212. The Nervous Control of Respiration. It is a matter of common experience that one's breath may be held for a short time, but the need of fresh air speedily gets the mastery, and a long, deep breath is drawn.

Hence the efforts of criminals to commit suicide by persistent restraint of their breathing, are always a failure. At the very worst, unconsciousness ensues, and then respiration is automatically resumed.

Thus a wise Providence defeats the purpose of crime. The movements of breathing go on without our attention. In sleep the regularity of respiration is even greater than when awake. There is a particular part of the nervous system that presides over the breathing function. It is situated in that part of the brain called the medulla oblongata, and is fancifully called the "vital knot" (sec. 270). It is injury to this respiratory center which proves fatal in cases of broken neck.

From this nerve center there is sent out to the nerves that supply the diaphragm and other muscles of breathing, a force which stimulates them to regular contraction. This breathing center is affected by the condition of the blood. It is stimulated by an excess of carbon dioxid in the blood, and is quieted by the presence of oxygen.

Experiment 108. _To locate the lungs_. Mark out the boundaries of the lungs by "sounding" them; that is, by _percussion_, as it is called.

This means to put the forefinger of the left hand across the chest or back, and to give it a quick, sharp rap with two or three fingers. Note where it sounds hollow, resonant. This experiment can be done by the student with only imperfect success, until practice brings some skill.

Experiment 109. Borrow a stethoscope, and listen to the respiration over the chest on the right side. This is known as _auscultation_. Note the difference of the sounds in inspiration and in expiration. Do not confuse the heart sounds with those of respiration. The respiratory murmurs may be heard fairly well by applying the ear flat to the chest, with only one garment interposed.

Experiment 110. Get a sheep's lungs, with the windpipe attached.

Ask for the heart and lungs all in one ma.s.s. Take pains to examine the specimen first, and accept only a good one. Parts are apt to be hastily snipped or mangled. Examine the windpipe. Note the horseshoe-shaped rings of cartilage in front, which serve to keep it open.

Experiment 111. Examine one bronchus, carefully dissecting away the lung tissue with curved scissors. Follow along until small branches of the bronchial tubes are reached. Take time for the dissection, and save the specimen in dilute alcohol. Put pieces of the lung tissue in a basin of water, and note that they float.

The labored breathing of suffocation and of lung diseases is due to the excessive stimulation of this center, caused by the excess of carbon dioxid in the blood. Various mental influences from the brain itself, as the emotions of alarm or joy or distress, modify the action of the respiratory center.

Again, nerves of sensation on the surface of the body convey influences to this nerve center and lead to its stimulation, resulting in a vigorous breathing movement. Thus a dash of cold water on the face or neck of a fainting person instantly produces a deep, long-drawn breath. Certain drugs, as opium, act to reduce the activity of this nerve center. Hence, in opium poisoning, special attention should be paid to keeping up the respiration. The condition of the lungs themselves is made known to the breathing center, by messages sent along the branches of the great pneumogastric nerve (page 276), leading from the lungs to the medulla oblongata.

213. Effects of Respiration upon the Blood. The blood contains three gases, partly dissolved in it and partly in chemical union with certain of its const.i.tuents. These are oxygen, carbon dioxid, and nitrogen.

The latter need not be taken into account. The oxygen is the nourishing material which the tissues require to carry on their work. The carbon dioxid is a waste substance which the tissues produce by their activity, and which the blood carries away from them.

As before shown, the blood as it flows through the tissues loses most of its oxygen, and carbon dioxid takes its place. Now if the blood is to maintain its efficiency in this respect, it must always be receiving new supplies of oxygen, and also have some mode of throwing off its excess of carbon dioxid. This, then, is the double function of the process of respiration. Again, the blood sent out from the left side of the heart is of a bright scarlet color. After its work is done, and the blood returns to the right side of the heart, it is of a dark purple color. This change in color takes place in the capillaries, and is due to the fact that there the blood gives up most of its oxygen to the tissues and receives from them a great deal of carbon dioxid.

In brief, while pa.s.sing through the capillaries of the lungs the blood has been changed from the venous to the arterial blood. That is to say, the blood in its progress through the lungs has rid itself of its excess of carbon dioxid and obtained a fresh supply of oxygen.[36]

214. Effects of Respiration upon the Air in the Lungs. It is well known that if two different liquids be placed in a vessel in contact with each other and left undisturbed, they do not remain separate, but gradually mix, and in time will be perfectly combined. This is called diffusion of liquids. The same thing occurs with gases, though the process is not visible. This is known as the diffusion of gases. It is also true that two liquids will mingle when separated from each other by a membrane (sec. 129). In a similar manner two gases, especially if of different densities, may mingle even when separated from each other by a membrane.

In a general way this explains the respiratory changes that occur in the blood in the lungs. Blood containing oxygen and carbon dioxid is flowing in countless tiny streams through the walls of the air cells of the lungs.

The air cells themselves contain a mixture of the same two gases. A thin, moist membrane, well adapted to allow gaseous diffusion, separates the blood from the air. This membrane is the delicate wall of the capillaries and the epithelium of the air cells. By experiment it has been found that the pressure of oxygen in the blood is less than that in the air cells, and that the pressure of carbon dioxid gas in the blood is greater than that in the air cells. As a result, a diffusion of gases ensues. The blood gains oxygen and loses carbon dioxid, while the air cells lose oxygen and gain the latter gas.

[Ill.u.s.tration: Fig. 92.--Capillary Network of the Air Cells and Origin of the Pulmonary Veins.

A, small branch of pulmonary artery; B, twigs of the pulmonary artery anastomosing to form peripheral network of the primitive air cells; C, capillary network around the walls of the air sacs; D, branches of network converging for form the veinlets of the pulmonary veins.

The blood thus becomes purified and reinvigorated, and at the same time is changed in color from purple to scarlet, from venous to arterial. It is now evident that if this interchange is to continue, the air in the cells must be constantly renewed, its oxygen restored, and its excess of carbon dioxid removed. Otherwise the process just described would be reversed, making the blood still more unfit to nourish the tissues, and more poisonous to them than before.

215. Change in the Air in Breathing. The air which we exhale during respiration differs in several important particulars from the air we inhale. Both contain chiefly the three gases, though in different quant.i.ties, as the following table shows.

Oxygen. Nitrogen. Carbon Dioxid.

Inspired air contains 20.81 79.15 .04 Expired air contains 16.03 79.58 4.38

That is, expired air contains about five per cent less oxygen and five per cent more carbon dioxid than inspired air.

The temperature of expired air is variable, but generally is higher than that of inspired air, it having been in contact with the warm air pa.s.sages. It is also loaded with aqueous vapor, imparted to it like the heat, not in the depth of the lungs, but in the upper air pa.s.sages.

Expired air contains, besides carbon dioxid, various impurities, many of an unknown nature, and all in small amounts. When the expired air is condensed in a cold receiver, the aqueous product is found to contain organic matter, which, from the presence of _micro-organisms_, introduced in the inspired air, is apt to putrefy rapidly. Some of these organic substances are probably poisonous, either so in themselves, as produced in some manner in the breathing apparatus, or poisonous as being the products of decomposition. For it is known that various animal substances give rise, by decomposition, to distinct poisonous products known as _ptomaines_. It is possible that some of the const.i.tuents of the expired air are of an allied nature. See under "Bacteria" (Chapter XIV).

At all events, these substances have an injurious action, for an atmosphere containing simply one per cent of pure carbon dioxid has very little hurtful effect on the animal economy, but an atmosphere in which the carbon dioxid has been raised one per cent by breathing is highly injurious.

The quant.i.ty of oxygen removed from the air by the breathing of an adult person at rest amounts daily to about 18 cubic feet. About the same amount of carbon dioxid is expelled, and this could be represented by a piece of pure charcoal weighing 9 ounces. The quant.i.ty of carbon dioxid, however, varies with the age, and is increased also by external cold and by exercise, and is affected by the kind of food. The amount of water, exhaled as vapor, varies from 6 to 20 ounces daily. The average daily quant.i.ty is about one-half a pint.

216. Modified Respiratory Movements. The respiratory column of air is often used in a mechanical way to expel bodies from the upper air pa.s.sages. There are also, in order to secure special ends, a number of modified movements not distinctly respiratory. The following peculiar respiratory acts call for a few words of explanation.

A sigh is a rapid and generally audible expiration, due to the elastic recoil of the lungs and chest walls. It is often caused by depressing emotions. Yawning is a deep inspiration with a stretching of the muscles of the face and mouth, and is usually excited by fatigue or drowsiness, but often occurs from a sort of contagion.

Hiccough is a sudden jerking inspiration due to the spasmodic contraction of the diaphragm and of the glottis, causing the air to rush suddenly through the larynx, and produce this peculiar sound. Snoring is caused by vibration of the soft palate during sleep, and is habitual with some, although it occurs with many when the system is unusually exhausted and relaxed.

Laughing consists of a series of short, rapid, spasmodic expirations which cause the peculiar sounds, with characteristic movements of the facial muscles. Crying, caused by emotional states, consists of sudden jerky expirations with long inspirations, with facial movements indicative of distress. In sobbing, which often follows long-continued crying, there is a rapid series of convulsive inspirations, with sudden involuntary contractions of the diaphragm. Laughter, and sometimes sobbing, like yawning, may be the result of involuntary imitation.

Experiment 112. _Simple Apparatus to Ill.u.s.trate the Movements of the Lungs in the Chest_.--T is a bottle from which the bottom has been removed; D, a flexible and elastic membrane tied on the bottle, and capable of being pulled out by the string S, so as to increase the capacity of the bottle. L is a thin elastic bag representing the lungs.

It communicates with the external air by a gla.s.s tube fitted air-tight through a cork in the neck of the bottle. When D is drawn down, the pressure of the external air causes L to expand. When the string is let go, L contracts again, by virtue of its elasticity.

[Ill.u.s.tration: Fig. 93.]

Coughing is produced by irritation in the upper part of the windpipe and larynx. A deep breath is drawn, the opening of the windpipe is closed, and immediately is burst open with a violent effort which sends a blast of air through the upper air pa.s.sages. The object is to dislodge and expel any mucus or foreign matter that is irritating the air pa.s.sages.

Sneezing is like coughing; the tongue is raised against the soft palate, so the air is forced through the nasal pa.s.sages. It is caused by an irritation of the nostrils or eyes. In the beginning of a cold in the head, for instance, the cold air irritates the inflamed mucous membrane of the nose, and causes repeated attacks of sneezing.

217. How the Atmosphere is Made Impure. The air around us is constantly being made impure in a great variety of ways. The combustion of fuel, the respiration of men and animals, the exhalations from their bodies, the noxious gases and effluvia of the various industries, together with the changes of fermentation and decomposition to which all organized matter is liable,--all tend to pollute the atmosphere.

The necessity of external ventilation has been foreseen for us. The forces of nature,--the winds, sunlight, rain, and growing vegetation,--all of great power and universal distribution and application, restore the balance, and purify the air. As to the princ.i.p.al gases, the air of the city does not differ materially from that of rural sections. There is, however, a vastly greater quant.i.ty of dust and smoke in the air of towns.

The breathing of this dust, to a greater or less extent laden with bacteria, fungi, and the germs of disease, is an ever-present and most potent menace to public and personal health. It is one of the main causes of the excess of mortality in towns and cities over that of country districts.

This is best shown in the overcrowded streets and houses of great cities, which are deprived of the purifying influence of sun and air. The fatal effect of living in vitiated air is especially marked in the mortality among infants and children living in the squalid and overcrowded sections of our great cities. The salutary effect of sunshine is shown by the fact that mortality is usually greater on the shady side of the street.

218. How the Air is Made Impure by Breathing. It is not the carbon dioxid alone that causes injurious results to health, it is more especially the organic matter thrown off in the expired air. The carbon dioxid which accompanies the organic matter is only the index. In testing the purity of air it is not difficult to ascertain the amount of carbon dioxid present, but it is no easy problem to measure the amount of organic matter. Hence it is the former that is looked for in factories, churches, schoolrooms, and when it is found to exceed .07 per cent it is known that there is a hurtful amount of organic matter present.

The air as expelled from the lungs contains, not only a certain amount of organic matter in the form of vapor, but minute solid particles of _debris_ and bacterial micro-organisms (Chap. XIV). The air thus already vitiated, after it leaves the mouth, putrefies very rapidly. It is at once absorbed by clothing, curtains, carpets, porous walls, and by many other objects. It is difficult to dislodge these enemies of health even by free ventilation. The close and disagreeable odor of a filthy or overcrowded room is due to these organic exhalations from the lungs, the skin, and the unclean clothing of the occupants.

The necessity of having a proper supply of fresh air in enclosed places, and the need of removal of impure air are thus evident. If a man were shut up in a tightly sealed room containing 425 cubic feet of air, he would be found dead or nearly so at the end of twenty-four hours.

Long before this time he would have suffered from nausea, headache, dizziness, and other proofs of blood-poisoning. These symptoms are often felt by those who are confined for an hour or more in a room where the atmosphere has been polluted by a crowd of people. The unpleasant effects rapidly disappear on breathing fresh air.

219. The Effect on the Health of Breathing Foul Air. People are often compelled to remain indoors for many hours, day after day, in shops, factories, or offices, breathing air perhaps only slightly vitiated, but still recognized as "stuffy." Such persons often suffer from ill health.

The exact form of the disturbance of health depends much upon the hereditary proclivity and physical make-up of the individual. Loss of appet.i.te, dull headache, fretfulness, persistent weariness, despondency, followed by a general weakness and an impoverished state of blood, often result.

Persons in this lowered state of health are much more p.r.o.ne to surfer from colds, catarrhs, bronchitis, and pneumonia than if they were living in the open air, or breathing only pure air. Thus, in the Crimean War, the soldiers who lived in tents in the coldest weather were far more free from colds and lung troubles than those who lived in tight and ill-ventilated huts. In the early fall when typhoid fever is prevalent, the grounds of large hospitals are dotted with canvas tents, in which patients suffering from this fever do much better than in the wards.

This tendency to inflammatory diseases of the air pa.s.sages is aggravated by the overheated and overdried condition of the air in the room occupied.

This may result from burning gas, from overheated furnaces and stoves, hot-water pipes, and other causes. Serious lung diseases, such as consumption, are more common among those who live in damp, overcrowded, or poorly ventilated homes.

220. The Danger from Pulmonary Infection. The germ of pulmonary consumption, known as the bacillus tuberculosis, is contained in the breath and the sputa from the lungs of its victims. It is not difficult to understand how these bacilli may be conveyed through the air from the lungs of the sick to those of apparently healthy people. Such persons may, however, be predisposed, either const.i.tutionally or by defective hygienic surroundings, to fall victims to this dreaded disease. Overcrowding, poor ventilation, and dampness all tend to increase the risk of pulmonary infection.

It must not be supposed that the tubercle bacillus is necessarily transmitted directly through the air from the lungs of the sick to be implanted in the lungs of the healthy. The germs may remain for a time in the dust turn and _debris_ of damp, filthy, and overcrowded houses. In this congenial soil they retain their vitality for a long time, and possibly may take on more virulent infective properties than they possessed when expelled from the diseased lungs.[37]

[Ill.u.s.tration: Fig. 94. Example of a Micro-Organism--Bacillus Tuberculosis in Spotum. (Magnified about 500 diameters.)]