[Ill.u.s.tration: Fig. 85.--Larynx, Trachea, and the Bronchi. (Front view.)

A, epiglottis; B, thyroid cartilage; C, cricoid-thyroid membrane, connecting with the cricoid cartilage below, all forming the larynx; D, one of the rings of the trachea.

The walls of the windpipe are strengthened by a series of cartilaginous rings, each somewhat the shape of a horseshoe or like the letter C, being incomplete behind, where they come in contact with the sophagus.

Thus the trachea, while always open for the pa.s.sage of air, admits of the distention of the food-pa.s.sage.

204. The Bronchial Tubes. The lower end of the windpipe is just behind the upper part of the sternum, and there it divides into two branches, called bronchi. Each branch enters the lung of its own side, and breaks up into a great number of smaller branches, called bronchial tubes. These divide into smaller tubes, which continue subdividing till the whole lung is penetrated by the branches, the extremities of which are extremely minute. To all these branches the general name of bronchial tubes is given. The smallest are only about one-fiftieth of an inch in diameter.

[Ill.u.s.tration: Fig. 86.--Relative Position of the Lungs, Heart, and its Great Vessels.

A, left ventricle; B, right ventricle; C, left auricle; D, right auricle; E, superior vena cava; F, pulmonary artery; G, aorta; H, arch of the aorta; K, innominate artery; L, right common carotid artery; M, right subclavian artery; N, thyroid cartilage forming upper portion of the larynx; O, trachea.

Now the walls of the windpipe, and of the larger bronchial tubes would readily collapse, and close the pa.s.sage for air, but for a wise precaution. The horseshoe-shaped rings of cartilage in the trachea and the plates of cartilage in the bronchial tubes keep these pa.s.sages open.

Again, these air pa.s.sages have elastic fibers running the length of the tubes, which allow them to stretch and bend readily with the movements of the neck.

205. The Cilia of the Air Pa.s.sages. The inner surfaces of the windpipe and bronchial tubes are lined with mucous membrane, continuous with that of the throat, the mouth, and the nostrils, the secretion from which serves to keep the parts moist.

Delicate, hair-like filaments, not unlike the pile on velvet, called cilia, spring from the epithelial lining of the air tubes. Their constant wavy movement is always upwards and outwards, towards the mouth.

Thus any excessive secretion, as of bronchitis or catarrh, is carried upwards, and finally expelled by coughing. In this way, the lungs are kept quite free from particles of foreign matter derived from the air.

Otherwise we should suffer, and often be in danger from the acc.u.mulation of mucus and dust in the air pa.s.sages. Thus these tiny cilia act as dusters which Nature uses to keep the air tubes free and clean (Fig. 5).

[Ill.u.s.tration: Fig. 87.--Bronchial tube, with its Divisions and Subdivisions. (Showing groups of air cells at the termination of minute bronchial tubes.)]

206. The Lungs. The lungs, the organs of respiration, are two pinkish gray structures of a light, spongy appearance, that fill the chest cavity, except the s.p.a.ce taken up by the heart and large vessels. Between the lungs are situated the large bronchi, the sophagus, the heart in its pericardium, and the great blood-vessels. The base of the lungs rests on the dome-like diaphragm, which separates the chest from the abdomen. This partly muscular and partly tendinous part.i.tion is a most important factor in breathing.

Each lung is covered, except at one point, with an elastic serous membrane in a double layer, called the pleura. One layer closely envelops the lung, at the apex of which it is reflected to the wall of the chest cavity of its own side, which it lines. The two layers thus form between them a Closed Sac a serous cavity (see Fig. 69, also note, p. 176).

[Ill.u.s.tration: Fig. 88.--The Lungs with the Trachea, Bronchi, and Larger Bronchial Tubes exposed. (Posterior view.)

A, division of left bronchus to upper lobe; B, left branch of the Pulmonary artery; C, left bronchus; D, left superior pulmonary vein; E, left inferior pulmonary vein; F, left auricle; K, inferior vena cava; L, division of right bronchus to lower lobe; M, right inferior pulmonary vein; N, right superior pulmonary vein; O, right branch of the pulmonary artery; P, division of right bronchus to upper lobe; R, left ventricle; S, right ventricle.

In health the two pleural surfaces of the lungs are always in contact, and they secrete just enough serous fluid to allow the surfaces to glide smoothly upon each other. Inflammation of this membrane is called _pleurisy_. In this disease the breathing becomes very painful, as the secretion of glairy serum is suspended, and the dry and inflamed surfaces rub harshly upon each other.

The root of the lung, as it is called, is formed by the bronchi, two pulmonary arteries, and two pulmonary veins. The nerves and lymphatic vessels of the lung also enter at the root. If we only remember that all the bronchial tubes, great and small, are hollow, we may compare the whole system to a short bush or tree growing upside down in the chest, of which the trachea is the trunk, and the bronchial tubes the branches of various sizes.

207. Minute Structure of the Lungs. If one of the smallest bronchial tubes be traced in its tree-like ramifications, it will be found to end in an irregular funnel-shaped pa.s.sage wider than itself. Around this pa.s.sage are grouped a number of honeycomb-like sacs, the air cells[35] or alveoli of the lungs. These communicate freely with the pa.s.sage, and through it with the bronchial branches, but have no other openings. The whole arrangement of pa.s.sages and air cells springing from the end of a bronchial tube, is called an ultimate lobule. Now each lobule is a very small miniature of a whole lung, for by the grouping together of these lobules another set of larger lobules is formed.

[Ill.u.s.tration: Fig. 89.

A, diagrammatic representation of the ending of a bronchial tube in air sacs or alveoli; B, termination of two bronchial tubes in enlargement beset with air sacs (_Huxley_); C, diagrammatic view of an air sac.

a lies within sac and points to epithelium lining wall; b, part.i.tion between two adjacent sacs, in which run capillaries; c, elastic connective tissue (_Huxley_).

In like manner countless numbers of these lobules, bound together by connective tissue, are grouped after the same fashion to form by their aggregation the lobes of the lung. The right lung has three such lobes; and the left, two. Each lobule has a branch of the pulmonary artery entering it, and a similar rootlet of the pulmonary vein leaving it. It also receives lymphatic vessels, and minute twigs of the pulmonary plexus of nerves.

[Ill.u.s.tration: Fig. 90.--Diagram to ill.u.s.trate the Amounts of Air contained by the Lungs in Various Phases of Ordinary and of Forced Respiration.]

The walls of the air cells are of extreme thinness, consisting of delicate elastic and connective tissue, and lined inside by a single layer of thin epithelial cells. In the connective tissue run capillary vessels belonging to the pulmonary artery and veins. Now these delicate vessels running in the connective tissue are surrounded on all sides by air cells. It is evident, then, that the blood flowing through these capillaries is separated from the air within the cells only by the thin walls of the vessels, and the delicate tissues of the air cells.

This arrangement is perfectly adapted for an interchange between the blood in the capillaries and the air in the air cells. This will be more fully explained in sec. 214.

208. Capacity of the Lungs. In breathing we alternately take into and expel from the lungs a certain quant.i.ty of air. With each quiet inspiration about 30 cubic inches of air enter the lungs, and 30 cubic inches pa.s.s out with each expiration. The air thus pa.s.sing into and out of the lungs is called tidal air. After an ordinary inspiration, the lungs contain about 230 cubic inches of air. By taking a deep inspiration, about 100 cubic inches more can be taken in. This extra amount is called complemental air.

After an ordinary expiration, about 200 cubic inches are left in the lungs, but by forced expiration about one-half of this may be driven out.

This is known as supplemental air. The lungs can never be entirely emptied of air, about 75 to 100 cubic inches always remaining. This is known as the residual air.

The air that the lungs of an adult man are capable of containing is thus composed:

Complemental air 100 cubic inches.

Tidal " 30 " "

Supplemental " 100 " "

Residual " 100 " "

---- Total capacity of lungs 330 " "

If, then, a person proceeds, after taking the deepest possible breath, to breath out as much as he can, he expels:

Complemental air 100 cubic inches.

Tidal " 30 " "

Supplemental " 100 " "

---- 230

This total of 230 cubic inches forms what is called the vital capacity of the chest (Fig. 90).

209. The Movements of Breathing. The act of breathing consists of a series of rhythmical movements, succeeding one another in regular order.

In the first movement, inspiration, the chest rises, and there is an inrush of fresh air; this is at once followed by expiration, the falling of the chest walls, and the output of air. A pause now occurs, and the same breathing movements are repeated.

The entrance and the exit of air into the respiratory pa.s.sages are accompanied with peculiar sounds which are readily heard on placing the ear at the chest wall. These sounds are greatly modified in various pulmonary diseases, and hence are of great value to the physician in making a correct diagnosis.

In a healthy adult, the number of respirations should be from 16 to 18 per minute, but they vary with age, that of a newly born child being 44 for the same time. Exercise increases the number, while rest diminishes it. In standing, the rate is more than when lying at rest. Mental emotion and excitement quicken the rate. The number is smallest during sleep. Disease has a notable effect upon the frequency of respirations. In diseases involving the lungs, bronchial tubes, and the pleura, the rate may be alarmingly increased, and the pulse is quickened in proportion.

210. The Mechanism of Breathing. The chest is a chamber with bony walls, the ribs connecting in front with the breastbone, and behind with the spine. The s.p.a.ces between the ribs are occupied by the intercostal muscles, while large muscles clothe the entire chest. The diaphragm serves as a movable floor to the chest, which is an air-tight chamber with movable walls and floor. In this chamber are suspended the lungs, the air cells of which communicate with the outside through the bronchial pa.s.sages, but have no connection with the chest cavity. The thin s.p.a.ce between the lungs and the rib walls, called the pleural cavity, is in health a vacuum.

Now, when the diaphragm contracts, it descends and thus increases the depth of the chest cavity. A quant.i.ty of air is now drawn into the lungs and causes them to expand, thus filling up the increased s.p.a.ce. As soon as the diaphragm relaxes, returning to its arched position and reducing the size of the chest cavity, the air is driven from the lungs, which then diminish in size. After a short pause, the diaphragm again contracts, and the same round of operation is constantly repeated.

The walls of the chest being movable, by the contractions of the intercostals and other muscles, the ribs are raised and the breastbone pushed forward. The chest cavity is thus enlarged from side to side and from behind forwards. Thus, by the simultaneous descent of the diaphragm and the elevation of the ribs, the cavity of the chest is increased in three directions,--downwards, side-ways, and from behind forwards.

It is thus evident that inspiration is due to a series of muscular contractions. As soon as the contractions cease, the elastic lung tissue resumes its original position, just as an extended rubber band recovers itself. As a result, the original size of the chest cavity is restored, and the inhaled air is driven from the lungs. Expiration may then be regarded as the result of an elastic recoil, and not of active muscular contractions.

[Ill.u.s.tration: Fig. 91.--Diagrammatic Section of the Trunk. (Showing the expansion of the chest and the movement of the ribs by action of the lungs.) [The dotted lines indicate the position during inspiration.]]

211. Varieties of Breathing. This is the mechanism of quiet, normal respiration. When the respiration is difficult, additional forces are brought into play. Thus when the windpipe and bronchial tubes are obstructed, as in croup, asthma, or consumption, many additional muscles are made use of to help the lungs to expand. The position which asthmatics often a.s.sume, with arms raised to grasp something for support, is from the need of the sufferer to get a fixed point from which the muscles of the arm and chest may act forcibly in raising the ribs, and thus securing more comfortable breathing.

The visible movements of breathing vary according to circ.u.mstances. In infants the action of the diaphragm is marked, and the movements of the abdomen are especially obvious. This is called abdominal breathing. In women the action of the ribs as they rise and fall, is emphasized more than in men, and this we call costal breathing. In young persons and in men, the respiration not usually being impeded by tight clothing, the breathing is normal, being deep and abdominal.

Disease has a marked effect upon the mode of breathing. Thus, when children suffer from some serious chest disease, the increased movements of the abdominal walls seem distressing. So in fracture of the ribs, the surgeon envelops the overlying part of the chest with long strips of firm adhesive plaster to restrain the motions of chest respiration, that they may not disturb the jagged ends of the broken bones. Again, in painful diseases of the abdomen, the sufferer instinctively suspends the abdominal action and relies upon the chest breathing. These deviations from the natural movements of respiration are useful to the physician in ascertaining the seat of disease.