Organs of breath

The word “respiration” describes two processes. Internal or cellular respiration is the process by which glucose or other small molecules are oxidised to produce energy: this requires oxygen and generates carbon dioxide. External respiration (breathing) involves simply the stage of taking oxygen from the air and returning carbon dioxide to it.

The respiratory tract, where external respiration occurs, starts at the nose and mouth. (Description of respiratory tract from nose to trachea here from overheads) (There is a brief complication where the airstream crosses the path taken by food and drink in the pharynx: air flows on down the trachea where food normally passes down the oesophagus to the stomach. )

The trachea (windpipe) extends from the neck into the thorax, where it divides into right and left main bronchi, which enter the right and left lungs, breaking up as they do so into smaller bronchi and bronchioles and ending in small air sacs or alveoli, where gaseous exchange occurs.

The lungs are divided first into right and left, the left being smaller to accommodate the heart, then into lobes (three on the right, two on the left) supplied by lobar bronchi.

Bronchi, pulmonary arteries and veins (which supply deoxygenated blood and remove oxygenated blood), bronchial arteries and veins (which supply oxygenated blood to the substance of the lung itself) and lymphatics all enter and leave the lung by its root (or hilum). Lymph nodes blackened by soot particles can often be seen here and the substance of the lung itself may be blackened by soot in city dwellers or heavy smokers.

Each lobe of the lung is further divided into a pyramidal bronchopulmonary segments. Bronchopulmonary segments have the apex of the pyramid in the hilum whence they receive a tertiary bronchus, and appropriate blood vessels. The 10 segments of the right lung and eight of the left are virtually self contained units not in communication with other parts of the lung. This is of obvious use in surgery when appropriate knowledge will allow a practically bloodless excision of a diseased segment. Gaseous exchange relies on simple diffusion. In order to provide sufficient oxygen and to get rid of sufficient carbon dioxide there must be:

• a large surface area for gaseous exchange
• a very short diffusion path between alveolar air and blood
• concentration gradients for oxygen and carbon dioxide between alveolar air and blood.

The surface available in an adult is around 140 m² in an adult, around the area of a singles tennis court. The blood in the alveolar capillaries is separated from alveolar air by 0.6* in many places (1* = one thousandth of a mm). Diffusion gradients are maintained by

• ventilation (breathing) which renews alveolar air, maintaining oxygen concentration near that of atmospheric air and preventing the accumulation of carbon dioxide
• the flow of blood in alveolar capillaries which continually brings blood with low oxygen concentration and high carbon dioxide concentration

Haemoglobin in blood continually removes dissolved oxygen from the blood and binds with it. The presence of this tennis court, separated from the outside air by a very narrow barrier imposes demands on the respiratory tract.

Outside air:

• varies in temperature. At the alveolar surface it must be at body temperature
• varies from very dry to very humid. At the alveolar surface it must be saturated with water vapour
• contains dust and debris. These must not reach the alveolar wall
• contains micro-organisms, which must be filtered out of the inspired air and disposed of before they reach the alveoli, enter the blood and cause possible problems.

It is easy to see that the temperature and humidity of inspired air will increase as it passes down a long series of tubes lined with a moist mucosa at body temperature. The mechanisms for filtering are not so obvious.

Mucus. The respiratory tract, from nasal cavities to the smallest bronchi, is lined by a layer of sticky mucus, secreted by the epithelium assisted by small ducted glands. Particles which hit the side wall of the tract are trapped in this mucus. This is encouraged by: (a) the air stream changing direction, as it repeatedly does in a continually dividing tube. (b) random (Brownian) movement of small particles suspended in the airstream.

The first of these works particularly well on more massive particles, the second on smaller bits.

Length. The length of the respiratory tract helps in both bringing the air to the right temperature and humidity but hinders the actual ventilation, as a long tract has a greater volume of air trapped within it, and demands a large breath to clear out residual air.

Protection. The entry of food and drink into the larynx is prevented by the structure of the larynx and by the complicated act of swallowing. The larynx is protected by three pairs of folds which close off the airway. In man these have a secondary function, they vibrate in the airstream to produce sounds, the basis of speech and singing. Below the larynx the trachea is usually patent i.e. open, and kept so by rings of cartilage in its walls. However it may be necessary to ensure that this condition is maintained by passing a tube (endotracheal intubation) to maintain the airway, especially post operatively if the patient has been given a muscle relaxant. Another common surgical procedure, tracheotomy, involves a small transverse cut in the neck. If this is done with anatomical knowledge no major structure is disturbed and the opening may be used for a suction tube, a ventilator, or in cases of tracheal obstruction as a permanent airway.

Ventilation and perfusion . The gills of fish and the lungs of birds allow water and air receptively to flow continually over the exchanging surface. In common with all mammals humans ventilate their lungs by breathing in and out. This reciprocal movement of air is less efficient and is achieved by alternately increasing and decreasing the volume of the chest in breathing. The body’s requirements for oxygen vary widely with muscular activity. In violent exercise the rate and depth of ventilation increase greatly: this will only work in conjunction with increase in blood flow, controlled mainly by the rich innervation of the lungs.. Gas exchange can be improved by breathing enriched air, which produces significantly reduced times for track events. Inadequate gas exchange is common in many diseases, producing respiratory distress.

Breathing works by making the cage bigger: the pleural layers slide over each other and the pressure in the lung is decreased, so air is sucked in. Breathing out does the reverse, the cage collapses and air is expelled. The main component acting here is the diaphragm. This is a layer of muscle which is convex above, domed, and squashed in the centre by the heart. When it contracts it flattens and increases the space above it. When it relaxes the abdominal contents push it up again. The proportion of breathing which is diaphragmatic varies from person to person. For instance breathing in children and pregnant women is largely diaphragmatic, and there is said to be more diaphragmatic respiration in women than in men.

The process is helped by the ribs which move up and out also increasing the space available. The complexity of breathing increases as does the need for efficiency. In quiet respiration, say whilst lying on ones back, almost all movement is diaphragmatic and the chest wall is still. This will increase thoracic volume by 500-700ml. The expansion of the lung deforms the flexible walls of the alveoli and bronchi and stretches the elastic fibres in the lung. When the diaphragm relaxes elastic recoil and abdominal musculature reposition the diaphragm again.

Deeper respiration brings in the muscles of the chest wall, so that the ribs move too.  We must therefore understand the skeleton and muscular system of the thoracic wall. The 12 pairs of ribs pass around the thoracic wall, articulating via synovial joints with the vertebral column – in fact two per rib. The ribs then curve outwards then forwards and downwards and attach to the sternum via the flexible costal cartilages. The first seven pairs of ribs (true ribs) attach directly, the next five hitch a lift on each other and the last two float i.e. are unattached. Costal cartilages are flexible. The first rib is rather different, short, flattened above and below and suspended beneath a set of fairly hefty muscles passing up into the neck, the scalene muscles. Between the ribs run two sets of intercostal muscles, the external intercostals running forward and downwards, the internal intercostals running up and back. These two muscle sheets thus run between ribs with fibres roughly at right angles. When they contract each rib moves closer to its neighbours. Because the lowest ribs float, and the first rib is suspended from the scalene muscles contraction of the intercostal muscles tends to lift rib two towards rib 1, and so on. The ribs are all, therefore pulled up towards the horizontal, increasing anteroom-posterior and lateral thoracic diameters.

These movements are sometimes divided intopump handle movements, the rib abducting on its vertebral joints and bucket handle movements, the rib rotating on its axis around anterior and posterior attachments: these are not necessarily helpful. With more and more effort put into deeper and deeper breathing the scalene muscles of the neck contract, raising the first rib and hence the rest of the cage, then other neck muscles and even those of the upper limb become involved. A patient with difficulty in breathing often grips a table edge in order to stabilise the limbs so that their muscles can be used to help in moving the thoracic wall.

Apparatus Respiratorius; Respiratory System

The respiratory apparatus consists of the larynx, trachea, bronchi, lungs, and pleuræ. Development.—The rudiment of the respiratory organs appears as a median longitudinal groove in the ventral wall of the pharynx. The groove deepens and its lips fuse to form a septum which grows from below upward and converts the groove into a tube, the laryngo-tracheal tube, the cephalic end of which opens into the pharynx by a slit-like aperture formed by the persistent anterior part of the groove. The tube is lined by entoderm from which the epithelial lining of the respiratory tract is developed. The cephalic part of the tube becomes the larynx, and its next succeeding part the trachea, while from its caudal end two lateral outgrowths, the right and left lung buds, arise, and from them the bronchi and lungs are developed. The first rudiment of the larynx consists of two arytenoid swellings, which appear, one on either side of the cephalic end of the laryngo-tracheal groove, and are continuous in front of the groove with a transverse ridge (furcula of His) which lies between the ventral ends of the third branchial arches and from which the epiglottis is subsequently developed. After the separation of the trachea from the esophagus the arytenoid swellings come into contact with one another and with the back of the epiglottis, and the entrance to the larynx assumes the form of a T-shaped cleft, the margins of the cleft adhere to one another and the laryngeal entrance is for a time occluded. The mesodermal wall of the tube becomes condensed to form the cartilages of the larynx and trachea. The arytenoid swellings are differentiated into the arytenoid and corniculate cartilages, and the folds joining them to the epiglottis form the aryepiglottic folds in which the cuneiform cartilages are developed as derivatives of the epiglottis. The thyroid cartilage appears as two lateral plates, each chondrified from two centers and united in the mid-ventral line by membrane in which an additional center of chondrification develops. The cricoid cartilage arises from two cartilaginous centers, which soon unite ventrally and gradually extend and ultimately fuse on the dorsal aspect of the tube.

The opening of the pulmonary diverticulum lies between the two fifth arch masses and behind a “central mass” in the middle line—the proximal end of the diverticulum is compressed between the fifth arch masses. The fifth arch is joined by the fourth to form a “lateral mass” on each side of the opening, and these “lateral masses” grow forward and overlap the central mass and so form a secondary transverse cavity, which is really a part of the cavity of the pharynx. The two parts of the cavity of the larynx are separated in the adult by a line drawn back along the vocal fold and then upward along the border of the arytenoid eminence to the interarytenoid notch. The arytenoid and cricoid are developed in the fifth arch mass. The thyroid is primarily a fourth arch derivative, and if it has a fifth arch element this is a later addition. The epiglottis is derived from the “central mass,” and has a third arch element in its oral and upper aspect; the arch value of the “central mass” is doubtful.   

The right and left lung buds grow out behind the ducts of Cuvier, and are at first symmetrical, but their ends soon become lobulated, three lobules appearing on the right, and two on the left; these subdivisions are the early indications of the corresponding lobes of the lungs. The buds undergo further subdivision and ramification, and ultimately end in minute expanded extremities—the infundibula of the lung. After the sixth month the air-sacs begin to make their appearance on the infundibula in the form of minute pouches. The pulmonary arteries are derived from the sixth aortic arches. During the course of their development the lungs migrate in a caudal direction, so that by the time of birth the bifurcation of the trachea is opposite the fourth thoracic vertebra. As the lungs grow they project into that part of the celom which will ultimately form the pleural cavities, and the superficial layer of the mesoderm enveloping the lung rudiment expands on the growing lung and is converted into the pulmonary pleura.

The Larynx

The larynx or organ of voice is placed at the upper part of the air passage. It is situated between the trachea and the root of the tongue, at the upper and forepart of the neck, where it presents a considerable projection in the middle line. It forms the lower part of the anterior wall of the pharynx, and is covered behind by the mucous lining of that cavity; on either side of it lie the great vessels of the neck. Its vertical extent corresponds to the fourth, fifth, and sixth cervical vertebræ, but it is placed somewhat higher in the female and also during childhood. Symington found that in infants between six and twelve months of age the tip of the epiglottis was a little above the level of the fibrocartilage between the odontoid process and body of the axis, and that between infancy and adult life the larynx descends for a distance equal to two vertebral bodies and two intervertebral fibrocartilages. According to Sappey the average measurements of the adult larynx are as follows:   

                                                     In males.                     In females.

Length                                           44 mm.                         36 mm.

Transverse diameter                      43 mm.                         41 mm.

Antero-posterior diameter             36 mm.                        26 mm.

Circumference                            136 mm.                        112 mm. 

  Until puberty the larynx of the male differs little in size from that of the female. In the female its increase after puberty is only slight; in the male it undergoes considerable increase; all the cartilages are enlarged and the thyroid cartilage becomes prominent in the middle line of the neck, while the length of the rima glottidis is nearly doubled.   

 The Cartilages of the Larynx (cartilagines laryngis) are nine in number, three single and three paired, as follows: Thyroid.  Two Corniculate. Cricoid.  Two Cuneiform. Two Arytenoid.  Epiglottis.

Ossification of cartilages commences about the twenty-fifth year in the thyroid cartilage, and somewhat later in the cricoid and arytenoids; by the sixty-fifth year these cartilages may be completely converted into bone.

Ligaments.—The ligaments of the larynx are extrinsic, i. e., those connecting the thyroid cartilage and epiglottis with the hyoid bone, and the cricoid cartilage with the trachea; and intrinsic, those which connect the several cartilages of the larynx to each other.   

Extrinsic Ligaments.—The ligaments connecting the thyroid cartilage with the hyoid bone are the hyothyroid membrane, and a middle and two lateral hyothyroid ligaments.  

Intrinsic Ligaments.—Beneath the mucous membrane of the larynx is a broad sheet of fibrous tissue containing many elastic fibers, and termed the elastic membrane of the larynx. It is subdivided on either side by the interval between the ventricular and vocal ligaments, the upper portion extends between the arytenoid cartilage and the epiglottis and is often poorly defined; the lower part is a well-marked membrane forming, with its fellow of the opposite side, the conus elasticus which connects the thyroid, cricoid, and arytenoid cartilages to one another. In addition the joints between the individual cartilages are provided with ligaments.   

  The Conus Elasticus (cricothyroid membrane) is composed mainly of yellow elastic tissue. It consists of an anterior and two lateral portions. The anterior part or middle cricothyroid ligament (ligamentum cricothyreoideum medium; central part of cricothyroid membrane) is thick and strong, narrow above and broad below. It connects together the front parts of the contiguous margins of the thyroid and cricoid cartilages. It is overlapped on either side by the An articular capsule, strengthened posteriorly by a well-marked fibrous band, encloses the articulation of the inferior cornu of the thyroid with the cricoid cartilage on either side.   

  Each arytenoid cartilage is connected to the cricoid by a capsule and a posterior cricoarytenoid ligament. The capsule (capsula articularis cricoarytenoidea) is thin and loose, and is attached to the margins of the articular surfaces. The posterior cricoarytenoid ligament (ligamentum cricoarytenoideum posterius) extends from the cricoid to the medial and back part of the base of the arytenoid.   

  The thyroepiglottic ligament (ligamentum thyreoepiglotticum) is a long, slender, elastic cord which connects the stem of the epiglottis with the angle of the thyroid cartilage, immediately beneath the superior thyroid notch, above the attachment of the ventricular ligaments.

Movements.—The articulation between the inferior cornu of the thyroid cartilage and the cricoid cartilage on either side is a diarthrodial one, and permits of rotatory and gliding movements. The rotatory movement is one in which the cricoid cartilage rotates upon the inferior cornua of the thyroid cartilage around an axis passing transversely through both joints. The gliding movement consists in a limited shifting of the cricoid on the thyroid in different directions.

  The articulation between the arytenoid cartilages and the cricoid is also a diarthrodial one, and permits of two varieties of movement: one is a rotation of the arytenoid on a vertical axis, whereby the vocal process is moved lateralward or medialward, and the rima glottidis increased or diminished; the other is a gliding movement, and allows the arytenoid cartilages to approach or recede from each other; from the direction and slope of the articular surfaces lateral gliding is accompanied by a forward and downward movement. The two movements of gliding and rotation are associated, the medial gliding being connected with medialward rotation, and the lateral gliding with lateralward rotation. The posterior cricoarytenoid ligaments limit the forward movement of the arytenoid cartilages on the cricoid.

Interior of the Larynx.—The cavity of the larynx (cavum laryngis) extends from the laryngeal entrance to the lower border of the cricoid cartilage where it is continuous with that of the trachea. It is divided into two parts by the projection of the vocal folds, between which is a narrow triangular fissure or chink, the rima glottidis. The portion of the cavity of the larynx above the vocal folds is called the vestibule; it is wide and triangular in shape, its base or anterior wall presenting, however, about its center the backward projection of the tubercle of the epiglottis. It contains the ventricular folds, and between these and the vocal folds are the ventricles of the larynx. The portion below the vocal folds is at first of an elliptical form, but lower down it widens out, assumes a circular form, and is continuous with the tube of the trachea.

  The entrance of the larynx is a triangular opening, wide in front, narrow behind, and sloping obliquely downward and backward. It is bounded, in front, by the epiglottis; behind, by the apices of the arytenoid cartilages, the corniculate cartilages, and the interarytenoid notch; and on either side, by a fold of mucous membrane, enclosing ligamentous and muscular fibers, stretched between the side of the epiglottis and the apex of the arytenoid cartilage; this is the aryepiglottic fold, on the posterior part of the margin of which the cuneiform cartilage forms a more or less distinct whitish prominence, the cuneiform tubercle.

The Ventricular Folds (plicœ ventriculares; superior or false vocal cords) are two thick folds of mucous membrane, each enclosing a narrow band of fibrous tissue, the ventricular ligament which is attached in front to the angle of the thyroid cartilage immediately below the attachment of the epiglottis, and behind to the antero-lateral surface of the arytenoid cartilage, a short distance above the vocal process. The lower border of this ligament, enclosed in mucous membrane, forms a free crescentic margin, which constitutes the upper boundary of the ventricle of the larynx.

  The Vocal Folds (plicœ vocales; inferior or true vocal cords) are concerned in the production of sound, and enclose two strong bands, named the vocal ligaments (ligamenta vocales; inferior thyroarytenoid). Each ligament consists of a band of yellow elastic tissue, attached in front to the angle of the thyroid cartilage, and behind to the vocal process of the arytenoid. Its lower border is continuous with the thin lateral part of the conus elasticus. Its upper border forms the lower boundary of the ventricle of the larynx. Laterally, the Vocalis muscle lies parallel with it. It is covered medially by mucous membrane, which is extremely thin and closely adherent to its surface.

The Ventricle of the Larynx (ventriculus laryngis [Morgagnii]; laryngeal sinus) is a fusiform fossa, situated between the ventricular and vocal folds on either side, and extending nearly their entire length. The fossa is bounded, above, by the free crescentic edge of the ventricular fold; below, by the straight margin of the vocal fold; laterally, by the mucous membrane covering the corresponding Thyreoarytænoideus. The anterior part of the ventricle leads up by a narrow opening into a cecal pouch of mucous membrane of variable size called the appendix.

  The appendix of the laryngeal ventricle (appendix ventriculi laryngis; laryngeal saccule) is a membranous sac, placed between the ventricular fold and the inner surface of the thyroid cartilage, occasionally extending as far as its upper border or even higher; it is conical in form, and curved slightly backward. On the surface of its mucous membrane are the openings of sixty or seventy mucous glands, which are lodged in the submucous areolar tissue. This sac is enclosed in a fibrous capsule, continuous below with the ventricular ligament. Its medial surface is covered by a few delicate muscular fasciculi, which arise from the apex of the arytenoid cartilage and become lost in the aryepiglottic fold of mucous membrane; laterally it is separated from the thyroid cartilage by the Thyreoepiglotticus. These muscles compress the sac, and express the secretion it contains upon the vocal folds to lubricate their surfaces.

The Rima Glottidis is the elongated fissure or chink between the vocal folds in front, and the bases and vocal processes of the arytenoid cartilages behind. It is therefore subdivided into a larger anterior intramembranous part (glottis vocalis), which measures about three-fifths of the length of the entire aperture, and a posterior intercartilaginous part (glottis respiratoria). Posteriorly it is limited by the mucous membrane passing between the arytenoid cartilages. The rima glottidis is the narrowest part of the cavity of the larynx, and its level corresponds with the bases of the arytenoid cartilages. Its length, in the male, is about 23 mm.; in the female from 17 to 18 mm. The width and shape of the rima glottidis vary with the movements of the vocal folds and arytenoid cartilages during respiration and phonation. In the condition of rest, i. e., when these structures are uninfluenced by muscular action, as in quiet respiration, the intramembranous part is triangular, with its apex in front and its base behind—the latter being represented by a line, about 8 mm. long, connecting the anterior ends of the vocal processes, while the medial surfaces of the arytenoids are parallel to each other, and hence the intercartilaginous part is rectangular. During extreme adduction of the vocal folds, as in the emission of a high note, the intramembranous part is reduced to a linear slit by the apposition of the vocal folds, while the intercartilaginous part is triangular, its apex corresponding to the anterior ends of the vocal processes of the arytenoids, which are approximated by the medial rotation of the cartilages. Conversely in extreme abduction of the vocal folds, as in forced inspiration, the arytenoids and their vocal processes are rotated lateralward, and the intercartilaginous part is triangular in shape but with its apex directed backward. In this condition the entire glottis is somewhat lozenge-shaped, the sides of the intramembranous part diverging from before backward, those of the intercartilaginous part diverging from behind forward—the widest part of the aperture corresponding with the attachments of the vocal folds to the vocal processes.

Muscles.—The muscles of the larynx are extrinsic, passing between the larynx and parts around—these have been described in the section on Myology; and intrinsic, confined entirely to the larynx.

The intrinsic muscles are: Cricothyreoideus.  Cricoarytænoideus lateralis. Cricoarytænoideus posterior.  Arytænoideus. Thyroarytænoideus.

Actions.—In considering the actions of the muscles of the larynx, they may be conveniently divided into two groups, vix.: 1. Those which open and close the glottis. 2. Those which regulate the degree of tension of the vocal folds.   

  The Cricoarytœnoidei posteriores separate the vocal folds, and, consequently, open the glottis, by rotating the arytenoid cartilages outward around a vertical axis passing through the cricoarytenoid joints; so that their vocal processes and the vocal folds attached to them become widely separated.  

  The Cricoarytœnoidei laterales close the glottis by rotating the arytenoid cartilages inward, so as to approximate their vocal processes.   

  The Arytœnoideus approximates the arytenoid cartilages, and thus closes the opening of the glottis, especially at its back part.   

  The Cricothyreoidei produce tension and elongation of the vocal folds by drawing up the arch of the cricoid cartilage and tilting back the upper border of its lamina; the distance between the vocal processes and the angle of the thyroid is thus increased, and the folds are consequently elongated.

  The Thyreoarytœnoidei, consisting of two parts having different attachments and different directions, are rather complicated as regards their action. Their main use is to draw the arytenoid cartilages forward toward the thyroid, and thus shorten and relax the vocal folds. But, owing to the connection of the deeper portion with the vocal fold, this part, if acting separately, is supposed to modify its elasticity and tension, while the lateral portion rotates the arytenoid cartilage inward, and thus narrows the rima glottidis by bringing the two vocal folds together.  

The Trachea and Bronchi

The trachea or windpipe is a cartilaginous and membranous tube, extending from the lower part of the larynx, on a level with the sixth cervical vertebra, to the upper border of the fifth thoracic vertebra, where it divides into the two bronchi, one for each lung. The trachea is nearly but not quite cylindrical, being flattened posteriorly; it measures about 11 cm. in length; its diameter, from side to side, is from 2 to 2.5 cm., being always greater in the male than in the female. In the child the trachea is smaller, more deeply placed, and more movable than in the adult.

Relations.—The anterior surface of the trachea is convex, and covered, in the neck, from above downward, by the isthmus of the thyroid gland, the inferior thyroid veins, the arteria thyroidea ima (when that vessel exists), the Sternothyreoideus and Sternohyoideus muscles, the cervical fascia, and, more superficially, by the anastomosing branches between the anterior jugular veins; in the thorax, it is covered from before backward by the manubrium sterni, the remains of the thymus, the left innominate vein, the aortic arch, the innominate and left common carotid arteries, and the deep cardiac plexus. Posteriorly it is in contact with the esophagus. Laterally, in the neck, it is in relation with the common carotid arteries, the right and left lobes of the thyroid gland, the inferior thyroid arteries, and the recurrent nerves; in the thorax, it lies in the superior mediastinum, and is in relation on the right side with the pleura and right vagus, and near the root of the neck with the innominate artery; on its left side are the left recurrent nerve, the aortic arch, and the left common carotid and subclavian arteries

Structure.—The trachea and extrapulmonary bronchi are composed of imperfect rings of hyaline cartilage, fibrous tissue, muscular fibers, mucous membrane, and glands.  

The Lungs  (Pulmones)

The lungs are the essential organs of respiration; they are two in number, placed one on either side within the thorax, and separated from each other by the heart and other contents of the mediastinum. The substance of the lung is of a light, porous, spongy texture; it floats in water, and crepitates when handled, owing to the presence of air in the alveoli; it is also highly elastic; hence the retracted state of these organs when they are removed from the closed cavity of the thorax. The surface is smooth, shining, and marked out into numerous polyhedral areas, indicating the lobules of the organ: each of these areas is crossed by numerous lighter lines.   

At birth the lungs are pinkish white in color; in adult life the color is a dark slaty gray, mottled in patches; and as age advances, this mottling assumes a black color. The coloring matter consists of granules of a carbonaceous substance deposited in the areolar tissue near the surface of the organ. It increases in quantity as age advances, and is more abundant in males than in females. As a rule, the posterior border of the lung is darker than the anterior.

  The right lung usually weighs about 625 gm., the left 567 gm., but much variation is met with according to the amount of blood or serous fluid they may contain. The lungs are heavier in the male than in the female, their proportion to the body being, in the former, as 1 to 37, in the latter as 1 to 43.

  Each lung is conical in shape, and presents for examination an apex, a base, three borders, and two surfaces.   

  The apex (apex pulmonis) is rounded, and extends into the root of the neck, reaching from 2.5 to 4 cm. above the level of the sternal end of the first rib. A sulcus produced by the subclavian artery as it curves in front of the pleura runs upward and lateralward immediately below the apex.

  The base (basis pulmonis) is broad, concave, and rests upon the convex surface of the diaphragm, which separates the right lung from the right lobe of the liver, and the left lung from the left lobe of the liver, the stomach, and the spleen. Since the diaphragm extends higher on the right than on the left side, the concavity on the base of the right lung is deeper than that on the left. Laterally and behind, the base is bounded by a thin, sharp margin which projects for some distance into the phrenicocostal sinus of the pleura, between the lower ribs and the costal attachment of the diaphragm. The base of the lung descends during inspiration and ascends during expiration.   

Borders.—The inferior border (margo inferior) is thin and sharp where it separates the base from the costal surface and extends into the phrenicocostal sinus; medially where it divides the base from the mediastinal surface it is blunt and rounded.   

  The posterior border (margo posterior) is broad and rounded, and is received into the deep concavity on either side of the vertebral column. It is much longer than the anterior border, and projects, below, into the phrenicocostal sinus.

  The anterior border (margo anterior) is thin and sharp, and overlaps the front of the pericardium. The anterior border of the right lung is almost vertical, and projects into the costomediastinal sinus; that of the left presents, below, an angular notch, the cardiac notch, in which the pericardium is exposed. Opposite this notch the anterior margin of the left lung is situated some little distance lateral to the line of reflection of the corresponding part of the pleura.

The right lung is divided into three lobes, superior, middle, and inferior, by two interlobular fissures. One of these separates the inferior from the middle and superior lobes, and corresponds closely with the fissure in the left lung. Its direction is, however, more vertical, and it cuts the lower border about 7.5 cm. behind its anterior extremity. The other fissure separates the superior from the middle lobe. It begins in the previous fissure near the posterior border of the lung, and, running horizontally forward, cuts the anterior border on a level with the sternal end of the fourth costal cartilage; on the mediastinal surface it may be traced backward to the hilus. The middle lobe, the smallest lobe of the right lung, is wedge-shaped, and includes the lower part of the anterior border and the anterior part of the base of the lung.

  The right lung, although shorter by 2.5 cm. than the left, in consequence of the diaphragm rising higher on the right side to accommodate the liver, is broader, owing to the inclination of the heart to the left side; its total capacity is greater and it weighs more than the left lung.  

The Root of the Lung (radix pulmonis).—A little above the middle of the mediastinal surface of each lung, and nearer its posterior than its anterior border, is its root, by which the lung is connected to the heart and the trachea. The root is formed by the bronchus, the pulmonary artery, the pulmonary veins, the bronchial arteries and veins, the pulmonary plexuses of nerves, lymphatic vessels, bronchial lymph glands, and areolar tissue, all of which are enclosed by a reflection of the pleura. The root of the right lung lies behind the superior vena cava and part of the right atrium, and below the azygos vein. That of the left lung passes beneath the aortic arch and in front of the descending aorta; the phrenic nerve, the pericardiacophrenic artery and vein, and the anterior pulmonary plexus, lie in front of each, and the vagus and posterior pulmonary plexus behind each; below each is the pulmonary ligament.

  The chief structures composing the root of each lung are arranged in a similar manner from before backward on both sides, viz., the upper of the two pulmonary veins in front; the pulmonary artery in the middle; and the bronchus, together with the bronchial vessels, behind. From above downward, on the two sides, their arrangement differs, thus:

  On the right side their position is—eparterial bronchus, pulmonary artery, hyparterial bronchus, pulmonary veins, but on the left side their position is—pulmonary artery, bronchus, pulmonary veins. The lower of the two pulmonary veins, is situated below the bronchus, at the apex or lowest part of the hilus. 

Divisions of the Bronchi.—Just as the lungs differ from each other in the number of their lobes, so the bronchi differ in their mode of subdivision.

  The right bronchus gives off, about 2.5 cm. from the bifurcation of the trachea, a branch for the superior lobe. This branch arises above the level of the pulmonary artery, and is therefore named the eparterial bronchus. All the other divisions of the main stem come off below the pulmonary artery, and consequently are termed hyparterial bronchi. The first of these is distributed to the middle lobe, and the main tube then passes downward and backward into the inferior lobe, giving off in its course a series of large ventral and small dorsal branches. The ventral and dorsal branches arise alternately, and are usually eight in number—four of each kind. The branch to the middle lobe is regarded as the first of the ventral series.

  The left bronchus passes below the level of the pulmonary artery before it divides, and hence all its branches are hyparterial; it may therefore be looked upon as equivalent to that portion of the right bronchus which lies on the distal side of its eparterial branch. The first branch of the left bronchus arises about 5 cm. from the bifurcation of the trachea, and is distributed to the superior lobe. The main stem then enters the inferior lobe, where it divides into ventral and dorsal branches similar to those in the right lung. The branch to the superior lobe of the left lung is regarded as the first of the ventral series.

Structure.—The lungs are composed of an external serous coat, a subserous areolar tissue and the pulmonary substance or parenchyma.

  The serous coat is the pulmonary pleura; it is thin, transparent, and invests the entire organ as far as the root.

  The subserous areolar tissue contains a large proportion of elastic fibers; it invests the entire surface of the lung, and extends inward between the lobules.

  The parenchyma is composed of secondary lobules which, although closely connected together by an interlobular areolar tissue, are quite distinct from one another, and may be teased asunder without much difficulty in the fetus. The secondary lobules vary in size; those on the surface are large, of pyramidal form, the base turned toward the surface; those in the interior smaller, and of various forms. Each secondary lobule is composed of several primary lobules, the anatomical units of the lung. The primary lobule consists of an alveolar duct, the air spaces connected with it and their bloodvessels, lymphatics and nerves.   

  The intrapulmonary bronchi divide and subdivide throughout the entire organ, the smallest subdivisions constituting the lobular bronchioles. The larger divisions consist of: (1) an outer coat of fibrous tissue in which are found at intervals irregular plates of hyaline cartilage, most developed at the points of division; (2) internal to the fibrous coat, a layer of circularly disposed smooth muscle fibers, the bronchial muscle; and (3) most internally, the mucous membrane, lined by columnar ciliated epithelium resting on a basement membrane. The corium of the mucous membrane contains numerous elastic fibers running longitudinally, and a certain amount of lymphoid tissue; it also contains the ducts of mucous glands, the acini of which lie in the fibrous coat. The lobular bronchioles differ from the larger tubes in containing no cartilage and in the fact that the ciliated epithelial cells are cubical in shape. The lobular bronchioles are about 0.2 mm. in diameter.

  Each bronchiole divides into two or more respiratory bronchioles, with scattered alveoli, and each of these again divides into several alveolar ducts, with a greater number of alveoli connected with them. Each alveolar duct is connected with a variable number of irregularly spherical spaces, which also possess alveoli, the atria. With each atrium a variable number (2–5) of alveolar sacs are connected which bear on all parts of their circumference alveoli or air sacs.

  The alveoli are lined by a delicate layer of simple squamous epithelium, the cells of which are united at their edges by cement substance. Between the squames are here and there smaller, polygonal, nucleated cells. Outside the epithelial lining is a little delicate connective tissue containing numerous elastic fibers and a close net-work of blood capillaries, and forming a common wall to adjacent alveoli.   

The fetal lung resembles a gland in that the alveoli have a small lumen and are lined by cubical epithelium. After the first respiration the alveoli become distended, and the epithelium takes on the characters described above.   

The Pleuræ

Each lung is invested by an exceedingly delicate serous membrane, the pleura, which is arranged in the form of a closed invaginated sac. A portion of the serous membrane covers the surface of the lung and dips into the fissures between its lobes; it is called the pulmonary pleura. The rest of the membrane lines the inner surface of the chest wall, covers the diaphragm, and is reflected over the structures occupying the middle of the thorax; this portion is termed the parietal pleura. The two layers are continuous with one another around and below the root of the lung; in health they are in actual contact with one another, but the potential space between them is known as the pleural cavity. When the lung collapses or when air or fluid collects between the two layers the cavity becomes apparent. The right and left pleural sacs are entirely separate from one another; between them are all the thoracic viscera except the lungs, and they only touch each other for a short distance in front; opposite the second and third pieces of the sternum the interval between the two sacs is termed the mediastinum.   

Reflections of the Pleura.—Commencing at the sternum, the pleura passes lateralward, lines the inner surfaces of the costal cartilages, ribs, and Intercostales, and at the back part of the thorax passes over the sympathetic trunk and its branches, and is reflected upon the sides of the bodies of the vertebræ, where it is separated by a narrow interval, the posterior mediastinum, from the opposite pleura. From the vertebral column the pleura passes to the side of the pericardium, which it covers to a slight extent; it then covers the back part of the root of the lung, from the lower border of which a triangular sheet descends vertically toward the diaphragm. Above, its cupula projects through the superior opening of the thorax into the neck, extending from 2.5 to 5 cm. above the sternal end of the first rib; this portion of the sac is strengthened by a dome-like expansion of fascia (Sibson’s fascia), attached in front to the inner border of the first rib, and behind to the anterior border of the transverse process of the seventh cervical vertebra. This is covered and strengthened by a few spreading muscular fibers derived from the Scaleni.  

The free surface of the pleura is smooth, polished, and moistened by a serous fluid; its attached surface is intimately adherent to the lung, and to the pulmonary vessels as they emerge from the pericardium; it is also adherent to the upper surface of the diaphragm: throughout the rest of its extent it is easily separable from the adjacent parts.   

The Mediastinum  (Interpleural Space)

The mediastinum lies between the right and left pleuræ in and near the median sagittal plane of the chest. It extends from the sternum in front to the vertebral column behind, and contains all the thoracic viscera excepting the lungs. It may be divided for purposes of description into two parts: an upper portion, above the upper level of the pericardium, which is named the superior mediastinum; and a lower portion, below the upper level of the pericardium. This lower portion is again subdivided into three parts, viz., that in front of the pericardium, the anterior mediastinum; that containing the pericardium and its contents, the middle mediastinum; and that behind the pericardium, the posterior mediastinum.   

  The Superior Mediastinum is that portion of the interpleural space which lies between the manubrium sterni in front, and the upper thoracic vertebræ behind. It is bounded below by a slightly oblique plane passing backward from the junction of the manubrium and body of the sternum to the lower part of the body of the fourth thoracic vertebra, and laterally by the pleuræ. It contains the origins of the Sternohyoidei and Sternothyreoidei and the lower ends of the Longi colli; the aortic arch; the innominate artery and the thoracic portions of the left common carotid and the left subclavian arteries; the innominate veins and the upper half of the superior vena cava; the left highest intercostal vein; the vagus, cardiac, phrenic, and left recurrent nerves; the trachea, esophagus, and thoracic duct; the remains of the thymus, and some lymph glands.   

The Anterior Mediastinum exists only on the left side where the left pleura diverges from the mid-sternal line. It is bounded in front by the sternum, laterally by the pleuræ, and behind by the pericardium. It is narrow, above, but widens out a little below. Its anterior wall is formed by the left Transversus thoracis and the fifth, sixth, and seventh left costal cartilages. It contains a quantity of loose areolar tissue, some lymphatic vessels which ascend from the convex surface of the liver, two or three anterior mediastinal lymph glands, and the small mediastinal branches of the internal mammary artery.

The Middle Mediastinum is the broadest part of the interpleural space. It contains the heart enclosed in the pericardium, the ascending aorta, the lower half of the superior vena cava with the azygos vein opening into it, the bifurcation of the trachea and the two bronchi, the pulmonary artery dividing into its two branches, the right and left pulmonary veins, the phrenic nerves, and some bronchial lymph glands.   

  The Posterior Mediastinum is an irregular triangular space running parallel with the vertebral column; it is bounded in front by the pericardium above, and by the posterior surface of the diaphragm below, behind by the vertebral column from the lower border of the fourth to the twelfth thoracic vertebra, and on either side by the mediastinal pleura. It contains the thoracic part of the descending aorta, the azygos and the two hemiazygos veins, the vagus and splanchnic nerves, the esophagus, the thoracic duct, and some lymph glands.