INFLECTIONS OF THE PLEURE.

893

tinal partition, and are reflected each upon the root and over the entire free surface of the corresponding lung.

Each pleura consists of a visceral and a parietal portion: the visceral portion, pleura pulmonalis, covers the lung; and the parietal portion lines the ribs and intercostal spaces, pleura costalis, covers the upper convex surface of the diaphragm, enters into the formation of the mediastinum, and is reflected on the sides of the pericardium.

The mediastinum or partition between the two pleural cavities, is formed by the reflection of each pleura from the anterior wall of the chest backwards on the pericardium to the root of the lung, and from the back of the root of the lung to the vertebral column. Its division into anterior, middle,

Fig. 626.-TRANSVERSE SECTION OF THE CHEST OF A FETUS, ILLUSTRATING THE INFLECTIONS OF THE PERICARDIUM AND PLEURE (after Luschka and from nature).

The sketch represents the upper surface of the lower section; the division is carried nearly in a horizontal plane on a level with the interval in front between the fifth and sixth ribs. s, the sternum; c, the body of the seventh dorsal vertebra; h, the right, and ', the left ventricle; a, the oesophagus; pn, the left pneumogastric nerve; near these letters respectively, the root of the right and left lungs; the right pneumogastric nerve is behind the oesophagus; a, the aorta; va, the vena azygos; d, thoracic duct; 1, the cardiac pericardium; 2, the external pericardium; 2', the cavity of the pericardium; 3, the pulmonary pleura passing over the surface, and reflected at the roots of the lungs; 3', their cavity, and on the right side, the reflection at the mediastinum to the surface of the pericardium; 4, the external or costal pleurae; c, c, the walls of the chest inclosing the ribs, pectoral muscles, &c.

and posterior mediastina, and the position and contents of each, have been already described (p. 297).

At the root of each lung which is enclosed by its own pleura, the visceral and parietal portions of this membrane are continuous with each other; and commencing immediately at the lower border of the root, there is found a triangular fold or duplicature of the serous membrane, extending vertically between the inner surface of the lung and the posterior mediastinum, and reaching down to the diaphragm, to which it is attached by its extremity; this fold, which serves to attach the lower part of the lung, is named ligamentum latum pulmonis.

The upper part of each pleura, which receives the apex of the corresponding lung, projects in the form of a cul-de-sac through the superior aperture of the thorax into the neck, reaching an inch or even an inch and a half above the margin of the first rib, and passing up under cover of the scaleni muscles, a small slip of which, arising from the transverse process of the last cervical vertebra, is described by Sibson as expanding into a dome-like aponeurosis or fascia, which covers or strengthens the pleural cul-de-sac, and is attached to the whole of the inner edge of the first rib. The right pleura is generally stated to reach higher in the neck than the left, but in twenty observations recorded by Hutchinson, the right lung was higher in ten cases, and the left in eight, whilst in two the height was equal on the two sides. Anteriorly the pleural sacs of opposite sides come nearly or altogether into contact behind the second piece of the sternum, and continue so for some distance; but opposite the lower end of the sternum the right pleura passes beyond the middle line or remains close to it, while the left recedes to a variable distance from the sternum. Inferiorly the pleura do not pass quite down to the attachments of the diaphragm, but leave a portion of its circumference in contact with the costal parietes. Owing to the height of the diaphragm on the right side (corresponding with the greater convexity of the liver), the right pleural sac is shorter than the left; it is at the same time wider. According to Luschka the right pleura, opposite a line descending from the axilla, reaches down to the lower border of the ninth rib, while the left pleura in the same transverse vertical plane reaches to the lower border of the tenth rib.

Structure. The pleura possesses the usual characters of serous membrane. The costal part of the membrane is the thickest, and may be easily raised from the ribs and intercostal spaces. It is strengthened in these situations by a layer of subserous areolar tissue of considerable thickness. On the pericardium and diaphragm the pleura is thinner and more firmly adherent; but it is thinnest and least easily detached upon the surface of the lungs.

Luschka has described nerves in this membrane, with fine and coarse fibres, which are traceable to the phrenic and sympathetic. Kölliker states that in the pleura pulmonalis of man, branches of nerves may be seen accompanying the ramifications of the bronchial arteries.

THE LUNGS.

cone, having its base turned The base is broad, concave,

Form.-Each lung is shaped somewhat like a downwards, and its inner side much flattened. and of a semilunar form, and rests upon the arch of the diaphragm. It is bounded by a thin margin, which is received in the angle between the ribs and the diaphragm; and it reaches much lower down behind and at the outer side than in front and towards the middle line. The apex forms a blunted point, and, as already mentioned, reaches into the root of the neck, above the margin of the first rib, where it is separated from the first portion of the subclavian artery by the pleural membrane.

FORM AND SITUATION OF THE LUNGS.

895

The outer surface of the lung, which moves upon the thoracic parietes, is smooth, convex, and of great extent, corresponding with the arches of the ribs and costal cartilages. It is of greater depth behind than in front. The posterior border is obtuse or rounded, and is received into the deep groove formed by the ribs at the side of the vertebral coluinn; measured from above downwards, it is the deepest part of the lung. The anterior border is thin and overlaps the pericardium, forming a sharp margin, which touches the sides of the anterior mediastinum, and, opposite the middle of the sternum, is separated during inspiration from the corresponding margin of the opposite lung only by the two thin and adherent layers of the mediastinal septum. The inner surface is concave, and in part adapted to the convex pericardium. Upon this surface, somewhat above the middle of the lung, and considerably nearer to the posterior than the anterior border, is the root, where the bronchi and great vessels join the lung. Each lung is traversed by a long and deep fissure, which is directed from above and behind, downwards and forwards. It commences upon the posterior border of the lung, about three inches from the apex, and extends obliquely downwards to the anterior and inferior angle, penetrating from the outer surface to within some inches of the root of the organ. The upper lobe, the portion of lung which is situated above this fissure, is smaller than the portion below, and is shaped like a cone with an oblique base, whilst the lower and larger lobe is more or less quadrilateral. In the right lung there is a second and shorter fissure, which runs forwards and upwards from the principal fissure to the anterior margin, thus marking off a third small portion, or middle lobe, which appears like an angular piece separated from the anterior and lower part of the upper lobe. The left lung, which has no such middle lobe, presents a deep notch in its anterior border, into which the apex of the heart (enclosed in the pericardium) is received. This notch is formed by the rapid retreat of the anterior margin of the upper lobe from the middle line, opposite the lower half of the sternum; while inferiorly a tongue-like process of the lower lobe usually projects slightly inwards towards the middle line. Besides these differences in form which distinguish the lungs, it is to be noted that the right lung is shorter, but at the same time wider, than the left, the perpendicular measurement of the former being less, owing to the diaphragm rising higher on the right side to accommodate the liver, whilst the breadth of the left lung is narrowed, owing to the heart and pericardium encroaching on the left half of the thorax. On the whole, however, as is seen on a comparison of their weights, the right is the larger of the two lungs.

At the apices and posterior borders the extent of the lungs accurately corresponds with that of the pleural sacs which contain them, but at the anterior and inferior margins it is not so: the anterior margins pass forwards most completely between the mediastinal and costal pleura during inspiration, and retire to a variable degree from between them in expiration; and in like manner the inferior margins descend, during inspiration, between the costal and diaphragmatic pleuræ, while probably at no time do they ever descend completely to the line of reflection between those membranes.

Weight, Dimensions, and Capacity.-The lungs vary much in size and weight according to the quantity of blood, mucus, or serous fluid they may happen to contain, which is greatly influenced by the circumstances immediately preceding death, as well as by other causes. The weight of both lungs together, as generally stated, ranges from thirty to forty-eight ounces, the more prevalent weights being found between thirty-six and forty-two ounces. The proportion borne by the right lung to the left is nearly that of

twenty-two ounces to twenty, taking the combined weight of the two at forty-two ounces. The lungs are not only absolutely heavier in the male than in the female, but appear to be heavier in proportion to the weight of the body. The general ratio between the weight of the lungs and body, in the adult, fluctuates according to the estimate of Krause, between one to thirty-five and one to fifty.

The following tables, deduced from Reid's and Hutchinson's observations, show the average weight of the right and left lungs, and of both lungs together, and also the relative weight of the lungs to the body in a certain number of adults of both sexes.

AVERAGE WEIGHT IN TWENTY-NINE MALES AND TWENTY-ONE FEMALES.

-(REID.)

AVERAGE IN TWENTY-FIVE MALES AND THIRTEEN FEMALES,—(REID AND HUTCHINSON.)

Proportionate weight of the lungs to the body... 1 to 37...........1 to 43

The size and cubical dimensions of the lungs are influenced so much by their state of inflation, and are therefore so variable, that no useful application can be made of many of the statements given as to these measurements. It is important, however, to ascertain the quantity of air which they contain under different conditions. This subject has been investigated by many inquirers, whose statements on this point, however, are exceedingly various. The volume of air contained in the lungs after a forced expiration, has been estimated by Goodwyn at 109 cubic inches. After an ordinary expiration it would seem that at least 60 cubic inches more are retained in the chest, giving a total of 170 cubic inches in that condition of the lungs. The amount of air inhaled and expelled in ordinary breathing has been very differently estimated by different observers; it is most probably from 16 to 20 cubic inches. According to the extensive researches of Hutchinson, men of mean height, between five and six feet, after a complete inspiration, expel from the chest, by a forced expiration, on an average, 225 cubic inches of air, at a temperature of 60°. This quantity is called by Hutchinson the vital capacity of the lungs. If to it be added the average quantity found by Goodwyn to be retained in the lungs after complete expiration, viz. 109 cubic inches, the result will yield 334 cubic inches of air at 60°, as the average total capacity of the respiratory organs for air in an adult male of ordinary height.

The vital capacity (or difference between extreme expiration and extreme inspiration) is found by Hutchinson to bear a very uniform relation to the height of the individual, increasing at the rate of eight cubic inches for every additional inch of stature above five feet. Its relations with the weight of the body are not thus regularly progressive, for it appears to increase about one cubic inch for each additional pound between the weights of 105 lb. and 155 lb., or 74 stones and 11 stones, and to decrease at a similar rate between the weights of 11 and 14 stones, or 155 and 200 lbs. From the age of 15 to 35 years the vital capacity continues to advance with the growth and activity of the frame, but between the ages of 35 and 65 it diminishes at the rate of upwards of 1 cubic inch per annum. This differential or vital capacity is by no means in proportion to the size of the thorax, whether that be estimated by the cir cumference of the chest, or by the sectional area of its base, or by its absolute capacity, as ascertained by measuring its cubical contents after death. It is found rather, that the vital capacity is strictly commensurate with the extent of the thoracic movements, and with the integrity of the lungs themselves; so that in phthisis, for example, it becomes reduced by a quantity varying from 10 to 70 per cent., according to the stage of the disease. A change from the erect to the sitting posture is accompanied by a diminution of the vital capacity, which in one case fell from 260 to 255 cubic inches, and on lying down, it was farther diminished to 230 cubic inches in the supine, and 220 cubic inches in the prone position of the body. Lastly, it is lowered by from 12

GENERAL STRUCTURE OF THE LUNGS.

897

to 20 cubic inches by the presence of a full meal in the stomach. (Hutchinson, in Journal of Statistical Society, August, 1844; and in Medico-Chirurg. Transactions, vol. xxix., 1846; also, in the article" Thorax," in Cyclopædia of Anatomy and Physiology, and the article "Respiration," by Reid, in the same.)

Texture and consistence.-The substance of the lung is of a light porous spongy structure, and, when healthy, is buoyant in water: but in the foetus, before respiration has taken place, and also in certain cases of congestion, collapse, or consolidation from disease, the entire lungs, or portions of them, sink in that fluid. The specific gravity of a healthy lung, as found after death, varies from 345 to 746, water being 1000. When the lung is fully distended its specific gravity is 126, whilst that of the pulmonary substance, entirely deprived of air, is 1056. (Krause.) When pressed between the fingers, the lungs impart a crepitant sensation, which is accompanied by a peculiar noise, both effects being caused by the air contained in the tissue. On cutting into the lung, the same crepitation is heard, and there exudes from the cut surface a reddish frothy fluid, which is partly mucus from the air tubes and air cells, and partly a serous exudation, tinged with blood, and rendered frothy by the admixed air. This fluid escapes in largest quantity from the posterior portion of the lung.

The pulmonary tissue is endowed with great elasticity, in consequence of which the lungs collapse to about one-third of their bulk, when the thorax is opened, and the resistance offered by the walls of that cavity to the atmospheric pressure on their outer surface is in this way removed. Owing to this elasticity also, the lungs, if artificially inflated out of the body, contract to their previous volume, when the air is again allowed to escape.

Colour.-In infancy the lungs are of a pale rose-pink colour, which might be compared to blood-froth; but as life advances they become darker, and are mottled or variegated with spots, patches, and streaks of dark slate colour, which sometimes increase to such a degree as to render the surface almost uniformly black.

The dark colouring matter found in these streaks is in the form of granules and collections of granules, not inclosed in cells; it is deposited in the interstitial areolar tissue mostly near the surface of the lung, and is not found so abundantly in the deeper substance. It exists sometimes in the air cells, and on the coats of the larger vessels. Its quantity increases with age, and is said to be less abundant in females than in males. In persons who follow the occupation of quarriers, more especially colliers, the lungs are often intensely charged with black matter. The black colouring substance of the lung seems to be composed of a mixture of carbon and some animal matter. A black substance of precisely the same nature is found in the bronchial glands. In exceptional cases the adult lungs are found with only very slight streaks of pigment.

Root of the Lung.

The root of each lung is composed of the bronchus and the large bloodvessels, together with the nerves, lymphatic vessels, and glands, connected together by areolar tissue, and enclosed in a sheath of the pleura.

The root of the right lung lies behind the superior vena cava and part of the right auricle, and below the azygos vein, which arches over it to enter the superior cava. That of the left lung passes below the arch of the aorta, and in front of the descending aorta. The phrenic nerve descends in front of the root of each lung, and the pneumogastric nerve behind, whilst the ligamentum latum pulmonis is continued from the lower border. The bronchus, together with the bronchial arteries and veins, the lymphatics