Vessels and Nerves. It is abundantly supplied with blood by the superior and inferior thyroid arteries. Sometimes an additional artery [see p. 289,] is derived from the arteria innominata, and ascends upon the front of the trachea to be distributed to the gland. The wounding of the latter vessel, in tracheotomy, might be fatal to the patient. The nerves are derived from the superior laryngeal and from the middle and inferior cervical ganglia of the sympathetic.

THE LUNGS.

The lungs are two conical organs, situated one at each side of the chest, embracing the heart, and separated from each other by that organ and by a membranous partition, the mediastinum. On the external or thoracic side they are convex, and correspond with the form of the cavity of the chest; internally they are concave, to receive the convexity of the heart. Superiorly they terminate in a tapering cone, which extends above the level of the first rib into the root of the neck, and inferiorly they are broad and concave, and rest on the convex surface of the diaphragm. Their posterior border is rounded, broad and long; the anterior, sharp, short, and marked by one or two deep fissures; and the inferior border which surrounds the base is also sharp. The color of the lungs is pinkish-gray, mottled, and variously marked with black. The surface is figured with irregularly polyhedral outlines, which represent the lobules of the organ, and the area of each of these polyhedral spaces is crossed by lighter lines representing smaller lobules. The weight of the lungs is about forty ounces, the right lung being two ounces heavier than the left.

HEART AND LUNGS. 1. Right ventricle; the vessels to the left of the figure are the middle coronary artery and veins; those to its right, the left coronary artery and veins. 2. Left ventricle. 3. Right auricle. 4. Left auricle. 5. Pulmonary artery. 6. Right pulmonary artery. 7. Left pulmonary artery. 8. Ligament of the ductus arteriosus. 9. Arch of the aorta. 10. Superior vena cava. 11. Arteria innominata, and, in front of it, the right vena innominata. 12. Right subclavian vein, and, behind it, its corresponding artery. 13. Right common carotid artery and vein. 14. Left vena innominata. 15. Left carotid artery and vein. 16. Left subclavian vein and artery. 17. Trachea. 18. Right bronchus. 19. Left bronchus. 20, 20. Pulmonary veins; 18, 20, form the root of the right lung; and 7, 19, 20, the root of the left. 21. Superior lobe of the right lung. 22. Middle lobe. 23. Inferior lobe. 24. Superior lobe of the left lung. 25. Inferior lobe.

Each lung is divided into two lobes, by a long and deep fissure, which extends from the posterior surface of the upper part of the organ, downwards and forwards to near the anterior angle of its base. In the right lung the upper lobe is subdivided by a second fissure, which extends obliquely forwards from the middle of the preceding to the anterior border of the organ, and marks off a small triangular lobe. The left lung presents a deep notch in its anterior border, at a point corresponding with the apex of the heart.

The right lung is larger than the left, in consequence of the inclination of the heart to the left side. It is also shorter, from the great convexity of the liver, which presses the diaphragm upwards on the right side of the chest, considerably above the level of the left; and it has three lobes. The left lung is smaller, has only two lobes, but is longer than the right.

Each lung is retained in its place by its root, which is formed by the pulmo nary artery, pulmonary veins, and bronchial tubes, together with the bronchial vessels and pulmonary plexuses of nerves. The groove on the surface of the lung where the vessels enter its substance is the hilum pulmonis; and the position of the large vessels in the root of the lung, as follows: from before, backwards, they are placed in a similar order on both sides, viz.,

Pulmonary veins,

Pulmonary artery,

Bronchus.

From above downwards, on the right side, this order is exactly reversed; but on the left side, the bronchus has to stoop beneath the arch of the aorta, which alters its position to the vessels. They are thus disposed on the two sides:

:

(Bronchus, Right, Artery, Veins.

Artery,
Left, Bronchus,
Veins.

The special relations of the roots of the lung are, for the right, the descending cava, which lies in front, and the vena azygos, which arches over it from behind to terminate in the superior vena cava. The root of the left lung has the de-scending aorta lying behind it.

Structure. The lungs are composed of the ramifications of the bronchial tubes (bronchia), which terminate in intercellular passages and air-cells, of the ramifications of the pulmonary arteries and veins, bronchial arteries and veins, lymphatics, and nerves; the whole of these structures, being held together by areolar tissue, constitute the parenchyma. The parenchyma of the lungs, when examined on the surface or by means of a section, is seen to consist of small polyhedral divisions, or lobules, which are connected to each other by an interlobular areolar tissue. These lobules again consist of smaller lobules, and the latter are formed by a cluster of air-cells, in the parietes of which the capillaries of the pulmonary artery and pulmonary veins are distributed. Each lobule, taken alone, is provided with its separate bronchial tube, pulmonary artery, and vein, and is isolated from surrounding lobules by a process of areolar membrane derived from the subserous tissue; the entire lung is an assemblage of these lobules, so so separated and so connected, held together by the pleura.

The serous investing membrane of the lungs or pleura is connected with the surface of the lobules by means of a subserous areolar tissue, which forms a distinct layer, and being prolonged between the lobules, is the bond of adhesion between them. This layer contains elastic tissue, and is a chief source of the elasticity of the lungs; its interstices are moistened by a serous secretion, and are unincumbered with fat.

Bronchial Tubes. The two bronchi proceed from the bifurcation of the trachea opposite the third dorsal vertebra to their corresponding lungs. The right, about an inch long, takes its course nearly at right angles with the

trachea, and enters the upper part of the right lung; while the left, two inches in length, and smaller than the right, passes obliquely beneath the arch of the aorta, and enters the lung at about the middle of its root. Upon entering the lungs, they divide into two branches, and each of these divides and subdivides. dichotomously to their ultimate termination in the intercellular passages and air-cells.

FIG. 325.

According to Rainey,' the bronchial tubes continue to diminish in size until they attain a diameter of to of an inch, and arrive within of an inch of the surface of the lung. They then become changed in structure, and are continued onwards in the midst of air-cells, under the name of intercellular passages. Lastly, the intercellular passages, after several bifurcations, terminate each by a cæcal extremity or air-cell. The intercellular passages are at first cylindrical, like the bronchial tubes, but soon become irregular in shape from the great number of air-cells which open into them on all sides. The air-cells in the adult lung measure between 6 and of an inch, they are irregular in shape, and, most frequently, four-sided cavities, separated by thin septa, and communicating freely with the intercellular passages, and sparingly with the bronchial tubes."

In structure the bronchial tubes are composed of cartilages, fibrous membrane, muscular fibres, elastic fibres, and mucous membrane. The cartilages in the primary bronchi are six or eight in number in the right bronchus, and ten or twelve in the left

A DIAGRAM SHOWING THE DILATATION OF THE ULTIMATE BRON

CHIAL TUBES INTO INTERCELLULAR PASSAGES, and the enlargement of the latter near the surface of the

lung.

tubes

[and like those of the trachea are imperfect be- walls of which are seen opening hind]. In the subsequent divisions of the bronchial the air-cells. c, c. Air-cells near tubes, which are cylindrical in figure, the carti- the surface of the lung. lages assume the form of thin plates of irregular

shape and size, adapted to each other by their edges, and completely surrounding the tubes. The plates are most strongly developed around the bifurcations of the tubes, and the point of division is furnished with a cartilage of 1 semilunar form. They are found entering into the structure of the bronchial tubes until the latter are reduced to a quarter of a line in diameter, and are then lost.

The fibrous membrane, which enters largely into the formation of the trachea and bronchial tubes, is the principal coat of the smallest tubes, and is continued to their terminations in the intercellular passages and air-cells.

The muscular fibres, belong to the class of non-striated, smooth, or organic muscle, they are arranged in rings around the tube, and form a muscular coat which is placed externally to the cartilaginous plates and is continued as far as the extremity of the tubes, being absent in the intercellular passages and air-cells. The elastic fibres, arranged in longitudinal fasciculi, form a thin stratum next the mucous lining; this elastic coat is prolonged to the ends of the tubes, and scattered fibres are found around the intercellular passages and air-cells. The mucous membrane, lining the bronchial tubes, is provided with a ciliated columnar epithelium as far as their termination; but in the intercellular passages and air-cells it is altered in its characters, is thin and transparent, and coated with a squamous epithelium.

The capillaries of the lungs form plexuses which occupy the walls and septa of the air-cells and the walls of the intercellular passages, but are not continued into the bronchial tubes. The septa between the air-cells consist of a single layer of the capillary plexus inclosed in a fold of the mucous lining membrane.

'Medico-Chirurgical Transactions, vol. xxviii.

The air-cells of the central parts of the lung are most vascular, and at the same time smallest, whilst those of the periphery are less vascular and larger.

The pigmentary matter of the lungs is contained in the air-cells, as well as in the areolar tissue of the interlobular spaces and blood vessels; it is composed chiefly of carbon.

The Pulmonary artery, conveying the dark and impure venous blood to the lungs, terminates in capillary vessels, which form a dense network in the parietes of the intercellular passages and air-cells, and then converge to form the pulmonary veins, by which the arterial blood, purified in its passage through the capillaries, is returned to the left auricle of the heart.

The Bronchial arteries, branches of the thoracic aorta, ramify on the parietes of the bronchial tubes, and terminate partly in bronchial veins which convey the venous blood to the vena azygos on the right side, and the superior intercostal vein on the left; and partly in the pulmonary capillaries.

The Lymphatics, commencing on the surface and in the substance of the lungs, terminate in the bronchial glands. These glands, very numerous and often of large size, are placed at the roots of the lungs, around the bronchi, and at the bifurcation of the trachea. In early life they resemble lymphatic glands in other situations; but in old age, and often in the adult, are black, and filled with carbonaceous matter, and occasionally with calcareous deposits.

The Nerves are derived from the pneumogastric and sympathetic. They form two plexuses: anterior pulmonary plexus, situated upon the front of the root of the lungs, and composed chiefly of filaments from the deep cardiac plexus; and posterior pulmonary plexus, on the posterior aspect of the root of the lungs, composed principally of branches from the pneumogastric. The branches from these plexuses follow the course of the bronchial tubes, and are distributed to the intercellular passages and air-cells.

PLEURE.

Each lung is inclosed, and its structure maintained, by a serous membrane, the pleura [upa, the side], which invests it as far as the root, and is then reflected on the parietes of the chest. That portion of the membrane which is in relation with the lung is called pleura pulmonalis, and that in contact with the parietes, pleura costalis. The reflected portion, besides forming the internal lining to the ribs and intercostal muscles, also covers the diaphragm and thoracic surface of the vessels at the root of the neck, extending for somewhat more than an inch above the margin of the first rib. At the lower border of the root of the lung is a fold of the pleura, which extends down by the side of the posterior mediastinum to the diaphragm, and serves to retain the lower part of the lung in position. This fold is the broad ligament of the lung, ligamentum latum pulmonis.

On its external surface, where the pleura is connected with surrounding parts, it is rough; on its inner surface, smooth. At the right side, where the diaphragm is pressed upwards by the liver, the pleura is shorter than on the left, but extends higher into the neck; while the left pleura, in consequence of the encroachment of the heart on the left side of the chest, is narrower than the right

MEDIASTINUM.

The approximation of the two reflected pleura in the middle line of the thorax forms a septum, which divides the chest into the two pulmonary cavities. This is the mediastinum. The two pleura are not, however, in contact with each other at the middle line in the formation of the mediastinum, but have a space between them which contains all the viscera of the chest with the excep

tion of the lungs. The mediastinum is divided into an anterior, middle, and posterior portion.

The Anterior mediastinum is a triangular space, bounded in front by the sternum, and at each side by the pleura. It contains a quantity of loose areolar

AN OUTLINE OF A TRANSVERSE SECTION OF THE CHEST, SHOWING the relativE POSITION OF THE PLEURE TO THE THORAX AND ITS CONTENTS. 1, 1, 2, 2, 3. The skin covering the chest. 4, 4. The subcutaneous fat, and muscles on the outside of the thorax. 5, 5. Muscles in the vertebral grooves. 6. Fifth dorsal vertebra. 7. The Spinal canal. 8. Spinous process. 9, 9, 10, 10. Sections of the ribs and intercostal muscles. 11, 11. Costal cartilages. 12. The sternum. 13. The division of the pulmonary artery. 14. The external surface of the lungs. 15. Posterior border of the lungs. 16. Anterior border of the lungs. 17. Inner surface of the lungs. 18. Anterior surface of the heart covered by the pericardium. 19. Pulmonary artery. 21, 20. Its division into right and left branches. 22. Portion of the right auricle. 23. Descending cava cut off at the right auricle. 24. Section of the left bronchus. 25. Section of the right bronchus. 26. The esophagus. 27. The thoracic aorta. The space between figures 12 and 18 and the two 16's is the anterior mediastinum, and the space which contains 26 and 27 is the posterior mediastinum. The middle mediastinum is between the anterior and posterior mediastina. These spaces are formed by the reflexions of the pleuræ.]

tissue, in which are found some lymphatic vessels passing upwards from the liver, the remains of the thymus gland, the origin of the sterno-hyoid, sterno-thyroid, and left triangularis sterni muscle, and the left internal mammary vessels.

The Middle mediastinum contains the heart inclosed in its pericardium; the ascending aorta; superior vena cava; pulmonary arteries and veins; bifurcation of the trachea; and phrenic nerves.

The Posterior mediastinum is bounded behind by the vertebral column, iu front by the pericardium, and at each side by the pleura. It contains the aorta; the greater and lesser azygos vein, and superior intercostal veins; the thoracic duct; oesophagus and pneumogastric nerves; and the great splanchnic nerves.

ABDOMEN.

The abdomen is the inferior cavity of the trunk of the body; it is bounded in front and at the sides by the lower ribs and abdominal muscles; behind, by the vertebral column and abdominal muscles; above, by the diaphragm; and, below, by the pelvis; and contains, the alimentary canal, the organs subservient