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[Sidenote: 39. What can you state of the diffusive power of gases? The added influence of the winds?]

39. NATURE'S PROVISION FOR PURIFYING THE AIR.--We have seen that carbonic acid is heavier than air, and is poisonous. Why, then, does it not sink upon and overwhelm mankind with a silent, invisible wave of death? Among the gases there is a more potent force than gravity, which forever precludes such a tragedy. It is known as the diffusive power of gases. It acts according to a definite law, and with a resistless energy compelling these gases, when in contact, to mingle until they are thoroughly diffused.

The added influence of the winds is useful, by insuring more rapid changes in the air; air in motion being perfectly wholesome. The rains also wash the air.

[Sidenote: 40. How is the constant purity of the air secured? Explain the process?]

40. We have seen that the whole animal creation is constantly abstracting oxygen from the atmosphere, and as constantly adding to it vast volumes of a gas injurious alike to all, even in small quant.i.ties. How, then, does the air retain, unchanged, its life-giving properties? The constant purity of the air is secured by means of the vegetable creation. Carbonic acid is the food of the plants, and oxygen is its waste product. The leaves are its lungs, and under the stimulus of sunlight a vegetable respiration is set in motion, the effects of which are just the reverse of the function we have been considering. Thus nature purifies the air, and at the same time builds up beautiful and useful forms of life from elements of decay.

[Sidenote: 41. What process occurs in the sea? How is the fact ill.u.s.trated?]

41. In the sea, as in the air, the same circle of changes is observed.

Marine animals consume oxygen and give off carbonic acid; while marine plants consume carbonic acid and liberate oxygen. Taking advantage of this fact, we may so arrange aquaria with fishes and sea-plants, in their proper combinations, so that each supplies the needs of the other, and the water requires seldom to be renewed. This {142} affords us, on a small scale, an ill.u.s.tration of the mutual dependence of the two great kingdoms of nature; as well as of those compensating changes which are taking place on such a grand scale in the world about us.

[Sidenote: 42. Character of the external air? Of the air in our dwellings?

What becomes imperative? Imperfect ventilation of our dwellings?]

42. VENTILATION.--Since the external atmosphere, as provided by nature, is always pure, and since the air in our dwellings and other buildings is almost always impure, it becomes imperative that there should be a free communication from the one to the other. This we aim to accomplish by ventilation. As our houses are ordinarily constructed, the theory of ventilation, "to make the internal as pure as the external air," is seldom carried out. Doors, windows, and flues, the natural means of replenishing the air, are too often closed, almost hermetically, against the precious element. Special means, or special attention, must therefore be used to secure even a fair supply of fresh air. This is still more true of those places of public resort, where many persons are crowded together.

[Sidenote: 43. What hints are given for the ventilation of our dwellings?]

43. If there are two openings in a room, one as a vent for foul air, and the other an inlet for atmospheric air, and if the openings be large, in proportion to the number of air consumers, the princ.i.p.al object will be attained. Thus, a door and window, each opening into the outer air, will ordinarily ventilate a small apartment; or a window alone will answer, if it be open both above and below, and the open s.p.a.ce at each end be not less than one inch for each occupant of the room, when the window is about a yard wide. The direction of the current is generally from below upward, since the foul, heated air tends to rise; but this is not essential. Its rate need not be rapid; a "draught," or perceptible current, is never necessary to good ventilation. The temperature of the air admitted may be warm or cold. It is thought by many that if the {143} air is cold, it is pure; but this is an error, since cold air will receive and retain the same impurities as warm air.

[Sidenote: 44. State what Florence Nightingale says about inhaling night air?]

44. Shall we open our bedrooms to the night air? Florence Nightingale says, in effect, that night air is the only air that we can then breathe. "The choice is between pure air without and impure air within. Most people prefer the latter,--an unaccountable choice. An open window, most nights in the year, can hurt no one. In great cities, night air is the best and purest to be had in twenty-four hours. I could better understand, in towns, shutting the windows during the day than during the night."

[Sidenote: 45. Warmth of the bird as compared with that of the air? Of the fish and the water? Heat in animals and plants? How ill.u.s.trated with the thermometer?]

45. ANIMAL HEAT.--Intimately connected with respiration is the production of animal heat, or the power of maintaining the temperature of the body above that of the medium in which the creature moves; thus, the bird is warmer than the air, and the fish than the water. This elevation of temperature is a result of the various chemical changes which are constantly taking place in the system. Although common to all animals, in a greater or less degree, heat is not peculiar to them; since plants also generate it, especially at the time of sprouting and flowering. If a thermometer be placed in a cl.u.s.ter of geranium flowers, it will indicate a temperature several degrees above that of the surrounding air.

[Sidenote: 46. Amount of heat in animals, how apportioned? As regards the birds? Frogs, and other sluggish animals? Arrangement made by zoologists?]

46. Among animals great differences are noticed in this respect, but the degree of heat produced is always proportional to the activity of respiration and the amount of oxygen consumed. Accordingly, the birds, whose habits are extremely active, and whose breathing capacity is the greatest, have uniformly the highest temperature. Sluggish animals, on the contrary, as frogs, lizards, and snakes, have little need for oxygen, and have incompletely {144} developed lungs; these animals are cold to the touch, that is, they have relatively a lower temperature than man, and their positive temperature is but little above that of the external air.

Accordingly, zoologists have so arranged the animal kingdom that _warm-blooded_ animals, including man, the birds, and the quadrupeds, are cla.s.sified together; while the _cold-blooded_ animals, such as the fish, tortoise, frog, and all that have no vertebral column, are cla.s.sed by themselves.

[Sidenote: 47. State what is said respecting the temperature of the human body.]

47. The temperature of the human body is about 100 Fahrenheit, and remains about the same through winter and summer, in the tropics as well as in the frozen regions of the north. It may change temporarily within the range of about twelve degrees; but any considerable, or long-continued elevation or diminution of the bodily heat is certain to result disastrously.

[Sidenote: 48. Ability of man to adapt himself to different climates? In what does the power to resist cold consist? What is said about warm clothing?]

48. Man is able to adapt himself to all extremes of climate; and, in fact, by means of clothing, shelter, and food, is able to create for himself an artificial climate where-ever he choses to reside. The power to resist cold consists chiefly in preventing the heat which is generated by the vital processes of the body from being lost by radiation. Warm clothing, such as we wear in winter, has, in reality, the same temperature as that which is worn in summer; but, by reason of being thick and porous, it is a bad conductor of heat, and thus prevents the escape of that produced by the body. If woollen fabrics were intrinsically warm, no one would wrap a piece of flannel, or blanket, around a block of ice to prevent its melting in summer.

[Sidenote: 49. Men in an atmosphere above the boiling-point? In foundries and gla.s.s works?]

49. The faculty of generating heat explains how it is that we are enabled to resist the effects of cold; but how does the body withstand a temperature higher than its {145} own? Men have been known to remain several minutes in an atmosphere heated above the boiling-point of water, and yet the temperature of their own bodies was not greatly elevated. Those who labor in foundries and gla.s.s-works are habitually subjected to very high degrees of temperature, but they do not suffer in health more than those engaged in many other occupations.

[Sidenote: 50. The regulation of the temperature of the body. Give the explanation.]

50. The regulation of the temperature of the body is effected by means of perspiration, and by its evaporation. So long as the skin acts freely and the air freely absorbs the moisture, the heat of the body does not increase, for whenever evaporation takes place, it is attended by the abstraction of heat--that is, the part becomes relatively colder. This may be tested by moistening some part of the surface with cologne, ether, or other volatile liquid, and then causing it to evaporate rapidly by fanning.

The principle that evaporation produces cold has been ingeniously and practically employed, in the manufacture of ice, by means of freezing machines.

[Sidenote: 51, 52. State what is said of spontaneous combustion.]

51. SPONTANEOUS COMBUSTION.--Is it possible that the temperature of the living body can be so increased, that its tissues will burn spontaneously?

From time to time, cases have been reported in which, by some mysterious means, considerable portions of the human body have been consumed, apparently by fire, the victim being found dead, or incapable of explaining the occurrence. Hence, the theory has been current that, under certain conditions, the tissues of the body might become self-ignited; and the fact that this so-called _spontaneous combustion_ has ordinarily taken place in those who had been addicted to the use of alcoholic drinks, has given a color of probability to the opinion. It has been supposed that the flesh of these unfortunate persons becoming saturated with the inflammable properties of the alcohol thus taken into the {146} system, took fire upon being exposed to a flame, as of a lighted candle, or, indeed, without any external cause. But, whether this be possible or not, one thing is certain, this strange kind of combustion has never been actually witnessed by any one competent to give a satisfactory account of it.

52. The results that have been observed may be satisfactorily explained by the accidental ignition of the clothes, or other articles near the body, and by the supposition that the individual was at the time too much stupefied by intoxication, to notice the source of danger, and provide for his safety. The highest temperature that has been observed in the body, about 112 Fahrenheit, is too low to ignite the vapor of alcohol; much less will it cause the burning of animal tissues. It is undoubtedly true that when the tissues are filled with alcohol, combustion will more easily take place than when the body is in a normal state; but, under any condition, the combustion of the body requires a higher degree of heat than can be generated by the body itself, or the mere _proximity_ of a lighted candle, or any cause of a similar character. {147}

QUESTIONS FOR TOPICAL REVIEW.

PAGE 1. What is the object of respiration? 123 2. What are the special organs of respiration? 123 3. In what organs does a change in the blood take place? 123 4. What is the nature of the change? 123, 133 5. Where are the lungs situated, and what is the character of the substance of which they are composed? 123, 125 6. Describe the facilities provided for the lung movements. 124 7. Describe the trachea, or windpipe. 124, 125, 127, 128 8. Describe the bronchial tubes, and their uses. 125, 126 9. What can you state in relation to the epiglottis? 126, 127 10. What are the cilia and what use do they probably serve? 128 11. How may the lungs be affected by not being properly protected? 128 12. Describe the movements necessary to the act of perfect respiration. 128, 129 13. What is the diaphragm, and what is its office? 128, 129 14. How may the organs of respiration be so improved as to increase their capacity and power? 129, 137 15. What is stated in relation to the frequency of respiration? 129, 130 16. To what extent may the act of respiration be subjected to our wills? 130 17. What may be said to be the capacity of the lungs? 130, 131 18. How long does it take every particle of air in the lungs to be expelled and new air to take its place? 130 19. What would be the consequences, if the entire capacity of the lungs were constantly used? 130, 131 20. What would be the consequences to a fish put into water from which the air had been completely exhausted? Why? 131 21. What is the air, and what are its parts? 131, 136, 138 22. What is the character of the air that has been just breathed? 132 23. Why is it that such air is not fit for respiration? 132, 139 24. What are the effects, as recorded in notable cases, of confinement in places the air of which has been breathed "over and over?" 133 25. What can you state of changes in the blood from respiration? 133 26. What of the air, as an article of food? 133, 134 27. What, on the subject of interchange of gases in the lungs? 134 28. Explain the difference between arterial and venous blood. 134, 135 29. Explain, if you can, the cause of the difference. 135 30. State what you can in relation to blue blood. 135 31. In relation to the amount of labor exerted in respiration. 135, 136 32. In relation to the deleterious properties of different gases. 136, 137 33. In relation to the dust that floats in the air. 137, 138 34. What are the properties of carbonic acid gas? 132, 138, 141 35. In what places is carbonic acid gas commonly found? 132, 138, 139 36. Describe the effects of carbonic acid gas. 132, 138, 139, 141 37. What are the general effects of breathing any impure atmosphere? 139, 140 38. What are Nature's provisions for purifying the air? 141, 142 39. What hints and directions are given on the subject of ventilation? 142, 143 40. How does the temperature of the body compare with the medium in which it lives? 143 41. How is temperature of the body regulated and sustained? 143, 144, 145 42. State what you can on the subject of spontaneous combustion. 145, 146

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CHAPTER IX.

THE NERVOUS SYSTEM.

_Animal and Vegetative Functions--Sensation, Motion, and Volition--The Structure of the Nervous System--The White and Gray Substances--The Brain--Its Convolutions--The Cerebellum--The Spinal Cord and its System of Nerves--The Anterior and Posterior Roots--The Sympathetic System of Nerves--The Properties of Nervous Tissue--Excitability of Nervous Tissues--The Functions of the Spinal Nerves and Cord--The Direction of the Fibres of the Cord--Reflex Activity, and its Uses--The Functions of the Medulla Oblongata and the Cranial Ganglia--The Reflex Action of the Brain._

[Sidenote: 1. What processes are known as the vegetative functions? Why so called? What properties and functions does the plant possess? Their object?]

1. ANIMAL FUNCTIONS.--The vital processes which we have been considering, in the three previous chapters, of digestion, circulation, and respiration--belong to the cla.s.s of functions known as _vegetative_ functions. That is, they are common to vegetables as well as animals; for the plant, like the animal, can originate nothing, not even the smallest particle of matter; and yet it grows, blossoms, and bears fruit, by reason of obtaining and digesting the nutriment which the air and soil provide.

The plant has its circulatory fluid and channels, by which the nutriment is distributed to all its parts. It has, also, a curious apparatus in its foliage, by which it abstracts from the air those gaseous elements so necessary to its support; and thus it accomplishes vegetable respiration.

These vegetative functions have their beginning and end within the organism of the plant; and their object is the preservation of the plant itself, as well as of the entire species.

[Sidenote: 2. What second set of powers has the animal? What functions are mentioned? The advantage they give?]

2. The animal, in addition to these vegetative functions, has another set of powers, by the use of which he becomes conscious of a world external to himself, and brings {149} himself into active relations with it. By means of the vegetative processes, his life and species are maintained; while, by means of certain animal functions, he feels, acts, and thinks. These functions, among which are sensation, motion, and volition, not only distinguish the animal from the plant, but, in proportion to their development, elevate one creature above another; and it is by virtue of his pre-eminent endowment, in these respects, that man holds his position at the head of the animal creation.

[Sidenote: 3. Animals whose structure is simple? As we approach man?

Dependence of the animal functions of man?]

3. Among animals whose structure is very simple, the hydra, or fresh-water polyp, being an example, no special organs are empowered to perform separate functions; but every part is endowed alike, so that if the animal be cut into pieces, each portion has all the properties of the entire original; and, if the circ.u.mstances be favorable, each of the pieces will soon become a complete hydra. As we approach man, in the scale of beings, we find that the organs multiply, and the functions become more complete.

The function of motion, the instruments of which--the muscles and bones--have been considered in former chapters, and all the other animal functions of man, depend upon the set of organs known as the nervous system.

[Sidenote: 4. The nervous tissues, of what composed? When examined by the aid of the microscope? The white substance? The gray substance?]

4. THE NERVOUS SYSTEM.--The intimate structure of this system differs from any tissue which we have before examined. It is composed of a soft, pulpy substance, which, early in life, is almost fluid, but which gradually hardens with the growth of the body. When examined under the microscope, it is found to be composed of two distinct elements:--(1) the white substance, composing the larger proportion of the nervous organs of the body, which is formed of delicate cylindrical filaments, about 1/6000 of an inch in diameter, termed the nerve-fibres; and (2) the gray substance, composed of grayish-red, or {150} ashen-colored cells, of various sizes, generally possessing one or more off-shoots, which are continuous with the nerve-fibres just mentioned.

[Sidenote: 5. Nervous centres and ganglia? Nerves? What do they serve?

Cerebro-spinal system?]