« ПретходнаНастави »
eating frequently, to fill the cavity of the stomach; hence the folly and mischief of filling it with heterogeneous and unnatural substances; it being the object of nature simply to extract from the matter in the stomach a single uniform milky substance called chyle; no other juice but chyle being admitted into the animal system, the rest being rejected and expelled.
464. As animals were intended to move about, the perfect are therefore provided with eyes, to see objects which might endanger their existence; with ears to hear, for a similar reason; with a voice to warn others or to obtain assistance in danger.
Hence, also, they were provided with the senses of smelling and tasting, to discriminate the food which was proper for the stomach; and with the sense of feeling, or irritability, to secure their identities, and excite them to action.
And though things sensible be numberless,
And in those five all things their forms express, Which we can touch, taste, smell, or hear or see. 465. The organs of sense and the powers of voli. tion proceed from the head and brain, by the nerves, which direct the muscles and tendons ; but the functions of animal life are sustained by a simple, yet wonderful arrangement in the stomach and cavities of the body.
The heart is the centre of a thousand tubes, called arteries ; and by its never-ceasing contractions, it carries the blood through them, to al parts of the frame, diffusing every where warmth and life.
466. The blood of a man, thus driven by the contraction of the heart (a force like that by which water is driven out of a syringe or bladder, weighs thirty pounds; and, as this is the stock of the precious fluid possessed by each of us, and our lives depend on its constant circulation, it is not allowed to remain at the extremity of the arteries, but is there taken up by
another set of tubes called veins, and by them brought back again to the heart.
467. Thus, there is a constant circulation, outward and inward, of this same blood, at the rate of an ounce to each contraction, from the heart through the arteries and back to the heart by the veins. To warm, revive, nourish it, and keep up its quantity, there are various other wonderful, but very simple contrivances,
Were once the energy of air deni'd,
Nor back again pursue its curious way. 468. The heart consists of four cavities, from one of which called the left ventricle, the blood is driven into the arteries through the body'; by another, called the right auricle, it is received back again by the veins : it then passes into the right ventricle, whence it is forced into the lungs.
Having there been revivified by coming into contact with the air, it is carried back by a set of veins into the left auricle, and, from thence into the left ventricle, where it began its course : it is then again forced into the arteries, brought back by the veins, &c. till the end of life.
469. The lungs are large and spongy substance filling nearly the whole cavity of the chest, which rises as they fill and falls as they empty, in respiring air through the mouth or nostrils.
The act of respiration is performed about 20 times in a minute; and about 40 cubic inches of air are respired every time; of which 2 inches of oxygen are absorbed by the blood on the lungs, producing at the same instant, 98 degrees of vital heat, and restoring the veinous blood to its bright red colour.
Obs.--The lights used in families to feed cats, are the lungs of sheep or oxen, and are exactly similar to the lungs of man. On inspection, they will be found to be wonderfully adapted to their design of bringing the air in contaet with the blood. Any rupture in this tender fabric, or defect in their action, leads to that fatal elass of diseases, called Consumption.
470. Four thousand times in every hour, each caviity of the heart is called into action: and all the blood in the body passes through the heart 14 times in every hour.
The arteries, into which it is forced, branch in every direction through the body, like the roots, banches, and leaves of a tree, running through the substance of the bones, and every part of the animal substance, till they are lost in such fine tubes as to be wholly invisible.
471. In this manner, they distribute nourishment; supply perspiration ; and renew all the waste of the skin ; and, by passing through glands in every part of the body, all the various animal secretions are elabor*ated.
In the parts where the arteries are lost to the sight, the veins take their rise, and in their commencement are also imperceptible. The blood is then of a dark colour ; and, as it returns to the heart with a less impetus, there is always twice as much blood in the veins as in the arteries.
472. As the blood, in this discoloured state, has lost some of its vital power, it is driven through the lungs, and its colour is restored, but on its passage
back to the heart, it also receives a supply of a new fluid extracted from the food of the animal in the stomach and intestines.
The loss of weight in a human body by perspiration in 24 hours is about four pounds ; and what is gained by the inspiration of air into the lungs, is lost by the expulsion of moisture, and of gas generated in
473. The motion of the lungs is preserved by that of the chest containing them; that of the heart, may be felt on the left breast; and the circulation of the blood, by the action of the pulse in various parts of the body, and particularly at the wrist.
In children, the pulse gives 120 strokes in a minute ; at 20 years, about 75; at 30, about 70; and in old age, 60 or 50.
474. For the purpose of renewing and nourishing the blood, food is taken in at the mouth, macerated by the teeth, and mixed with the saliva; it is then carried into the stomach, (a bag like a highland bagpipe,) where it is dissolved into a soft pap by a powerful liquid called the gastric juice.
475. This pap is then drawn from the stomach into the intestines ; where, by means of a liquid called bile, it is separated into a white milky liquid called chyle, and into the excrement.
The chyle is taken up, or absorbed, by myriads o fine tubes called the lacteals, which carry
it to a main pipe called the thoracic duct. This pipe ascends to the throat, where it empties the chyle into a large vein, and being mixed with the blood, is conveyed to the heart.
476. Of such subtle and wonderful contrivance is the organization of man! Similar, also, is the construction of the whole of animated nature, from the greatest to the smallest.
Within the package of the skin, and essential to life and comfort, are numerous bones for strength; hundreds of muscles and tendons for action; nerves spread every where for sensation; hundreds of arteries, to carry out the blood; hundreds of veins to bring it back again; and hundreds of glands performing all kinds of secretions ; besides an infinite number of tubes called lacteals and lymphatics, to absorb and convey nutriment to the blood.
477. Such being the complex construction of animal bodies, is it not rather wonderful that we last 70 or 80 years, than that we endure no longer ! When it is considered also, that a muscle or a bone out of place, a vein or artery stopt in its circulation, or a nerve unduly acted upon, creates disease, pain, and misery ; is it not wonderful, that we enjoy so large a portion of health and pleasure ?
Should not such considerations teach us the value of prudence and temperance ?
Thick, in yon stream of light, a thousand ways,
THOMSON. 478. The nerves are soft white chords which arise from the brain, the focus of sensation, and disperse themselves in branches through all parts of the body. Impressions are recieved by the brain from the adjacent organs of sense; and the brain exercises its commands over the muscles and limbs by means of the
Thus, the body is enabled to avoid what is hurtful, to flee from danger, and to pursue every thing useful and agreeable.
Obs.-Trees and vegetables have no nerves or sensorium ; because, as they are unable to move and avoid danger, they could be of no use to them. The proper object of vegetable organization, appears to be to supply food to animated na. ture; and the wisdom of Providence is in nothing more evident than in the variety, wholesomeness, and abundance of vegetable provisions.
479. The car is placed in the most convenient part of the body near the brain, the common seat of all the senses, to give more speedy information.
In man it is of a form proper for the erect posture of his body ; n birds, of a form proper for flight, and not protuberait ; in quadrupeds, its form is, in some, large, erect, ard open ; in others, covered ; in subter