digestion.
All articles used for food necessarily undergo several changes before they are fitted to constitute a part of the body. In the process of digestion, four different changes should be noticed. More might be specified.
1. Mastication.—The first step in the preparation of food for imparting nourishment to the system consists in proper mastication, or chewing. Food should be thoroughly masticated before it is taken into the stomach. This is necessary in order to break it up and reduce it to a sufficient degree of fineness for the efficient action of the gastric juice. Besides, the action of chewing and the presence of nutrient food constitute a healthful stimulus to the salivary glands, situated in the mouth. By this means, also, the food not only becomes well masticated, but has blended with it a proper amount of saliva, upon both of which conditions the healthy action of the stomach depends. We have here another illustration of the beneficence of the Creator, who has kindly so arranged that the very act of mastication gratifies taste, the mouth being the seat of this sensation. But if we disregard these benevolent laws, and introduce unmasticated food into the stomach, the gastric juice can act only upon its surface, and changes of a purely chemical nature frequently commence in food thus swallowed before digestion can take place. Hence frequently arise—and especially in children and persons of delicate constitution—pains, nausea, and acidity, consequent on the continued presence of undigested aliment in the stomach.
2. Chymification.—As soon as food has been thoroughly masticated and impregnated with saliva, it is ready for transmission to the stomach. This interesting part of the process of digestion, called deglutition or swallowing, is most easily and pleasantly performed, when the alimentary morsel has been well masticated and properly softened, not by drink, which should never be taken at this time, but by saliva. When the food reaches the stomach, it is converted into a soft, pulpy mass, called chyme; and the process by which this change is effected is called chymification. This is the second principal step in digestion, and is effected immediately by the action of the gastric juice. This powerful solvent is secreted by the gastric glands, which are excited to action by the presence of food in the stomach. In health, the gastric secretion always bears a direct relation to the quantity of aliment required by the system. If too much food is taken into the stomach, indigestion is sure to follow, for the sufficient reason that the gastric juice is unable to dissolve it. This is true even when food has been well masticated; but it becomes strikingly apparent when a full meal has been hastily swallowed, both mastication and insalivation having been imperfectly performed.
The time usually occupied in the process of chymification, when food has been properly masticated, varies from three to four hours. Digestion is sometimes effected in less time, as in the case of rice, and pigs' feet soused; but it more commonly requires a longer period, as in the case of salt pork and beef, and many other articles of food, both animal and vegetable.
By the alternate contraction and relaxation of