This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
1. Nutrition may be considered as the completion of the functions of digestion. It is that process by which the waste of the organs is repaired, and by which their development and growth are maintained. Respiration, digestion, circulation, absorption, and secretion, are but separate links in the chain of nutrition ; which would be instantly destroyed by the absence of any one of them.
2. In the construction of a machine, or an instrument, designed to last for many years, the mechanist seeks for the most durable materials. In making a watch, for instance, he forms the wheels of brass, the spring and barrel chain of steel, and for the pivot, which is subject to incessant friction, he employs the hardest of all materials-the diamond. The necessity for this arises from the fact, that such instruments do not contain, within themselves, the power of repairing their own losses.
3. But far different is the case with the animal machine. In order to qualify it for exercising the functions of life, it must be so constructed as to render it capable of continual alterations, displacements and adjustments; and these subject to continual variation, according to the stage of growth, and also to the different circumstances in which it may be placed. Instead, therefore, of a few elementary bodies, or their simpler combinations, nature has employed such compounds as admit of greater change, and a more variable proportion of ingredients, and greater diversity in the mode of combination. It is nutrition that moulds these plastic materials, and forms these ever changing compounds ; and so preserves the animal machine, amid all the varying changes of condition to which it is subject.
4. No one can doubt that the system is continually undergoing changes. This is proved by the losses to which it is subjected; by the necessity of frequent supplies of aliment ; by the rapid wasting of flesh on the withdrawal of food ; and by the emaciation caused by sickness and old age. It is also shewn by an experiment, which has often been made, viz. of giving madder to animals mixed with their food; which in a short time tinges their bones of a red colour. If the madder be withdrawn, the red colour in a few days disappears from the bones; evidently from the effects of absorption.
5. Every part of the body is subject to this constant change of matter. While one set of vessels, the lymphatics, are taking to pieces and carrying away the various parts of which the machine is composed, another set, viz., the capillaries, are constantly at work, repairing the loss, depositing bone, muscle, cartilage, nerve, tendon, fat, membrane, ligament, hair, nails, etc. where each is wanted, and this with such regularity and order, as to preserve the shape, size, and appearance of every organ, so that, though after an interval of a few years there may not remain in the body a single particle of which, at the former period, it was made up; still, the individual preserves the same form and features ; his personal identity is never lost.
6. Those animals, which are the most complicated in their structure, and are distinguished by the greatest variety of vital manifestations, are subject to the most rapid changes of matter. Such animals require more frequent, and more abundant supplies of food, and in proportion as they are exposed to a greater number of external impressions, so will be the rapidity in this change of matter. The frog, for instance, has been dug from the earth many feet below its surface ; and even taken from cavities in solid limestone, where he had been shut up probably for centuries, and still exhibited signs of life when exposed to the open air. As he was so situated as to lose nothing, by secretion or evaporation, of course he required nothing to supply any loss, but how the vital principle was preserved for such a length of time, is a mystery not easy to be explained.
7. The blood contains all the materials of nutrition. The process by which the food is changed into blood has been already explained. As it goes the round of circulation, the nutrient capillary vessels select and secrete those parts which are similar to the nature of the structure, and the other portions pass on ; so that every tissue takes up and converts to its own use the very principles which it requires for its growth; or in other words, as the vital current approaches each organ, the particles appropriate to it, feel its attractive force ; obey it; quit the stream ; mingle with the substance of its tissue, and are changed into its own true and proper nature.
8. Before the body has attained its full growth, the function of nutrition is very active ; a large amount of food is taken, being not only sufficient to supply the place of what is lost by the action of the absorbents, but also to contribute to the growth of the body. In middle age, nutrition and absorption are more equal ; but in old age the absorbents are more active than the nutrient vessels ; the size consequently diminishes ; the parts grow weaker; the bones more brittle ; the body bends forward; and every function exhibits marks of decay and dissolution.
9. A few years ago, a man by the name of Calvin Edson, of Vermont, commonly called the living skeleton, exhibited himself through the country for money. From having been a large man, he had wasted away by degrees, so that instead of his usual weight, he weighed but sixty pounds. He had been gradually losing flesh for eighteen years ; and he attri buted it to having taken cold from sleeping on the ground. This emaciation was owing to the absorbent vessels being more active than those of nutrition; whatever may have been the cause of the loss of balance.
10. On the other hand, when the nutrient vessels are the most active, the person grows fleshy and corpulent, as in the case of Daniel Lambert, who weighed seven hundred and thirty nine pounds at the age of forty ; or in that of a London butcher, who weighed eight hundred pounds. There are several cases on record, where men weighed eight hundred pounds.
11. The degree of nutrition, depends much on the quanti ty and quality of the food. A person who confines himself chiefly to animal diet, and drinks freely of ale and other malt liquors, will usually grow fat; but this does not indicate strength but weakness. It shows that there is not only a weakness of the absorbents, which are not able to take up and remove the fat; but also a muscular debility, and a want of force in the circulation. Motion is impeded ; the heart is loaded and oppressed; the breathing is laborious; the blood accumulates in the brain, and the person is every moment exposed to apoplexy. In all such cases, a sparing diet of vegetables, with proper exercise, will prove an effectual remedy. By sweating, horse riding, and a low diet, jockeys have not unfrequently reduced themselves 15 or 20 pounds, in a week or ten days.
12. Large accumulations of fat, it is said, sometimes take place, as the sudden effect of the influence of the atmosphere. Thus, in the short space of twenty four hours, it is stated by writers on natural history, that a mist will occasionally fatten thrushes, robins, etc, to such a degree, than they can hardly get out of the way of the sportsman's gun. This however is not fat, but the appearance is owing to a fulness of the vessels, from a suspension of evaporation.
13. The hump of the camel appears to form a sort of reserve, by which, the Arabs say, he is nourished, during his long journeys. In a period of plenty, the rapid secretion of fat converts it into a pyramid, equalling a fourth of the animal's entire bulk; but a journey through the desert gradually lowers it, so that it becomes scarcely visible. The camel then gives out, and can travel no further till the store is replenished by rest and food.*
* Burkhardt's travels.
14. Tumours, wens, and other morbid growths are the consequence of an error in nutrition. The nutritious vessels deposite fat where it is not wanted, and occasionally bony matter, where fibrin should be left or something else. In this way only can we account for the bony concretions and scales which are sometimes met with about the heart and blood vessels ; for the chalky deposits about the joints, in cases of gout and rheumatism, and even for horny projections, which have in a few cases been known to sprout out from the head. A few years ago, there were exhibited in London, several individuals called the porcupine family, who were all covered with dark colored horny excrescences; which they shed annually in the autumn or winter. These curious organic peculiarities, resembled the quills of the porcupine, and were two or three inches in length. But these are only exceptions to the usual regularity of nature's operations, and ought, instead of lessening, to increase our admiration at the admirable symmetry and uniformity that prevail, through every department of organized being.
15. The activity of muscular nutrition, depends much on exercise. The arm of a blacksmith, or a stone cutter, for instance, is generally large and brawny, because their muscles are almost in constant use. The same is true of the muscles of the leg, in rope dancers and tumblers, also in great walkers. Let any one examine the muscles of the Ravel Family, so celebrated for strength and agility in all gymnastic exercises, and he will find them not only unusually developed, but also hard and firm. If a person meet with an accident so that he is unable to walk, although his appetite remains, his muscles dwindle away for want of exercise. If then, a person who leads a sedentary life be corpulent, the excess is not to be considered sound muscle or flesh, but fat, which, I have before stated, is a sign of weakness.
What is nutrition ? What is sought for in constructing a machine ? Why the necessity for this ? How is it in the animal machine ? Is the system constantly undergoing changes ? How is this shown ? What vessels repair the losses of the system ? What animals are subject to the most rapid changes of matter ? What contains the materials of nutrition ? When is nutrition most active ?-when Jeast so ? What is the consequence ? What does the degree of nutrition depend on ? Does fatness indicate strength ? Why not ? What use does the hump of the camel serve ? What are tumours and wens -owing to ? What causes muscular nutrition ?