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 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, 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, 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 analogy 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 illustration 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 analogy 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 constitutional liability to pulmonary disease, and especially to consumption; yet, continues he, no condition is less attended to in forming matrimonial engagements.
Another requisite to the well-being of the lungs, and to the free and salutary exercise of respiration, is a due supply of rich and healthy blood. When, from defective food or impaired digestion, the blood is impoverished in quality, and rendered unfit for adequate nutrition, the lungs speedily suffer, and that often to a fatal extent. The free and easy expansion of the chest is also indispensable to the full play and dilation of the lungs. Whatever interferes with or impedes it, either in dress or in position, is obviously prejudicial to health. On the other hand, whatever favors the free expansion of the chest equally promotes the healthy action of the respiratory organs. Stays and corsets, and tight vests and waistbands, operate most injuriously, compressing as they do the thoracic cavity, and interfering with the healthy dilation of the lungs.