Fig. 50.—Diagrammatic representation of the termination of a bronchial tube in a group of infundibula: B, Bronchial tube; LB, bronchiole; A, atrium; I, infundibulum; C, alveoli. (de Nancrede.)
Roughly speaking, the lungs begin at the sterno-clavicular articulation above, the apex coming up above the level of the first rib, and extend downward together to the fourth cartilage, where the lower margins gradually separate, the lowest lung limit being the eleventh rib in the vertebral region. Each lung is conical. The apices extend upward and the bases, which are broad and concave, rest upon the diaphragm. The right lung is divided by a fissure into three lobes, the left into two. The root consists of a bronchus and pulmonary arteries, veins, lymphatics, and nerves. The tissue itself is composed of an aggregation of lobules, each consisting of a terminal bronchiole with its alveoli or air cells, blood-vessels, and nerves, a lung in miniature. The blind pouches which the air cells surround are called infundibula and are separated by delicate membranous septa in which lie the capillaries of the pulmonary artery, thus exposing the blood to the air on two sides. The lung itself is supplied by the bronchial arteries from the thoracic aorta and by branches of the sympathetic and pneumogastric nerves.
At birth the lungs are pinkish-white in color but in later life they are marked with slate-colored patches, due to the deposit in the lung tissue of particles of dirt breathed in. They are light, spongy, and highly elastic, and will float in water, crepitating upon pressure owing to the air in the tissue.
At birth, also, the lungs are solid, so that the first air has to overcome adhesions between the collapsed walls of the bronchioles and air sacs, but after they are thus gradually unfolded, in that they are of extensible material and open to the air above, atmospheric pressure from within keeps them distended to the full extent of the chest, which is air tight. They never collapse afterwards unless puncture of the chest wall, as in stabbing, causes collapse, in which case the lung shrivels into a small ball.
Respiration.—That the organic materials used by the body as food may give up their energy they must be broken up, and for this oxygen is needed. The supply of oxygen for the purpose is brought to the tissues by the blood, which acquires it in the lungs, and the waste product of combustion, carbon dioxide, is carried off in the same manner. The lungs are, therefore, adapted to take in large quantities of air and to keep up a rapid exchange of oxygen and carbon dioxide in the blood. This process of supplying oxygen to the tissues and of removing carbon dioxide and other waste is ordinarily an involuntary act, though it can be regulated temporarily, and is known as respiration or breathing.
There are two periods to respiration: 1. inspiration or the drawing in of air, and 2. expiration or the expulsion of air from the lungs, the former process being a little shorter than the latter. A pause follows each expiration before there is another inspiration. At birth the normal rate of respiration is 42, but it grows slower as the child grows older, being 26 at the age of five or six, while in the adult it averages 17 to 20 times a minute. It is slower during sleep and more rapid during physical activity. The average amount of air taken in with every inspiration is 30 cubic inches and the minimum air space per individual should be 3000 cubic feet per hour.
Breathing is of two kinds, diaphragmatic or abdominal and chest or rib breathing, the former usually being more pronounced in men than in women, probably because of centuries of tight dressing on the part of the latter. As a rule, however, both diaphragm and ribs come into play; for in inspiration, which is an active movement, the thorax becomes enlarged from before backward, laterally, and vertically. The ribs are raised by the external intercostals chiefly, though the internal intercostals aid somewhat, and swinging out upon the vertebræ, widen the chest as well as deepen it. The diaphragm, which is dome-like when relaxed, becomes flattened in contraction and so increases the size of the chest from above downward. As the chest enlarges, the lungs expand, the air in them becomes rarefied, and more air rushes in. When the lungs are full they relax and the muscles relax after their contraction, so that expiration is a passive movement, due largely to the elastic relaxation of lungs and muscles, the air being driven out by the lessened capacity of the lungs.
Difficult Breathing.—In heart and lung troubles, where too little oxygen is carried to the tissues, dyspnœa or difficult breathing results and may even advance to asphyxia, a condition in which no air is obtained. In difficult or labored respiration the pectoral muscles are used in inspiration and the scaleni, which pass from the vertebræ of the neck to the sternum, develop and become powerful. The levatores of the ribs may also assist, and even the muscles of the neck and arms may help out, while in forced expiration the abdominal muscles are called into play. The glottis opens and closes rhythmically as the air enters and leaves the lungs, and the nostrils add their mite in the struggle for oxygen. Finally there may be scarcely a muscle in the body that is not striving to aid the respiration, and general convulsions may result, followed by exhaustion and death.
Air.—In ordinary breathing the lungs are not used to their full capacity and the air ordinarily used is known as tidal air. In forced inspiration the lungs are filled to their fullest extent and the air then taken in in excess of the tidal air is known as complemental air. In like manner, the difference between the air ordinarily breathed out and that breathed out in forced expiration is known as supplemental air. The sum of these three is the vital capacity of the lungs, while beyond this there is probably some air that is never expelled, the stationary or residual air.