THE BONES.--The bones const.i.tute the frame-work of the system. They consist of two substances, being formed of both _animal_ and _earthy_ matter. To the former belongs every thing connected with their _life_ and _growth_, while the latter gives to them _solidity_ and _strength_.

The proportions of the animal and earthy elements of which the bones are composed vary at different ages. In childhood and early youth, when but _little strength_ is needed, and _great growth_ of bone is required, the animal part preponderates. As growth advances the animal part _decreases_, and the earthy part _increases_. In middle life, when growth is finished and the strength is greatest, and when nutrition is required only to repair waste, the proportions are changed, and the solid or earthy part exceeds the vital or animal; and in extreme old age, the earthy part so predominates as to cause the bones to become very brittle.

The bones, like other parts of the system, require exercise. If properly used, they increase in size and strength. But while a due degree of exercise is beneficial, it ought to be remarked that severe and continued labor should not be required of children and youth; for its tendency is to increase the deposition of earthy matter to a hurtful extent. It is by this means that many children are made dwarfs for life, their bones being consolidated by an undue amount of exercise and excessive labor before they have attained their full growth. Mult.i.tudes of children in our country, from this and kindred causes, fail of attaining the size of their ancestors. These remarks may be turned to a practical account in the family and in the school. At birth, many of the bones are scarcely more than cartilage; yet children are frequently urged to stand and walk long before the bones become sufficiently strong to sustain the pressure; and, as a consequence, their legs become crooked, and they are perhaps other ways deformed for life. Children ought always, when seated, to be able to rest their feet upon the floor.

When they occupy a seat that is too high, and especially when they are unable to reach their feet to the floor, the thigh bones very frequently become curved. If, in addition to high seats, the back is not supported, children become round shouldered, their chests contract, their const.i.tutions become permanently enfeebled, and they become peculiarly susceptible to pulmonary disease. The back to the seat should afford a pleasant and agreeable support to the small of the back, but it ought not to reach to the shoulder blades.

Parents and teachers should never forget that children are as susceptible to physical training as to intellectual or moral culture.

And here, especially, they should be "trained _up_ in the way they should go." Physical uprightness is next to moral. If children are allowed to contract bad physical habits, they are liable not only to grow crooked, but to become deformed in various ways. But so great is the power of education, that by it even the physically crooked may be made straight; the chest may be enlarged, the general health may be improved, and much may be done in many ways to fortify those who have inherited feeble const.i.tutions against the attacks of disease. The benefits resulting from maintaining an upright form, and a free and open chest, have already been considered, and I shall have occasion to refer to them again. The chest of most adults, although _incased with bone_, may be increased several inches by drawing the arms back in the use of _nature's own shoulder-braces_, and at the same time taking deep inhalations of air, and filling the lungs to their utmost capacity.

Hundreds of individuals in different parts of the country have borne testimony to the efficacy of this treatment in the improvement of their health. The good results of such discipline in childhood are still more manifest.

A stooping posture is frequently induced by sitting at tables and desks that are too low. It has been erroneously maintained by some that the top of the desk should be on the same plane with the elbow when the arm hangs by the side. When the desk is higher, it has been said the tendency is to elevate one shoulder, to depress the other, and to produce a permanent curvature of the spinal column. Although this may have been frequently the result of sitting at a high desk, yet it is not a _necessary result_. To prevent the projection of one shoulder, and the consequent spinal curvature, _both of the arms must be kept on the same level_. For this purpose, there should be room to support them equally; and care should be taken to see that this support is regularly sought.

If this be not done, the right arm will be apt to rise above the left, from its more constant use and elevation. A physician, highly celebrated for the success that has attended his treatment for lung affections, after dwelling upon the injury to the health that frequently results from sitting at too low desks, remarks, that "every parent should go to the school-rooms, and know for a certainty that the desks at which his children write or study are fully up to the arm-pits, and in no case allow them to sit stooping, or leaning the shoulders forward on the chest. If fatigued by this posture, they should be called to stand, or go out of doors and run about." The height of table I find most conducive to comfort for my own use is midway between the two; that is, half way from the elbow (as the arm hangs by the side) to the arm-pit.

It is necessary, however, to rest both arms equally upon the table. The secret of posture consists in avoiding all bad positions, and in not continuing any one position too long. The ordinary carriage of the body is an object worthy of the attention of every parent and instructor. The more favorable impression which a man of erect and commanding att.i.tude is sure to make, should not be overlooked. But there is a greater good than this; for he who _walks erect_, enjoys better health, possesses increased powers of usefulness, realizes more that _he is a man_, and has more to call forth grat.i.tude to a beneficent Creator, than he who adopts an _oblique_ posture. It was just remarked that "physical uprightness is next to moral." Physical _obliquity_, it may be added, is akin to _moral_. If they are not German-cousins, there can be little doubt but that, considered in all its bearings, the tendency of the former is to induce the latter.

Important as an erect posture and a well-developed chest are to gentlemen, they are in some respects even more so to the fairer s.e.x; for, in addition to the advantages already considered, which both enjoy in common, these impart to them a peculiar charm, that to men of sense is far greater than pretty faces, which Nature has not given to all.

"For a great number of years, it has been the custom in France to give young females, of the earliest age, the habit of holding back the shoulders, and thus expanding the chest. From the observations of anatomists lately made, it appears that the clavicle or collar bone is actually longer in females of the French nation than in those of the English. As the two nations are of the same race, as there is no remarkable difference in their bones, and this is peculiar to the s.e.x, it must be attributed, as I believe, to the habit above mentioned, which, by the extension of the arms, has gradually produced an elongation of this bone. Thus we see that habit may be employed to alter and improve the solid bones. The French have succeeded in the development of a part in a way that adds to health and beauty, and increases a characteristic that distinguishes the human being from the brute."[9]

[9] Quoted into the Schoolmaster (a work published in London under the superintendence of the Society for the Diffusion of Useful Knowledge) from a lecture delivered by Dr. J. C. Warren before the American Inst.i.tute of Instruction, August, 1830.

THE MUSCLES.--The muscles consist of compact bundles of fleshy fibers, which are found in animals on removing the skin. They const.i.tute the red fleshy part of meat, and give form and symmetry to the body. In the limbs they surround and protect the bones, while in the trunk they spread out and const.i.tute a defensive wall for the protection of the vital parts beneath. The muscles have been divided into _three parts_, of which the middle and fleshy portion, called the _belly_, is most conspicuous. The other two parts are the opposite ends, and are commonly called the _origin_ and _insertion_ of the muscle. The _origin_ is usually fastened to one bone, and the _insertion_ is attached to another. By the contraction of the _belly_ of the muscle, the _insertion_, which is _movable_, is drawn toward the _origin_, which is _fixed_, and brings with it the bone to which it is attached. This any one can see ill.u.s.trated in bending the arm. The muscle which performs this function lies between the elbow and the shoulder. It is attached to the shoulder by its _origin_, and to one of the bones of the fore-arm, just below the elbow, by its _insertion_. By grasping the arm midway between the shoulder and the elbow with the opposite hand, and then bending the arm, the enlargement of the belly of the muscle by the contraction will be at once perceived. Then, by moving the hand down on the inside of the arm toward the elbow, the lessening muscle may be readily traced until it terminates in a _tendon_, of much less size than the muscle, but of great strength, which is inserted into the bone just below the elbow. As the fore-arm is drawn up, and especially if there be a weight in the hand, the _tendon_ may be felt just within the elbow-joint, running toward the point of insertion. Extend the arm at the elbow, and the muscle on the outside of the arm will swell and become firm, while the inside muscle, and its tendon at the elbow, will be relaxed. This example well ill.u.s.trates the principle on which all the joints of the system are moved. Those who are acquainted with mechanics will readily perceive that the action just described is an example of the "_third_ kind of lever," where the power is applied between the weight and the fulcrum. The elbow is the fulcrum, the hand contains the weight, and the tendon, inserted into the bone just below the elbow, is the power. This kind of lever requires the power to be greater than the weight, and acts under what is called a _mechanical disadvantage_. What is lost in power, however, is compensated in increased velocity.

There are upward of four hundred muscles in the human body. Some of these are _voluntary_ in their motions, as those I have described, while others are _involuntary_, as the action of the heart and the respiratory muscles. Had the action of these depended upon the will, as does the action of the muscles of locomotion, the circulation of the blood and the process of breathing would cease, and life would become extinct whenever sleep or any other cause should overcome the attention. Here, then, we have another beautiful ill.u.s.tration of the wisdom and beneficence of the Creator in so ordering that those muscles which are essential to the continuation of life shall perform their functions without the control or attention of the individual.

The study of the muscular system involves an exposition of the principles by which exercise should be regulated, and can scarcely fail to excite the attention of the general reader, and especially of those who, as parents or teachers, are interested in the education of the young.

The muscles enable us to move the frame-work of the system. Their chief purpose obviously is to enable us to carry into effect the various resolutions and designs which have been formed by the mind. But, while fulfilling this grand object, their active exercise is, at the same time, highly conducive to the well-being of many other important functions. By muscular contraction, the blood is gently a.s.sisted in its course through the smaller vessels to the more distant parts of the body; and by it the important processes of digestion, respiration, secretion, absorption, and nutrition are promoted; and by it the health of the whole body is immediately and greatly influenced. The mind itself is exhilarated or depressed by the proper or improper use of muscular exercise. It thus becomes a point of no slight importance to establish general principles by which that exercise may be regulated.

In every part of the animal economy, the muscles are proportioned in size and structure to the efforts required of them. Whenever a muscle is called into frequent use, its fibers increase in thickness within certain limits, and become capable of acting with greater force and readiness. On the other hand, when a muscle is little used, its volume and power decrease in a corresponding degree.

In order to secure the most beneficial results from exercise, reference should be had to the time at which it is taken. Those who are in perfect health may engage in it at almost any hour except immediately after a meal; but those who are not robust ought to confine their hours of exercise within narrower limits. To a person in full vigor, a good walk, or other brisk exercise before breakfast may be highly beneficial and exhilarating, while to an invalid or delicate person it will be likely to prove detrimental. In order to prove beneficial, exercise must be resorted to only when the system is sufficiently vigorous to be able to meet it. This is usually the case after a lapse of from two to four hours after a moderate meal. The forenoon, then, will generally be found the best time for exercise for persons whose habits are sedentary. If exercise be delayed till the system feels exhaustion from want of food, its tendency will be to dissipate the strength that remains and impair digestion; while, if taken at the proper time, it will invigorate the system and promote digestion. The reasons are obvious; for exercise of every kind causes increased action and waste in the organ, and if there be not materials and vigor enough in the system to keep up that action and supply the waste, nothing but increased debility can reasonably be expected.

Active exercise immediately _before_ meals is injurious. The reasons are apparent, for muscular exercise directs a flow of blood and nervous energy to the surface and extremities; and it is an established law in physiology, that energetic action can not be kept up in two distant parts of the system at the same time. Hence, whenever a meal is taken immediately after vigorous exercise, the stomach is taken at disadvantage, and, from want of the necessary action in its vessels and nerves, is unable to carry on digestion with success. This is very obviously the case where the exercise has been severe or protracted.

Active exercise ought to be equally avoided immediately _after_ a heavy meal, for then the functions of the digestive organs are in the highest state of activity. If the muscular system be called into vigorous action under such circ.u.mstances, it will cause a withdrawal of the vital stimuli of the blood and nervous influence from the stomach to the extremities, which can not fail greatly to r.e.t.a.r.d the digestive process.

In accordance with this well-established fact, there is a natural and marked aversion to active pursuits after a full meal. A mere stroll, which requires no exertion and does not fatigue, will not be injurious before or after eating; but exercise beyond this limit is at such times hurtful. All, therefore, who would preserve and improve their health, will find it to their advantage to observe faithfully this important law, otherwise they will deprive themselves of most of the benefits that are usually attendant upon judicious exercise. All, then, who are forced to much exertion immediately after eating, should satisfy themselves with partaking of a very moderate meal. These remarks apply to both physical and mental exercise; for if the intellect be intently occupied in profound and absorbing thought, the nervous energy will be concentrated in the brain, and any demands made on it by the stomach or muscles will be very imperfectly attended to. So, also, if the stomach be actively engaged in digesting a full meal, and some subject of thought be presented to the mind, considerable difficulty will be felt in pursuing it, and most probably both thought and digestion will be disturbed.

Another law of the muscular system requires that relaxation and contraction should alternate; or, in other words, that rest should follow exercise. In accordance with this law, it is easier to walk than to stand; and in standing, it is easier to change from one foot to the other than to stand still. To require a child to extend his arm and hold a book in his hand, or even to keep the arm extended but a short time, is a violation of this law which should never be permitted. Akin to this is the very injudicious practice, which is sometimes resorted to in schools, of requiring a boy to stoop over, and, placing his finger upon a nail in the floor, "hold it in." Teachers who are disposed to inflict punishments like these ought first to try the experiment themselves.

Such protracted tension of the muscles enfeebles their action, and ultimately destroys their power of contraction.

These remarks sufficiently explain why small children, after sitting a while in school, become restless. Proper regard for this organic law requires that the smaller children in school be allowed a recess as often, at least, as once an hour; and that all be allowed and encouraged frequently to change their position. I fully concur in the opinion expressed by Dr. Caldwell, who says, "It would be infinitely wiser and better to employ suitable persons to superintend the exercises and amus.e.m.e.nts of children under seven years of age, in the fields, orchards, and meadows, and point out to them the richer beauties of nature, than to have them immured in crowded school-rooms, in a state of inaction, poring over torn books and primers, conning words of whose meaning they are ignorant, and breathing foul air."

A change of position calls into action a different set of muscles, and relieves those that are exhausted. The object of exercise is to employ all the muscles of the body, and especially to strengthen those that are weak. It ought hence to be frequently varied, and always adapted to the peculiarities of individuals. Different kinds of exercise will therefore be found to suit different const.i.tutions. Sedentary persons best enjoy, and will be most profited by, that kind of exercise which brings into action the greatest number of muscles.

To give exercise its greatest value, it should be taken at the same hour every day. This is well-nigh as important as the rule that requires meals to be taken regularly. If exercise be taken irregularly, one day in the morning, another day at noon, and another day at night, if at all, it is possible that good may result from it, but its beneficial effects would be greatly increased if the same amount of exercise were taken every day at the same hours. Give the system an opportunity of establishing _good habits_ in this respect, and it will derive great advantage from them; but it is difficult for it to derive any benefit from a _habit of irregularity_, if such may be called a habit. Students, teachers, and all persons who lead sedentary lives, should have their regular times for exercise as well as for meals, and if they find it necessary to do without one, they will generally find it advantageous to dispense with the other also.

Walking, it has been said, agrees with every body. But as it brings into play chiefly the lower limbs and muscles of the loins, and affords little scope for the play of the arms and muscles of the chest, it is of itself insufficient to const.i.tute adequate exercise. To render it most beneficial, the shoulders should be drawn back, and the chest should be enlarged by taking deep inspirations of pure air. The muscles of the chest, and of every part of the body, should be free to move and unconfined by tight clothing. Fencing, shuttlec.o.c.k, and such other useful sports as combine with them free movements of the upper part of the body, are doubly advantageous, for they not only exercise the muscles of the whole body, but possess the additional advantage of animating the mind and increasing the nervous stimulus, by which exercise is rendered easy, pleasant, and invigorating. For the purpose of developing the chest, physiologists generally concur in recommending _fencing_ as a good exercise for boys. Shuttlec.o.c.k is a very beneficial exercise for females, calling into play, as it does, the muscles of the chest, trunk, and arms. It ought to be practiced in the open air. When played with both hands, as it may be after a little practice, it is very useful in preventing curvature, and in giving vigor to the spine. It is an excellent plan to play with a battledore in each hand, and to strike with them alternately. The graces is another play well adapted for expanding the chest, and giving strength to the muscles of the back, and has the advantage of being practicable in the open air. It is very important that the muscles of the back be strengthened by due exercise, for their proper use contributes to both health and beauty.

When managed with due regard to the natural powers of the individual, and so as to avoid effort and fatigue, _reading aloud_ becomes a very useful and invigorating exercise. In forming and undulating the voice, not only the chest, but also the diaphragm and abdominal muscles are in constant action, and communicate to the stomach and bowels a healthy and agreeable stimulus. Where the voice is raised and the elocution is rapid, the muscular effort becomes fatiguing; but when care is taken not to carry reading aloud so far at one time as to excite a sensation of soreness or fatigue in the chest, and the exercise is duly repeated, it is extremely useful in developing and giving tone to the organs of respiration and to the general system.

"Vocal music is also very useful, by its direct effect on the const.i.tution. It was the opinion of Dr. Rush, that young ladies especially, who, by the custom of society, are debarred from many kinds of salubrious exercise, should cultivate singing, not only as an accomplishment, but as a means of preserving health. He particularly insists that it should never be neglected in the education of females; and states that, besides its salutary operation in enabling them to soothe the cares of domestic life, and quiet sorrow by the united a.s.sistance of the sound and sentiment of a properly chosen song, it has a still more direct and important effect. 'I here introduce a fact,' he remarks, 'which has been suggested to me by my profession, and that is, that the exercise of the organs of the breast by singing contributes very much to defend them from those diseases to which the climate and other causes expose them. The Germans are seldom afflicted with consumption, nor have I ever known but one instance of spitting blood among them. This, I believe, is in part occasioned by the strength which their lungs acquire by exercising them frequently in vocal music, for this const.i.tutes an essential branch of their education. The music-master of our academy has furnished me with an observation still more in favor of this opinion. He informed me that he had known several instances of persons who were strongly disposed to consumption, who were restored to health by the exercise of their lungs in singing.'"[10]

[10] Mr. Woodbridge's lecture before the American Inst.i.tute of Instruction, 1830.

Bathing or ablution, when conducted as recommended on pages 60 and 61, is not only a means of cleanliness and of exciting a healthy action in the skin, but it const.i.tutes, at the same time, a most _admirable exercise_. If a lodging-room has been properly ventilated by leaving open windows, or otherwise, so that the air is pure and healthful in the morning, ten or fifteen minutes spent in bathing and friction, with a proper exercise of the muscles of the back and abdomen, will contribute more to invigorate the system and promote the general health than twice the amount of exercise taken at any other time or in any other way.

From the foregoing remarks, it appears that the most perfect of all exercises are those which combine the free play of all the muscles of the body, mental interest and excitement, and the unrestrained use of the voice.

CHAPTER IV.

THE LAWS OF HEALTH. PHILOSOPHY OF RESPIRATION.

We instinctively shun approach to the dirty, the squalid, and the diseased, and use no garment that may have been worn by another. We open sewers for matters that offend the sight or the smell, and contaminate the air. We carefully remove impurities from what we eat and drink, filter turbid water, and fastidiously avoid drinking from a cup that may have been pressed to the lips of a friend. On the other hand, we resort to places of a.s.sembly, and draw into our mouths air loaded with effluvia from the lungs, skin, and clothing of every individual in the promiscuous crowd--exhalations offensive, to a certain extent, from the most healthy individuals; but when arising from a living ma.s.s of skin and lungs in all stages of evaporation, disease, and putridity, they are in the highest degree deleterious and loathsome.--BIRNAN.

Respiration is usually defined as the process by which air is taken into the lungs and expelled from them. It explains the changes that take place in these organs, in the conversion of _chyle_ and _venous_, or worn-out blood, into _arterial_ or nutrient blood. In order to be clearly understood, I must premise a few observations on the circulation of the blood.[11] The blood circulating through the body is of two different kinds; the one _red_ or _arterial_, and the other _dark_ or _venous_ blood. The former alone is capable of affording nourishment and supporting life. It is distributed from the _left_ side of the heart all over the body by means of a great _artery_, which subdivides in its course, and ultimately terminates in myriads of very minute ramifications closely interwoven with, and in reality const.i.tuting a part of, the texture of every living part. On reaching this extreme point of its course, the blood pa.s.ses into equally minute ramifications of the _veins_, which in their turn gradually coalesce, and form larger and larger trunks, till they at last terminate in two large veins, by which the whole current of the venous blood is brought back in a direction contrary to that of the blood in the arteries, and poured into the _right_ side of the heart. On examining the quality of the blood in the arteries and veins, it is found to have undergone a great change in its pa.s.sage from the one to the other. The florid hue which distinguished it in the arteries has disappeared, and given place to the dark color characteristic of venous blood. Its properties, too, have changed, and it is now no longer capable of sustaining life.

[11] Taken, with slight alterations, from the description of Dr. A.

Combe.

Two conditions are essential to the reconversion of venous into arterial blood, and to the restoration of its vital properties. The first is an adequate provision of _new materials_ from the _food_ to supply the place of those which have been expended in nutrition, and the second is the free exposure of the _venous blood_ to the _atmospheric air_. The first condition is fulfilled by the chyle, or nutrient portion of the food, being regularly poured into the venous blood just before it reaches the right side of the heart, and the second by the important process of _respiration_, which takes place in the air-cells of the lungs. The venous blood, having arrived at the right side of the heart, is propelled by the contraction of that organ into a large artery, leading directly, by separate branches, to the two lungs, and hence called the _pulmonary_ artery. In the innumerable branches of this artery expanding themselves throughout the substance of the lungs, the dark blood is subjected to the contact of the air inhaled in breathing, and a change in the composition both of the blood and of the inhaled air takes place, in consequence of which the former is found to have rea.s.sumed its florid or arterial hue, and to have regained its power of supporting life. The blood then enters minute venous ramifications, which gradually coalesce into larger branches, and at last terminate in four large trunks in the left side of the heart, whence the blood, in its arterial form, is again distributed over the body, to pursue the same course and undergo the same change as before.

It will be perceived that there are two distinct circulations, each of which is carried on by its own system of vessels. The one is from the _left_ side of the _heart_ to _every part of the body_, and back to the _right_ side of the _heart_. The other is from the _right_ side of the _heart_ to the _lungs_, and back to the _left_ side of the _heart_. The former has for its object nutrition and the maintenance of life; and the latter, the restoration of the deteriorated blood, and the _animalization_ or _a.s.similation_ of the _chyle_ from which the _blood_ is formed. This process has already been referred to as the _completion of digestion_; for _chyle_ is not fitted to nourish the system until, by its exposure to the atmospheric air in the lungs, it is converted into _arterial blood_.

As the food can not become a part of the living animal, or the venous blood regain its lost properties until they have undergone the requisite changes in the air-cells of the lungs, the function of respiration by which these are effected is one of pre-eminent importance in the animal economy, and well deserves the most careful examination.

The term respiration is frequently restricted to the mere inhalation and expiration of air from the lungs, but more generally it is employed to designate the whole series of phenomena which occur in these organs. The term _sanguification_ is occasionally used to denote that part of the process in which the blood, by exposure to the action of the air, pa.s.ses from the venous to the arterial state. As the chyle does not become a.s.similated to the blood until it has pa.s.sed through the lungs, this term, which signifies _blood-making_, is not unaptly used.

The _quant.i.ty_ and _quality_ of the blood have a most direct and material influence upon the condition of every part of the body. If the _quant.i.ty_ sent to the arm, for example, be diminished by tying the artery through which it is conveyed, the arm, being then imperfectly nourished, wastes away, and does not regain its plumpness till the full supply of blood be restored. In like manner, when the _quality_ of that fluid is impaired by deficiency of food, bad digestion, impure air, or imperfect sanguification in the lungs, the body and all its functions become more or less disordered. Thus, in consumption, death takes place chiefly in consequence of respiration not being sufficiently perfect to admit of the formation of proper blood in the lungs. A knowledge of the structure and functions of the lungs, and of the conditions favorable to _their_ healthy action, is therefore very important, for on their welfare depends that of every organ of the body.

The exposure of the blood to the action of the air seems to be indispensable to every variety of animated creatures. In man and the more perfect of the lower animals, it is carried on in the lungs, the structure of which is admirably adapted for the purpose. In many animals, however, the requisite action is effected without the intervention of lungs. In fishes, for example, that live in water and do not breathe, the blood circulates through the gills, and in them is exposed to the air which the water contains. So necessary is the atmospheric air to the vitality of the blood in all animals, that the want of it inevitably proves fatal. A fish can no more live in water deprived of air, than a man could in an atmosphere devoid of oxygen, which is the element that unites with the blood in the lungs in sanguification.

In man the lungs are those large, light, spongy bodies which, along with the heart, completely fill up the cavity of the chest. They vary much in size in different persons; and as the chest is formed for their protection, it is either large and capacious, or the reverse, according to the size of the lungs.

The substance of the lungs consists of bronchial tubes, air-cells, blood-vessels, nerves, and cellular membrane. The bronchial tubes are merely continuations and subdivisions of the windpipe, and serve to convey the external air to the air-cells of the lungs. The air-cells const.i.tute the chief part of the lungs, and are the termination of the smaller branches of the bronchial tubes. When fully distended, they are so numerous as in appearance to const.i.tute almost the whole lung. They are of various sizes, from the twentieth to the hundredth of an inch in diameter, and are lined with an exceedingly fine, thin membrane, on which the minute capillary branches of the pulmonary arteries and veins are copiously ramified. It is while circulating in the small vessels of this membrane, and there exposed to the air, that the blood undergoes the change from the venous to the arterial state. So numerous are these air-cells, that the aggregate extent of their lining membrane in man has been computed to exceed twenty thousand square inches, or about ten times the surface of the human body. Some writers place the estimate considerably higher.

A copious _exhalation_ of moisture takes place in breathing, which presents a striking a.n.a.logy to the exhalation from the surface of the skin already described. In the former as in the latter instance, the exhalation is carried on by the innumerable minute capillary vessels in which the small arterial branches terminate in the air-cells. Pulmonary exhalation is, in fact, one of the chief outlets of waste matter from the system; and the air we breathe is thus vitiated, not only by the subtraction of its oxygen and the addition of carbonic acid gas, but also by animal effluvia, with which it is loaded when returned from the lungs. In some individuals this last source of impurity is so great as to render their vicinity offensive, and even insupportable. It is this which gives the disagreeable, sickening smell to crowded rooms. The air which is expired from the lungs is rendered offensive by various other causes. When spirituous liquors are taken into the stomach, for example, they are absorbed by the veins and mixed with the venous blood, in which they are carried to the lungs to be expelled from the body. In some instances, when persons have drank copiously of spirits, their breath has been so saturated with them as actually to _take fire_ and _burn_.

An instance of this kind has recently been communicated to me by several reliable witnesses, in which the flame was extinguished by closing the mouth and nose, thus excluding the pure air that supported the combustion, until the unfortunate experimenter could remove the candle by which his breath had taken fire. This ill.u.s.tration will explain how the odor of different substances is frequently perceptible in the breath long after the mouth is free from them.

The lungs not only exhale waste matter, but _absorption_ takes place from their lining membrane. In both of these respects there is a striking a.n.a.logy between the functions performed by the lungs and the skin. When a person breathes an atmosphere loaded with the fumes of spirits, tobacco, turpentine, or of any other volatile substance, a portion of the fumes is taken up by the absorbing vessels of the lungs, and carried into the system, and there produces precisely the same effects as if introduced into the stomach. Dogs, for example, have been killed by being made to inhale the fumes of prussic acid for a few minutes. The lungs thus become a ready inlet to contagion, miasmata, and other poisonous influences diffused through the air we breathe.

From this general explanation of the structure and uses of the lungs, it is obvious that several conditions which it is our interest to know and observe are essential to the healthy performance of the important function of respiration. The first among these is a healthy original formation of the lungs. No fact in medicine is better established, says Dr. Combe, than that which proves the hereditary transmission, from parents to children, of a const.i.tutional liability to pulmonary disease, and especially to consumption; yet, continues he, no condition is less attended to in forming matrimonial engagements.