19. CHANGES IN THE BLOOD FROM RESPIRATION.--The most striking change which the blood undergoes by its pa.s.sage through the lungs, is the change of color from a dark blue to bright red. That this change is dependent upon respiration has been fully proved by experiment. If the trachea, or windpipe, of a living animal be so compressed as to exclude the air from the lungs, the blood in the arteries will gradually grow darker, until its color is the same as that of the venous blood. When the pressure is removed the blood speedily resumes its bright hue. Again, if the animal be made to breathe an atmosphere containing more oxygen than atmospheric air, the color changes from scarlet to vermilion, and becomes even brighter than arterial blood. This change of color is not of itself a very important matter, but it indicates a most important change of composition.

[Sidenote: 20. What does the air lose and gain by respiration? What, the blood? Air as food?]

20. The air, as we have seen, by respiration loses oxygen and gains carbonic acid: the blood, on the contrary, gains oxygen and loses carbonic acid. The oxygen is the food of the blood corpuscles; while the articles we eat and drink belong more particularly to the plasma of the blood. The air, then, it is plain, is a sort of food, and we should {134} undoubtedly so regard it, if it were not for the fact that we require it constantly, instead of taking it at stated intervals, as is the case with our articles of diet. Again, as the demand of the system for food is expressed by the sensation of hunger, so the demand for air is marked by a painful sensation called suffocation.

[Sidenote: 21. Moist animal membranes? How shown with the bladder?]

21. INTERCHANGE OF GASES IN THE LUNGS.--As the air and the blood are not in contact, they being separated from each other by the walls of the air-cells and of the blood-vessels, how can the two gases, oxygen and carbonic acid, exchange places? Moist animal membranes have a property which enables them to transmit gases through their substance, although they are impervious to liquids. This may be beautifully shown by suspending a bladder containing dark blood in a jar of oxygen. At the end of a few hours the oxygen will have disappeared, the blood will be brighter in color, and carbonic acid will be found in the jar.

[Sidenote: 22. Gaseous diffusion? If oxygen be not received? If carbonic acid be retained?]

22. If this interchange takes place outside of the body, how much more perfectly must it take place within, where it is favored by many additional circ.u.mstances! The walls of the vessels and the air-cells offer no obstacle to this process, which is known as gaseous diffusion. Both parts of the process are alike of vital importance. If oxygen be not received, the organs cease to act; and if carbonic acid be retained in the blood, its action is that of a poison; unconsciousness, convulsions, and death following.

[Sidenote: 23. Difference in the appearance and composition of the blood?

Temperature of the blood? The blood while pa.s.sing through the lungs? The consequence?]

23. DIFFERENCE BETWEEN ARTERIAL AND VENOUS BLOOD.--The following table presents the essential points of difference in the appearance and composition of the blood, before and after its pa.s.sage through the lungs:-- {135}

_Venous Blood._ _Arterial Blood._

Color, Dark blue, Scarlet.

Oxygen, 8 per cent., 18 per cent.

Carbonic Acid, 15 to 20 per cent., 6 per cent., or less.

Water, More, Less.

The temperature of the blood varies considerably; but the arterial stream is generally warmer than the venous. The blood imparts heat to the air while pa.s.sing through the lungs, and consequently the contents of the right side of the heart has a higher temperature than the contents on the left side.

[Sidenote: 24. What do we learn by means of the spectroscope? "Carriers of oxygen?" Blue blood in the system?]

24. By means of the spectroscope, we learn that the change of color in the blood has its seat in the corpuscles; and that, according as they retain oxygen, or release it, they present the spectrum of arterial or venous blood. There evidently exists, on the part of these little bodies, an affinity for this gas, and hence they have been called "carriers of oxygen." It was long ago thought that blue blood was a trait peculiar to persons of princely and royal descent, and boastful allusions to the "_sang azure_" of kings and n.o.bles are quite often met with. Physiology, however, informs us that blue blood flows in the veins of the low as well as the high, and that so far from its presence indicating a mark of purity, it, in reality, represents the waste and decay of the system.

[Sidenote: 25. The amount of air that pa.s.ses in and out of the lungs?]

25. AMOUNT OF RESPIRATORY LABOR.--During ordinary calm respiration, we breathe eighteen times in a minute; and twenty cubic inches of air pa.s.s in and out of the lungs with every breath. This is equivalent to the use of three hundred and sixty cubic inches, or more than ten pints of air each minute. From this we calculate that the quant.i.ty of air which hourly traverses the lungs is about thirteen cubic feet, or seventy-eight gallons; and daily, not {136} less than three hundred cubic feet, an amount nearly equal to the contents of sixty barrels.

[Sidenote: 26. Air absorbed in its transit through the lungs? The loss?

Carbonic acid exhaled? Effect of excitement or exertion? What estimate?]

26. Of this large volume of air five per cent. is absorbed in its transit through the lungs. The loss thus sustained is almost wholly of oxygen, and amounts to fifteen cubic feet daily. The quant.i.ty of carbonic acid exhaled by the lungs during the day is somewhat less, being twelve cubic feet.

Under the influence of excitement or exertion, the breathing becomes more frequent and more profound; and then the internal respiratory work increases proportionately, and may even be double that of the above estimate. It has been estimated that in drawing a full breath, a man exerts a muscular force equal to raising two hundred pounds placed upon the chest.

[Sidenote: 27. Importance of the oxygen in the atmosphere? Injurious character of gases?]

27. IMPURITIES OF THE AIR.--The oxygen in the atmosphere is of such prime importance, and its proportion is so nicely adjusted to the wants of man, that any gas or volatile substance which supplants it must be regarded as a hurtful impurity. All gases, however, are not alike injurious. Some, if inhaled, are necessarily fatal; _a.r.s.enuretted hydrogen_ being one of these, a single bubble of which destroyed the life of its discoverer, Gehlen.

Others are not directly dangerous, but by taking the place of oxygen, and excluding it from the lungs, they become so. Into this latter cla.s.s we place carbonic acid.

[Sidenote: 28. Pungency of gases? The inference? Our safeguard?]

28. Most of the actively poisonous gases have a pungent or offensive odor; and, as may be inferred, most repugnant odors indicate the presence of substances unfit for respiration. Accordingly, as we cannot see or taste these impurities, the sense of smell is our princ.i.p.al safeguard against them; and we recognize the design which has planted this sense, like a sentinel at the proper entrance of the {137} air-pa.s.sages, the nostrils, to give us warning of approaching harm. Take, as an example, the ordinary illuminating gas of cities, from which so many accidents happen. How many more deaths would it cause if, when a leak occurs, we were not able to discover the escape of the gas by means of its disagreeable odor.

[Sidenote: 29. The air of rooms in which fever-sick persons are confined?]

29. Organic matters exist in increased measure in the expired breath of sick persons, and impart to it, at times, a putrid odor. This is especially true in diseases which, like typhus and scarlet fever, are referable to a blood poison. In such cases the breath is one of the means by which nature seeks to expel the offending material from the system. Hence, those who visit or administer to fever-sick persons should obey the oft-repeated direction, "not to take the breath of the sick." At such times, if ever, fresh air is demanded, not alone for the sick, but as well for those who are in attendance.

[Sidenote: 30. Animalcula in the water? Dust in the air?]

30. DUST IN THE AIR.--Attention has lately been directed to the dust, or haze, that marks the ray of sunshine across a shaded room. Just as, many years ago, it was discovered that myriads of animalcula infested much of the water we drank, so now the microscope reveals "the gay motes that dance along a sunbeam" to be, in part, composed of mult.i.tudes of animal and vegetable forms of a very low grade, the germs of fermentation and putrefaction, and the probable sources of disease.

[Sidenote: 31. The best air filter? The remarks of Prof. Tyndall?]

31. It is found that the best filter by which to separate this floating dust from the air is cotton wool, although a handkerchief will imperfectly answer the same purpose. In a lecture on this subject by Prof. Tyndall, he remarks that, "by breathing through a cotton wool respirator, the noxious air of the sick room is restored to practical purity. Thus filtered, attendants may breathe the air unharmed. {138} In all probability, the protection of the lungs will be the protection of the whole system. For it is exceedingly probable that the germs which lodge in the air-pa.s.sages are those which sow epidemic disease in the body. If this be so, then disease can certainly be warded off by filters of cotton wool. By this means, so far as the germs are concerned, the air of the highest Alps may be brought into the chamber of the invalid."

[Sidenote: 32. Carbonic acid in volcanic regions? In Java? At Lake Avernus?

In mines?]

32. CARBONIC ACID IN THE AIR.--We have already spoken of this gas as an exhalation from the lungs, and a source of impurity; but it exists naturally in the atmosphere in the proportion of one half part per thousand. In volcanic regions it is poured forth in enormous quant.i.ties from fissures in the earth's surface. Being heavier than air, it sometimes settles into caves and depressions in the surface. It is stated that in the island of Java, there is a place called the "Valley of Poison," where the ground is covered with the bones of birds, tigers, and other wild animals, which were suffocated by carbonic acid while pa.s.sing. The Lake Avernus, the fabled entrance to the infernal regions, was, as its name implies, bird-less, because the birds, while flying over it, were poisoned by the gas and fell dead into its waters. In mines, carbonic acid forms the dreaded _choke-damp_, while carburetted hydrogen is the _fire-damp_.

[Sidenote: 33. In the open air? Amount of carbonic acid exhaled by a man? A gas-burner? A room fire? From furnaces?]

33. In the open air, men seldom suffer from carbonic acid, for, as we shall see presently, nature provides for its rapid distribution, and even turns it to profitable use. But its ill effects are painfully evident in the abodes of men, in which it is liable to collect as the waste product of respiration and of that combustion which is necessary for lighting and warming our homes. A man exhales, during repose, not less than one-half cubic foot of carbonic acid per hour. One gas-burner liberates five cubic feet in the {139} same time, and spoils about as much air as ten men. A fire burning in a grate or stove emits some gaseous impurity, and at the same time abstracts from the air as much oxygen as twelve men would consume in the same period, thus increasing the relative amount of carbonic acid in the air. From furnaces, as ordinarily constructed, this gas, with other products of combustion, is constantly leaking and vitiating the air of tightly-closed apartments.

[Sidenote: 34. Effects of inhaling carbonic acid alone? In small quant.i.ties?]

34. EFFECTS OF IMPURE AIR.--Carbonic acid, in its pure form, is irrespirable, causing rapid death by suffocation. Air containing forty parts per thousand of this gas (the composition of the expired breath) extinguishes a lighted candle, and is fatal to birds; when containing one hundred parts, it no longer yields oxygen to man and other warm-blooded animals; and is of course at once fatal to them. In smaller quant.i.ties, this gas causes headache, labored respiration, palpitation, unconsciousness, and convulsions.

[Sidenote: 35. Effects of the air in crowded and badly ventilated rooms?]

35. In crowded and badly ventilated apartments, where the atmosphere relatively contains from six to ten times the natural amount of carbonic acid, the contaminated air causes dulness, drowsiness, and faintness; the dark, impure blood circulating through the brain, oppressing that organ and causing it to act like a blunted tool. This is a condition not uncommon in our schools, churches, court-rooms, and the like, the places of all others where it is desirable that the mind should be alert and free to act; but, unhappily, an unseen physiological cause is at work, dispensing weariness and stupor over juries, audience, and pupils.

[Sidenote: 36. A cause of consumption? How was the fact ill.u.s.trated?]

36. Another unmistakable result of living in and breathing foul air is found in certain diseases of the lungs, especially consumption. For many years the barracks of {140} the British army were constructed without any regard to ventilation; and during those years the statistics showed that consumption was the cause of a very large proportion of deaths. At last the government began to improve the condition of the buildings, giving larger s.p.a.ce and air-supply; and as a consequence, the mortality from consumption has diminished more than one-third.

[Sidenote: 37. How, in the case of the lower animals? Tendency of certain occupations?]

37. The lower animals confined in the impure atmosphere of menageries, contract the same diseases as man. Those brought from a tropical climate, and requiring artificial warmth, generally die of consumption. In the Zoological gardens of Paris, this disease affected nearly all monkeys, until care was taken to introduce fresh air by ventilation; and then it almost wholly disappeared. The tendency of certain occupations to shorten life is well known; disease being occasioned by the fumes and dust which arise from the material employed, in addition to the unhealthful condition of the workshop or factory where many hours are pa.s.sed daily.

[Sidenote: 38. Give the fact as set forth in the table.]

38. The following table shows the comparative amount of carbonic acid in the air under different conditions and the effects sometimes produced:--

PROPORTION OF CARBONIC ACID. In 1000 parts of Air.

Air of country. .4 " " city. .5 In hospital, well ventilated. .6 In school, church, etc., fairly ventilated. 1.2 to 2.5 In court-house, factory, etc., without ventilation. 4. to 40.

In bedroom, before being aired. 4.5 In bedroom, after being aired. 1.5 Constantly breathed, causing ill health. 2.

Occasionally breathed, causing discomfort. 3.

Occasionally breathed, causing distress. 10.

Expired air. 40.

Air no longer yielding oxygen 100.