Besides the heart and the great vessels the chest contains the lungs, the chief organ of respiration, which, with the rest of the respiratory system, will now be treated. The nose and mouth, through which the air first enters the body, have already been spoken of. From them the air passes through the larynx to the trachea, thence to the bronchi, and so to the lungs, where the supply of oxygen for the tissues is taken from the air by the hemoglobin of the blood.

The Larynx.—The larynx lies in front of the pharynx at the upper and fore part of the neck, where it causes a considerable projection, known as Adam’s apple. It is a triangular box, base up, flattened at the back, in front, and at the sides, but becoming cylindrical below. Above it opens into the bottom of the pharynx and below into the trachea. It is lined with mucous membrane. Its opening at the base of the tongue is closed during swallowing by a little door-like valve of fibro-cartilage, the epiglottis, to prevent the entrance of food.

Nine cartilages go to make up the larynx, of which the most important are the thyroid and cricoid cartilages and the epiglottis already mentioned. The thyroid is the largest and is open behind, its two alæ or wings meeting in an acute angle in front and forming the Adam’s apple, always more prominent in the male than in the female. It is attached above to the hyoid bone and has cornua or horns on either side, top and bottom. The cricoid or ring-like cartilage resembles a seal ring with the stone placed posteriorly. It is stronger than the thyroid and forms the lower part of the cavity of the larynx. Inside and resting on the upper border of the cricoid are the two smaller arytenoid or pitcher-like cartilages, pyramidal in shape, and surmounting these again the two cornicula laryngis. The two cuneiform cartilages are in the free borders of the folds of mucous membrane which extend from the apex of the arytenoids to the sides of the epiglottis. Numerous small muscles serve to bind these various cartilages together.

At the angle of the thyroid cartilage in front are attached the epiglottis at the top and just below that the superior or false vocal cords, two folds of mucous membrane enclosing the superior thyro-arytenoid ligaments. Lower still are found the inferior or true vocal cords, which are formed by the inferior thyro-arytenoid ligaments covered with a thin, tightly fitting mucous membrane. Both sets of vocal cords as well as the epiglottis may be seen by means of a head and a throat mirror. Between the true vocal cords is a narrow triangular interval called the glottis. It is by means of the vibrations of these cords that sound is produced. The false vocal cords cannot produce sound, though they can modify it indirectly. Quality of voice, as treble, base, etc., depends upon the size of the larynx and the length and elasticity of the vocal cords. Modulation is produced by changing the form of the cavity of the mouth and nose. In whispering the lips take the place of the vocal cords and produce sound by the vibration of their muscular walls.

Instead of tracheotomy laryngotomy is sometimes done in the depression between the thyroid and the cricoid, which may be felt on the living. Foreign bodies sometimes get into the larynx and have to be removed, or the mucous membrane may become inflamed, causing laryngitis. Syphilis attacks the larynx, and tuberculosis and cancer of the larynx occur, these last two being generally fatal. Edema of the glottis may also occur.

The Trachea.—The trachea is a membranous tube extending down from the larynx for about four and a half inches to the fourth or fifth dorsal vertebra, where it divides into the right and left bronchi. It is formed of sixteen to twenty imperfect cartilaginous rings, open behind, enclosed in a double elastic fibrous membrane, and is lined with ciliated mucous membrane. The rings are for strength and in the interval at the back where they are wanting there is one layer of longitudinal and another of transverse unstriped muscle fibers. The passage is kept clear by the action of the cilia, which sweep up and out any particles of dust that become entangled in the mucus. Tracheotomy is generally done about one inch below the cricoid, just above the sternal notch, incision being made through the cartilage.

Extending up on either side of the upper trachea in the neck are the two lobes of the thyroid gland, the isthmus, which connects the lobes, covering the trachea below anteriorly. The function of the gland is obscure, but it has an internal secretion of great importance in the metabolic processes. Its removal or disease is followed by general disturbances of mind and body. The injection of thyroid extract has proved effective as treatment. In goiter the gland becomes enlarged. The thymus gland lies below the thyroid gland at birth, in front of and at the sides of the trachea, and runs down behind the sternum. It is largest at the end of the second year, after which it atrophies, being almost absent at puberty. It, too, is ductless and its function is not well understood.

The Bronchi.—The two bronchi, of which the right is the larger and shorter, resemble the trachea in structure. As they enter the root of the lung they divide, the right into three and the left into two branches, one for each lobe, after which they divide and subdivide, the bronchioles becoming smaller and smaller and finally ending in the infundibula, pouch-like places lined with air cells, in which cilia keep the air in motion. As they grow smaller the bronchioles gradually become wholly membranous.

The Lungs.—The lungs themselves, two in number, lie each in a serous sac or pleura, similar in structure to the pericardium and serving a like purpose. The outer layer of the pleura is reflected back over the thoracic wall and diaphragm. There is no pleural cavity in health between the two layers of the pleura, the two surfaces being in close contact, though moistened with lymph to prevent friction during respiration. In inflammation of the pleuræ or pleurisy, they become thickened and roughened and friction results, as is shown by the sounds heard through the stethoscope. Friction causes effusion and fluid collects. This generally absorbs again, but occasionally the serous fluid becomes pustular and empyema results.

In front, between the two pleuræ, which are wholly separate, is the mediastinal space or mediastinum, which extends from the sternum to the spinal column and contains all the thoracic viscera except the lungs and heart, that is, the trachea, esophagus, thoracic duct, and many large vessels and nerves.

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.

Respiratory Sounds.—The entrance and exit of the air is accompanied by respiratory sounds or murmurs, which vary according to their position in the trachea, the bronchi, or the bronchioles and are modified in diseases of the lungs and bronchi, when they are often called râles.

Changes in Air in Lungs.—In passing through the nose and the rest of the respiratory tract the air is warmed to body temperature and saturated with moisture. After its entrance into the lungs various changes take place in it through the mingling of the tidal with the residual air. Thus, it gives up about 4 or 5 per cent. of its oxygen and acquires some 4 per cent. additional carbon dioxide, while the amount of nitrogen remains about the same. By its giving up more oxygen than it receives carbon dioxide, its volume is slightly diminished. Exhaled air also contains traces of ammonia and certain organic matters, generally the results of decomposition, which give a bad odor to the breath and are more dangerous in a close room than the mere lack of oxygen or the presence of carbon dioxide. Indeed, the amount of oxygen may be very much diminished, being reduced even to 5 or 6 per cent. instead of the normal 21 per cent., without being noticed or giving rise to any immediate bad results. Yet the importance of ventilation is very evident.

Effect on Blood.—Respiration causes changes also in the blood, the venous blood being purple and the arterial bright red. This difference in color is due to the absence or presence of oxygen, which is not absorbed or dissolved by the blood but forms a rather unstable compound, oxyhemoglobin, with the hemoglobin of the blood. As the oxygen is removed in the passage of the blood through the body, there results in venous blood reduced hemoglobin, which is of a purplish color. Upon exposure to the air, however, it absorbs oxygen once more and resumes its scarlet color. If carbon monoxide gets into the blood, as in cases of gas poisoning, it drives off the oxygen and forms a more stable compound with the hemoglobin, whence the difficulty in restoring a person so poisoned.

Nervous Mechanism.—Nervously, respiration is controlled in three ways: 1. by the phrenic nerve to the diaphragm; 2. by some fibers of the vagus or pneumogastric, and 3. by the respiratory center in the bulbous portion of the spinal cord. Injury to the respiratory center means the ceasing of respiration and death. Stimulation of the respiratory center seems to depend upon the character of the blood. If it is well oxygenized, the breathing is slow and quiet; if there is a lack of oxygen, dyspnœa results. Probably certain chemical substances in the blood, which are ordinarily rapidly burned up by the oxygen but which accumulate in its absence, serve to stimulate the respiratory center, thus adjusting the effort to get oxygen to the need of it. Respiration may be stopped by stimulating the mucous membrane of the nose, as with strong ammonia.

Variations.—Certain variations from the ordinary respiration might be mentioned here. A deep inspiration followed by a long expiration is known as a sigh and a very deep inspiration through the mouth only as a yawn. Hiccough results from a sudden inspiratory contraction of the diaphragm during which the glottis is suddenly closed. In sobbing the inspirations are short and rapid with a prompt closing of the glottis between. Both coughing and sneezing consist of a deep inspiration followed by complete closure of the glottis and then its sudden opening and the forcible expulsion of air. Coughing, however, is generally caused by an irritation or obstruction of the larynx or trachea and the air is expelled through the mouth, while sneezing is caused by irritation of the nasal passages and the air is driven out through the nose. Laughing and crying also resemble one another in that each is an inspiration followed by a series of short, spasmodic expirations, during which the glottis is open and the vocal cords in characteristic vibration. They differ, however, in rhythm and in the facial expression that accompanies them.