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 wo 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 assembly, 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 mass 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 wornout blood, into arterial or nutrient blood. In order to be clearly understood, I must premise a few observa

tions on the circulation of the blood.* 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 constituting a part of, the texture of every living part. On reaching this extreme point of its course, the blood passes 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 passage 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.

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 import

*

Taken, with slight alterations, from the description of Dr. A. Combe.

ant process of respiration, which takes place in the aircells 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 reassumed 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 assimilation 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 un

til 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, passes from the venous to the arterial state. As the chyle does not become assimilated to the blood until it has passed through the lungs, this term, which signifies blood-making, is not unaptly used.

The quantity and quality of the blood have a most direct and material influence upon the condition of every part of the body. If the quantity 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 constitute 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 constitute 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,