1. THE OBJECT OF RESPIRATION.--In one set of capillaries, or hair-like vessels, the blood is impoverished for the support of the different members and organs of the body. In another capillary system the blood is refreshed and again made fit to sustain life. The former belongs to the greater or _systemic_ circulation; the latter to the lesser or _pulmonary_, so called from _pulmo_, the lungs, in which organs it is situated. The blood, as sent from the right side of the heart to the lungs, is venous, dark, impure, and of a nature unfit to circulate again through the tissues. But, when the blood returns from the lungs to the left side of the heart, it has become arterial, bright, pure, and no longer hurtful to the tissues. This marvellous purifying change is effected by means of the very familiar act of respiration, or breathing.

[Sidenote: 2. What are the lungs? How many lungs are there? Lung-substance?

Its properties? The pleura?]

2. THE LUNGS.--The lungs are the special organs of respiration. There are two of them, one on each side of the chest, which cavity they, with the heart, almost wholly occupy. The lung-substance is soft, elastic, and sponge-like. Under pressure of the finger, it _crepitates_, or crackles, and floats when thrown into water; these properties being {124} due to the presence of air in the minute air-cells of the lungs. To facilitate the movements necessary to these organs, each of them is provided with a double covering of an exceedingly smooth and delicate membrane, called the _pleura_. One layer of the pleura is attached to the walls of the chest, and the other to the lungs; and they glide, one upon the other, with utmost freedom. Like the membrane which envelops the heart, the pleura secretes its own lubricating fluid, in quant.i.ties sufficient to keep it always moist.

[Ill.u.s.tration: FIG. 34.--ORGANS OF THE CHEST.

A, Lungs. B, Heart. D, Pulmonary Artery. E, Trachea.]

{125}

[Ill.u.s.tration: FIG. 35.--LARYNX, TRACHEA, AND BRONCHIAL TUBES.]

[Ill.u.s.tration: FIG. 36.--DIAGRAM AND SECTION OF THE AIR-CELLS.]

[Sidenote: 3. Communication of the lungs with the external air? Bronchial tubes?]

3. THE AIR-Pa.s.sAGES. --The lungs communicate with the external air by means of certain air-tubes, the longest of which, the _trachea_, or windpipe, runs along the front of the neck (Fig. 34, E, and 35). Within the chest this tube divides into two branches, one entering each lung; these in turn give rise to numerous branches, or bronchial tubes, as they are called, which gradually diminish in size until they are about one-twenty-fifth of an inch in diameter. Each of these terminates in a cl.u.s.ter of little pouches, or "air-cells," having very thin walls, and covered with a capillary network, the most intricate in the body (Fig. 36).

[Sidenote: 4. Office of the bronchial tubes? What further can you state of them?]

4. These tubes are somewhat flexible, sufficiently so to bend when the parts move in which they are situated; but they are greatly strengthened by bands or rings of cartilage which keep the pa.s.sages always open; otherwise there would be a constantly-recurring tendency to collapse after every breath. The lung-substance essentially consists of these bronchial tubes and terminal air-cells, with the blood-vessels ramifying about them (Fig.

37). At the top of the trachea is the larynx, a sort of {126} box of cartilage, across which are stretched the vocal cords. Here the voice is produced chiefly by the pa.s.sage of the respired air over these cords, causing them to vibrate.

[Ill.u.s.tration: FIG. 37.--SECTION OF THE LUNGS.]

[Sidenote: 5. The epiglottis? When it does not close in time, what is the consequence?]

5. Over the opening of the larynx is found the _epiglottis_, which fits like the lid of a box at the entrance to the lungs, and closes during the act of swallowing, so that food and drink shall pa.s.s backward to the oesophagus, or gullet (Fig. 38). Occasionally it does not close in time, and some substance intrudes within the larynx, when we at once discover, by a choking sensation, that "something has gone the wrong way," and, by coughing, we attempt to expel the unwelcome intruder. The epiglottis is one of the many safeguards furnished by nature for our security and {127} comfort, and is planned and put in place long before these organs are brought into actual use in breathing and in taking food.

[Ill.u.s.tration: FIG. 38.--SECTION OF MOUTH AND THROAT.

A, The Tongue. B, The Uvula C, Vocal Cord. E, Epiglottis. L, Larynx.

N, Trachea. O, Oesophagus.]

[Sidenote: 6. Lining of the air-pa.s.sages? Ciliated cells? Their uses? The three diseases of the lungs?]

{128}

[Ill.u.s.tration: FIG. 39.--CILIATED CELLS.]

6. The air-pa.s.sages are lined through nearly their whole extent with mucous membrane, which maintains these parts in a constantly moist condition. This membrane has a peculiar kind of cells upon its outer surface. If examined under a powerful microscope, we may see, even for a considerable time after their removal from the body, that these cells have minute hair-like processes in motion, which wave like a field of grain under the influence of a breeze (Fig. 39). This is a truly beautiful sight; and since it is found that these little _cilia_, as they are called, always produce currents in one direction, from within outward, it is probable that they serve a useful purpose in catching and carrying away from the lungs dust and other small particles drawn in with the breath (Fig. 39). The three diseases which more commonly affect the lungs, as the result of exposure, are pneumonia, or inflammation of the lungs, implicating princ.i.p.ally the air-cells; bronchitis, an inflammation of the large bronchial tubes; and pleurisy, an inflammation of the investing membrane of the lungs, or pleura. Among the young, an affection of the trachea takes place, known as croup.

[Sidenote: 7. The act of breathing? Extension of the chest by breathing?]

7. THE MOVEMENTS OF RESPIRATION.--The act of breathing has two parts--(1), _inspiration_, or drawing air into the lungs, and (2), _expiration_, or expelling it from the lungs again. In inspiration, the chest extends in its length, breadth, and height, or width. We can prove that this is the case as regards the two latter, by observing the effect of a deep breath. The ribs are elevated by means of numerous muscles, some of which occupy the entire s.p.a.ces between those bones. But the increase in length, or vertically, is not so apparent, as it is caused by a muscle within the body called the _diaphragm_, it being the thin part.i.tion which separates the chest from the abdomen, rising like a dome within the chest. (Fig. 16).

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[Sidenote: 8. Contraction of the diaphragm? Power of the diaphragm? Effects of extending the walls of the chest? The habit of taking frequent and deep inspirations?]

8. With every inspiration, the diaphragm contracts, and in so doing, approaches more nearly a plane, or horizontal, surface, and thus enlarges the capacity of the chest. Laughing, sobbing, hiccoughing, and sneezing are caused by the spasmodic or sudden contraction of the diaphragm. The special power of this muscle is important in securing endurance, or "long wind," as it is commonly expressed; which may be obtained mainly by practice. It is possessed in a marked degree by the mountaineer, the oarsman, and the trained singer. As the walls of the chest extend, the lungs expand, and the air rushes in to fill them. This const.i.tutes an inspiration. The habit of taking frequent and deep inspirations, in the erect position, with the shoulders thrown back, tends greatly to increase the capacity and power of the organs of respiration.

[Sidenote: 9. Expiration? The mechanism of expiration?]

9. EXPIRATION is a less powerful act than inspiration. The diaphragm relaxes its contraction, and ascends in the form of a dome; the ribs descend and contract the chest; while the lungs themselves, being elastic, a.s.sist to drive out the air. The latter pa.s.ses out through the same channels by which it entered. At the end of each expiration there is a pause, or period of repose, lasting about as long as the period of action.

[Sidenote: 10. Frequency of respiration? Effect of hurried action of the heart?]

10. FREQUENCY OF RESPIRATION.--It is usually estimated that we breathe once during every four beats of the heart, or about eighteen times in a minute.

There is, of course, a close relation between the heart and lungs, and whatever modifies the pulse, in like manner affects the breathing. When the action of the heart is hurried, a larger amount of blood is sent to the lungs, and, as the consequence, they must act more rapidly. Occasionally, the heart beats so very forcibly that the lungs cannot keep pace with it, and then we experience a peculiar sense of {130} distress from the want of air. This takes place when we run until we are "out of breath." At the end of every fifth or sixth breath, the inspiration is generally longer than usual, the effect being to change more completely the air of the lungs.

[Sidenote: 11. Respiration controlled by the will? Advantage of the knowledge to us?]

11. Although, as a general rule, the work of respiration goes on unconsciously and without exertion on our part, it is nevertheless under the control of the will. We can increase or diminish the frequency of its acts at pleasure, and we can "hold the breath," or arrest it altogether for a short time. From twenty to thirty seconds is ordinarily the longest period in which the breath can be held; but if we first expel all the impure air from the lungs, by taking several very deep inspirations, the time may be extended to one and a half or even two minutes. This should be remembered, and acted upon, before pa.s.sing through a burning building, or any place where the air is very foul. The arrest of the respiration may be still further prolonged by training and habit; thus it is said, the pearl-fishers of India can remain three or four minutes under water without being compelled to breathe.

[Sidenote: 12. Capacity of the lungs? Time required to renovate the air in the lungs? In tranquil respiration? Importance of the provision?]

12. CAPACITY OF THE LUNGS.--The lungs are not filled and emptied by each respiration. For while their full capacity, in the adult, is three hundred and twenty cubic inches, or more than a gallon, the ordinary breathing air is only one-sixteenth part of that volume, or twenty cubic inches, being two-thirds of a pint. Accordingly, a complete renovation, or rotation, of the air of the lungs does not take place more frequently than about once in a minute; and by the gradual introduction of the external air, its temperature is considerably elevated before it reaches the delicate pulmonary capillaries. In tranquil respiration, less than two-thirds of the breathing power is {131} called into exercise, leaving a reserve capacity of about one hundred and twenty cubic inches, equivalent to three and one half pints. This provision is indispensable to the continuation of life; otherwise, a slight embarra.s.sment of respiration, by an ordinary cold, for instance, would suffice to cut off the necessary air, and the spark of life would be speedily extinguished.

[Sidenote: 13. The atmosphere? How high or deep? How essential to life?

Marine life in perfectly pure water and air?]

13. THE AIR WE BREATHE.--The earth is enveloped on all sides by an invisible fluid, called the atmosphere. It forms a vast and sh.o.r.eless ocean of air, forty-five miles deep, encircling and pervading all objects on the earth's surface, which is absolutely essential for the preservation of all vegetable and animal life,--in the sea, as well as on the land and in the air. At the bottom, or in the lower strata of this aerial ocean, we move and have our being. Perfectly pure water will not support marine life, for a fish may be drowned in water from which the air has been exhausted, just as certainly as a mouse, or any other land animal, will perish if put deeply into the water for a length of time. The cause is the same in both cases: the animal is deprived of the requisite amount of air. It is also stated, that if the water-supply of the plant be deprived of air, its vital processes are at once checked.

[Sidenote: 14. Composition of the air? Properties of the two gases?]

14. The air is not a simple element, as the ancients supposed, but is formed by the mingling of two gases, known to the chemist as oxygen and nitrogen, in the proportion of one part of the former to four parts of the latter. These gases are very unlike, being almost opposite in their properties: nitrogen is weak, inert, and cannot support life; while oxygen is powerful, and incessantly active; and is the essential element which gives to the atmosphere its power to support life and combustion. The discovery of this fact was made by the French chemist, Lavoisier, in 1778.

{132}

[Sidenote: 15. Air once breathed? An animal in it? A candle? a.n.a.lysis of expired air? Change in volume?]

15. CHANGES IN THE AIR FROM RESPIRATION.--Air that has been once breathed is no longer fit for respiration. An animal confined within it will sooner or later die; so too, a lighted candle placed in it will be at once extinguished. If we collect a quant.i.ty of expired air and a.n.a.lyze it, we shall find that its composition is not the same as that of the inspired air. When the air entered the lungs it was rich in oxygen; now it contains twenty-five per cent. less of that gas. Its volume, however, remains nearly the same; its loss being replaced by another and very different gas, which the lungs exhaled, called _carbonic acid_, or, as the chemist terms it, _carbon dioxide_.

[Sidenote: 16. What else has the expired air gained? When and where noticed?]

16. The expired air has also gained moisture. This is noticed when we breathe upon a mirror, or the window-pane, the surface being tarnished by the condensation of the watery vapor exhaled by the lungs. In cold weather, this causes the fine cloud which is seen issuing from the nostrils or mouth with each expiration, and contributes in forming the feathery crystals of ice which decorate our window-panes on a winter's morning.

[Sidenote: 17. Nature of the watery vapor? Its effects upon animals?]

17. This watery vapor contains a variable quant.i.ty of animal matter, the exact nature of which is unknown; but when collected it speedily putrefies and becomes highly offensive. From the effects, upon small animals, of confinement in their own exhalations, having at the same time an abundant supply of fresh air, it is believed that the organic matters thrown off by the lungs and skin are direct and active poisons; and that to such emanations from the body, more than to any other cause, are due the depressing and even fatal results which follow the crowding of large numbers of persons into places of limited capacity. {133}

[Sidenote: 18. Give some of the instances furnished by history.]

18. History furnishes many painful instances of the ill effects of overcrowding. In 1756, of one hundred and forty-six Englishmen imprisoned in the Black Hole of Calcutta, only twenty-three, at the end of eight hours, survived. After the battle of Austerlitz, three hundred prisoners were crowded into a cavern, where, in a few hours, two-thirds of their number died. On board a steam-ship, during a stormy night, one hundred and fifty pa.s.sengers were confined in a small cabin, but when morning came, only eighty remained alive.

[Sidenote: 19. Change in the blood from blue to red. Upon what does the change depend? How shown?]