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throughout the body, where, in consequence, animal heat is produced, the process being chemically analogous to ordinary combustion, in which oxygen is consumed and carbonic acid set free.
When we consider the amount of action attending even moderate exercise, and going on in the living processes even during sleep, and the consequent waste giving rise to the constantly recurring demands for food, it is evident that a rapid process of Change is going on in our bodies. It is interesting to think of this, and at the same time of the similarity of the new to the old. Like the stream of the river, there is, as it were, a slow current of matter passing through us, lodging only for a short time and then swept away. As an ancient philosopher has well remarked, we cannot cross the same river twice at the same place ; and so the body we inhabit to-day is not altogether the same that we inhabited yesterday, possibly none of it the same that we inhabited a year ago.
Like the water of the river, all is new, and yet we speak of the same river and the same body.
The rapidity of the change will depend on the activity of each part, the more active will change most frequently, and we change more rapidly at one season than another. While the blacksmith and the tailor change their hands oftener than their feet, it will be the opposite with the postman ; and the right hand will be renewed oftener than the left. Thinking men will have new brains, and teachers and orators new tongues, while the thoughtless and the silent are still going on with the old ones.
The only parts perhaps that do not change, or at least do not seem capable of renewal, are the little shells of enamel which cover the exposed part of the teeth.
It is this inherent power of renewal, even more than the wisdom of the invention, which distinguishes the Creator's work from man's. The best way to preserve our handiwork is to lay it by, but nature's machinery improves with use, nay, will even disappear unless used. The clothes we make for ourselves get thin and ragged with that use which makes the skin below them grow thicker and healthier ; and every stroke on the blacksmith’s anvil lightens the hammer but strengthens the arm that wields it. And so nature's renewing process goes on until it is arrested by the law of death, when the forsaken body resolves itself into its component elements, and returns to the dust from which it came.
In connexion with the subject of waste and repair, it is important to recognise the effect of Exercise on the health. As has been already remarked, use is necessary to the preservation of living texture, and has the effect of increasing and strengthening it, provided, of course, that it is not so excessive as to induce exhaustion or over-action. In active exercise, the muscles are directly employed, and the effect of the exercise is to enlarge them, and also to enlarge the bones, thus giving us greater strength of limb and body generally. Compare, in illustration of this, the size and power of the blacksmith's or labourer's arm with that of the sedentary clerk or the invalid. But independently of increased strength, exercise brings a healthy condition by circulating the blood more rapidly, and by rendering the blood itself more pure. When the muscles act they compress the veins, and, as these are provided with valves which prevent regurgitation, the current is driven rapidly forward to the heart. Increased action of the heart results, and this, again, is followed by increased respiration. The want of sufficient breath which we soon feel on starting suddenly to run, is owing to the venous blood being sent forward more rapidly than the lungs can or will at first let it through. But if we begin the race more gradually, the blood becomes so purified that the lungs offer no obstruction. The effect of the increased force and rapidity of the circulation is to send the blood more freely into every part, carrying off the waste and laying down new matter more rapidly. The blood is rendered more pure by the increased frequency and volume of respiration. A more highly oxygenated blood therefore is circulated throughout the body, by which every organ benefits, and none more so than the brain, on which venous or impure blood acts as a poison. Hence the importance of exercise being taken in the open air (not to speak of the freedom which it permits), that we may inhale air free from the impurities with which the confined atmosphere of inhabited rooms is quickly saturated. Exercise in games, too, and in the society of others, is much preferable to solitary and monotonous walking or working, as in the latter the same muscles are continuously used, and there is no amusement or excitement to lead the mind away from dwelling on the cares of business and study. Gymnastic training in schools and public games for adults, should be much more general than they are.
It must not be overlooked that Repose is a law of health as well as exercise. All action induces more or less exhaustion, and, as waste has been going on more actively than renewal, repose is necessary in order that renewal may proceed more actively than waste. The ordinary fatigue attending exercise is chiefly in the muscles, whose power of contraction becomes exhausted, but is restored by rest. Simple cessation from labour appears to be sufficient for this, while sleep is essentially the repose of the nervous system and more especially of the brain. The reflex actions of the spinal cord and lower parts of the brain, alone remain in operation to carry on respiration and those other reflex actions which go on during sleep independently of sensation and conscious
But in profound sleep, all sensation and consciousness of mental action and existence are suspended, as much so as if for the time they were dead. We have already seen that the mind uses the brain as it works, and, accordingly, the brain must have its rest like
This it may have by changing the subject of study, but periods of entire relaxation and ample sleep are no less necessary for those whose labour is mental, than for those who labour with the hand. Sleep, therefore, in which man spends about a third part of his life, is not lost time or a thing to be neglected, but is that intended period of repose and restoration which enables us to act and work during the other two-thirds.
LET us now, in concluding this brief sketch, take a view of the position of man in the general scheme of creation.
Taking a wide survey, we find a successive dependence of the great kingdoms of nature : the mineral, or inorganic, kingdom -the earth, the air, and the water-supports the vegetable kingdom ; after which, and dependent on it, follows the animal kingdom. Next, within the latter, we trace the gradual development of a brain, an organ fitted to become the instrument of intelligence. Lastly comes man, made as we are told in the image of his Maker —that is, in mind ; and, as the human problem was to unite a mind to a body, we find an enormous brain provided as the seat of the mysterious union. Thus the whole scheme of nature rises up till it finds its completion in the human brain, and may be likened to a great pyramid in three successive zones, the mineral, vegetable, and animal, towering upwards to a summit, on the top of which is placed a human brain, the habitation of the mind, there fitly placed as the connecting link between the material and mortal below and the spiritual and immortal above.
But the human figure is not distinguished merely by its great brain. Man is placed upright, on two limbs, and, though not the only biped, is the only straight or erect biped ; and he is erect chiefly that his other two limbs may be set free to be used as hands. To these two great central ideas, the large brain and the erect posture, all the lesser characteristics are referable. The erect posture requires the plantigrade foot, and this, again, the great inner toe. The long lower limbs, forming one-half of his entire length, enable him to take long steps and elevate him, as on two pillars of observation. His broad chest and shoulders enable him not only to balance himself in the erect posture, but to lie comfortably on his back; and they also support conveniently his active working instrument, the hand, which is there attached to his body. The great brain gives the forehead, and the no less characteristic hind-head ; and by his free hand, with intelligence to guide it, he is enabled to dispense with long jaws and sunken snout, and to have in their place a chin and prominent nose, peculiar characteristics of the “ human face divine."
Man, then, is intended to work, as well as to feel and think. He alone is progressive, and engaged in intelligently operating upon and modifying the earth's surface, round and upon which myriads of human hands are working. Without the senses, he could not be educated ; and without hands, he might be knowing, but would be very useless. The object of his existence may be typified in three organs, the eye, the hand, and the brain ;—the eye, with which, sun-like, he scans the world ; the hand, with which he hammers the world ; and the brain, with which he knows the world, and by which he becomes, as Shakspere well defines him, " the Being of large discourse looking before and after.” And yet we must not forget that the brain is only the instrument, that the mind itself has a constitution ; and it is this mind which -guided by the revealed word of God, and strengthened and enlightened by the contemplation of the works of God in naturetells us of the long past, and of the eternal future, and is the source of all true pleasure in the present exercise of the parts of our double nature.
Enough, it is trusted, has been said to lead the pupil to follow the study of the human body farther than the prescribed limits of this treatise permit. Much might be said in advocacy of the wide diffusion of such knowledge. It acquaints us with the laws of health, and puts it in our power, if we choose, to avoid much disease, and suffering, and loss of time. The Creator manages all this for the lower animals through their instincts, which they blindly but securely follow. And while much of this is fixed for us also, much is left to us to find out through reason and experience ; and thus it is for man himself to choose whether he shall fall below the level of the beasts by allowing the body to overcome the mind, or shall rise to all the high capabilities of his physical nature, by subjecting the body to his reason, and by the careful observance of the laws of health. That we cannot, either as individuals or communities, transgress the laws of health with impunity, is in itself a sufficient reason for our studying and obeying them ; to which we may add the grave consideration that, as these laws are of God's making, obedience to them becomes a duty to our Maker as well as to ourselves.
But, perhaps, no argument is of more direct weight than the consideration of the influence which our bodily health has upon the mind. Our mental progress and disposition, on which may hang also the happiness of others, are largely influenced by attention to the health of the body. When we think of this, it becomes evident that temperance, cleanliness, exercise, breathing fresh air, and sufficient repose, are virtues and duties of no small importance.?
1 Some knowledge of our own bodies will serve also to protect us from the impositions of the charlatan; and that this is no small matter will be evident when we think of the vast sum of money annually spent in the mere advertising of quack medicines, to say nothing of their consumption, or of the profits of those who thus prey upon the credulity of their fellow creatures. Education, in the general sense, affords no protection against this; unless it includes a knowledge of physiology ; for this alone can enable us, for instance, to detect the bone-setter who professes to have replaced a bone which does not even exist, or to understand the absurdity of professing to cure every disease by one medicine or one means. It is such knowledge alone which can enable the public to distinguish between the bold and therefore (with the ignorant) successful promises of the empiric, and the rational and moderate statements of the qualified medical man; and to comprehend bow it is that cures are effected by working through and upon nature's processes. Trust in the medical practitioner, instead of being a kind of blind superstition, then becomes an intelligent faith.