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the line of diaphragmatic reflection is carried downwards and laterally across the extremities of the bony portions of the ninth and tenth ribs. As it crosses the tenth rib, or, it may be, as it proceeds across the tenth intercostal space, the line of pleural reflection reaches its lowest point, and it is important to observe that this point lies in the mid-lateral line (i.e. in a vertical line drawn on the side of the chest, midway between vertebral column and sternum). Thence, as it curves dorsally towards the vertebral column, it passes slightly upwards. Thus it cuts across the eleventh rib and reaches the twelfth rib. The relation which it presents to the twelfth rib varies in accordance with the length of that rib. When the last rib is not abnormally short the pleura clothes its medial half, and the line of reflection falls below that portion of the rib, so as to reach the vertebral column, midway between the capitulum of the last rib and the transverse process of the first lumbar vertebra (Fig. 866). There, therefore, the line of diaphragmatic reflection falls below the inferior border of the thoracic wall; and this is a point of practical importance, because in operations upon the kidney the incision cannot be carried above the level of the transverse process of the first lumbar vertebra and the lateral lumbo-costal arch without the risk of wounding the pleura. On the right side the line of diaphragmatic pleural reflection differs from that on the left chiefly ventrally (Fig. 864, p. 1086). There it passes at a lower level, and proceeds laterally and downwards from the dorsum of the xiphoid process along the dorsal aspect of the seventh costal cartilage, and then behind the eighth costal arch, as a rule at the same point as on the left side, viz., the junction of its cartilaginous and bony parts. From that point to the vertebral column the relations are so similar to those of the left side that a separate description is unnecessary.

It is commonly stated that the left pleural sac reaches a lower level than the right. In certain cases there is no doubt that it does, but this condition is by no means the rule. In those cases where the two pleural sacs do not reach the same level at their lowest points, it is sometimes the right and sometimes the left pleura which oversteps the mark.

As already stated, the lowest point which the pleura attains is usually found, on both sides, in the mid-lateral line where the diaphragmatic reflection-line crosses the tenth rib or the tenth intercostal space. That point can be very readily ascertained on the surface by drawing a horizontal line round the trunk at the level of the lowest part of the extremity of the spinous process of the first lumbar vertebra, and noting where it is intersected by the mid-lateral line. In the majority of cases the point of intersection will correspond with the lowest part of the pleural sac. Another horizontal line opposite the spine of the twelfth thoracic vertebra will give the level of the diaphragmatic pleural reflection in the mamillary line.

Along the line of the diaphragmatic reflection a strong fascia passes from the uncovered part of the diaphragm, and from the costal cartilages to the surface of the costal pleura, so as to hold it firmly in its place. It may be termed the phrenicopleural fascia.

MEDIASTINUM.

The term mediastinum is applied to the interval between the mediastinal portions of the two pleural sacs. Ventrally it is bounded by the sternum, and dorsally by the vertebral column. It is customary to subdivide this space in a purely arbitrary manner into four portions, termed respectively the superior or cranial, the ventral or anterior, the middle, and the dorsal or posterior part, according to the relations which they present to the pericardium.

The superior mediastinum is that part of the general area which lies above the level of the pericardium. Its boundaries are as follows :- Ventrally, the manubrium sterni, with the attached sterno-hyoid and sterno-thyreoid muscles ; dorsally, the bodies of the first four thoracic vertebræ; below, an imaginary and oblique plane, which extends from the inferior border of the manubrium sterni to the inferior border of the fourth thoracic vertebra ; laterally, the mediastinal pleura.

Within the superior mediastinum are placed (1) the aortic arch and the three

The above description represents the average results which have been obtained from the study of the pleura in a large number of subjects, eight of which were specially hardened by formalin or other re-agents for the purpose. For many of the dissections I have to thank my former assistant, Dr. H. St. J. Brooks, and for others I am indebted to Professor C. J. Patten of Sheffield.-D. J. C.

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great arteries which spring from it; (2) the innominate veins and part of the vena cava superior; (3) the trachea, oesophagus, and thoracic duct; (4) the phrenic, vagi, and cardiac nerves, and the left recurrent nerve; (5) the thymus.

The middle mediastinum is the wide part of the area which contains the pericardium, and lies below the superior mediastinum. In addition to the pericardium and its contents the middle mediastinum contains the phrenic nerves and their accompanying vessels.

The ventral mediastinum is that part of the interpleural region which lies between the pericardium dorsally and the body of the sternum ventrally In its superior

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FIG. 867.—SAGITTAL SECTION THROUGH THE THORAX OF AN OLD Man. The superior border of the

manubrium sterni and the bifurcation of the trachea are lower than in the average adult. part this region can hardly. be said to exist, seeing that there the two pleural sacs come into contact with each other on the ventral aspect of the pericardium; but below the level of the sternal ends of the fourth costal cartilages the left pleura falls short of the right pleura, and an interval is apparent. The only contents to be noticed in the ventral mediastinum are a few lymph glands and some areolar tissue, in which ramify some lymph vessels, and some minute twigs from the internal mammary artery.

The dorsal mediastinum is that part of the interpleural region which is situated dorsal to the pericardium. It may be regarded as an inferior continuation of the more dorsal part of the superior mediastinum, and many of the structures in the one are prolonged into the other. The arbitrary superior limit of the dorsal mediastinum is the inferior border of the fourth thoracic vertebra. Ventrally it is bounded by the pericardium and the vertical part of the diaphragm. Dorsally it is limited by the bodies of the last eight thoracic vertebræ and on each lateral side by the mediastinal pleura. It contains the descending

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thoracic aorta, the aortic intercostal arteries, the azygos, hemiazygos and accessory hemiazygos veins, the thoracic duct and the oesophagus, with the two vagi.

Structure of the Pleura.--The pleura on each side is a closed sac, and, like other serous membranes, is attached to the wall of the cavity which it lines and to the surface of the viscus which it covers. It is composed of a thin connective-tissue stratum, in which bundles of fibres cross each other in various directions, and intermixed with which there is a considerable quantity of elastic tissue. On the internal surface of this there is a continuous coating of thin endothelial cells placed edge to edge. The pleura so formed is attached to the parts which it lines and invests by a small amount of areolar tissue termed the subserous layer. In the case of the pulmonary pleura the subserous tissue is continuous with the areolar tissue in the substance of the lung, and this accounts for the tight manner in which the membrane is bound down.

The pleura is plentifully supplied with blood. This is conveyed to it by minute twigs from the intercostal arteries, the internal mammary artery, and the bronchial arteries. Lymph vessels are also particularly abundant in the pleura and in the subserous layer, and it is by these that excess of Auid is conveyed from its cavity. Many lymph vessels communicate directly with the cavity by means of excessively minute orifices termed stomata. Dybkowsky has shown that the lymph vessels and stomata of the pleura are not equally distributed throughout the membrane. Over the ribs and on the niediastinal pleura they are absent.

PULMONES.

The Lungs.—When healthy and sound each lung lies free within the corresponding pleural cavity, and is attached only by its root and the ligamentum pulmonale. It is uncommon, however, in the dissecting room, to meet with a perfectly healthy lung. Adhesions between the pulmonary and parietal layers of pleura, due to pleurisy, are generally present.

Like the cavities in which they are placed, the two lungs are not precisely alike. The right lung is slightly larger than the left, in the proportion of about 11 to 10. The right lung is also shorter and wider than the left lung. This difference is due partly to the great bulk of the right lobe of the liver, which forces the right cupola of the diaphragm to a higher level than the left cupola, and partly to the heart and pericardium projecting more to the left than to the right, thus diminishing the width of the left lung.

The lung is light, soft, and spongy in texture; when pressed between the finger and thumb it crepitates, and when placed in water it floats. The elasticity of the pulmonary tissue is very remarkable. A striking demonstration of this is afforded when the thoracic cavity is opened, and the atmospheric pressure acting upon the interior and exterior of the lung is equalised. Under these conditions the organ immediately collapses to about one-third of its original bulk, and it becomes impossible in such a specimen to study its proper form and dimensions.

The surface of the adult lung presents a mottled appearance. The ground colour is a light slate-blue, but scattered over this there are numerous dark patches of various sizes, and also fine dark intersecting lines. The coloration of the lung differs considerably at different periods of life. In early childhood the lung is rosy-pink, and the darker colour and the mottling of the surface, which appear later, are due to the pulmonary substance, and particularly its interstitial areolar tissue, becoming impregnated, more or less completely, with atmospheric dust and minute particles of soot.

At every breath foreign matter of this kind is inhaled, but only a small proportion of it reaches the lung tissue. The greater part of it becomes entangled in the mucus which coats the mucous membrane of the larger air - passages, and is gradually got rid of along with the mucus through the activity of the cilia attached to the lining epithelium. By the constant upward sweep of these a current towards the pharynx is established. The fine dust and soot particles which reach the finer recesses of the lungs, and ultimately the interstitial tissue, are partly conveyed away by the lymph vessels to the bronchial glands, which in consequence become, in many cases, quite black. The colour of the lung, therefore, depends, to some extent, upon the purity of the atmosphere which is inhaled, and it thus happens thai in coalminers the surface of the lung may be very nearly uniformly black.

The fætal lung differs in a marked degree from the lung of an individual who has breathed. After respiration is fully established, the lung soon comes to occupy almost the whole space allotted to it in the pleural cavity; in the fætus, on the other hand, the lung is packed away at the dorsal aspect, and occupies a relatively much smaller amount of space in the thoracic cavity. Further, it is firm to the touch, and sinks in water. It is only when air and an increased supply of blood are

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Fig. 868.—DISSECTION OF THORAX AND RooT OF THE NECK FROM THE FRONT TO SHOW THE RELATIONS

OF THE LUNGS, PERICARDIUM, AND THYMUS.

introduced into the lung that it assumes the soft spongy and buoyant qualities which are characteristic of the adult lung.

Form of the Lungs.—The lungs are accurately adapted to the walls of the pleural chambers in which they are placed, and in the natural state they bear on the surface impressions and elevations which are an exact counterpart of the irregularities on the walls of the cavity in which they lie.

When care has been taken to harden it in situ, each lung presents for examination an apex, diaphragmatic, mediastinal, and costal surfaces, and ventral (anterior) and inferior borders.

The apex pulmonis is blunt and rounded, and rises above the level of the oblique first costal arch to the full height of the cupula pleuræ. It therefore protrudes, above, through the superior aperture of the thorax, into the root of the

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neck. The subclavian artery arches laterally on its medial and ventral aspects a short distance below its sum

Trachea mit, and a groove, the sulcus Subclavian sulcus subclavius, corresponding to

Subclavian sulcus Groove caused by the first rib

Groove caused the vessel, is apparent upon it.

by the first rib At a lower level on the apex pulmonis a shallower and wider groove upon its medial and ventral aspects marks the position of the innominate vein. Although these vessels impress the lung they are separated from it by the cupula pleuræ.

The diaphragmatic surface, or base of the lung, presents a semilunar outline, being curved around the base of the pericardium. It is adapted to the thoracic surface of the diaphragm, and is consequently

Lower lobe

Lower lobe deeply hollowed out. As the

Fig. 869.-THE TRACHEA, BRONCHI, AND LUNGS OF A CHILD, right cupola of the diaphragm

HARDENED BY FORMALIN INJECTION. passes further upwards than the left, the basal concavity of the right lung is deeper than that of the left

lung. Laterally and Groove for left subclavian artery

dorsally, the diaGroove for left

phragmatic surface of innominate vein

each lung is limited

by a thin salient mar. Groove for first rib gin, called the inferior

border or margin, which Groove for arch of aorta

Groove for tissue

extends downwards for in mediastinum, some distance in a thymus, etc.

narrow pleural recess, the sinus

sinus phrenicoLeft pulmonary

costalis, between the artery Upper left

Groove for diaphragm and

the pulmonary vein Left bronchus

chest - wall. This inarteriosus

ferior border of the Lower left

lung extends further pulmonary

downwards on the vein

lateralsideand dorsally Depression

than it does ventrally, Pulmonary

but it falls considerligament

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costal sinus. Thus, notch

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reaches the inferior cesophagus

border of the sixth rib in the mamillary line; the eighth rib, in the

axillary or mid-lateral Fig. 870.--MEDIAL SURFACE OF LEFT LUNG HARDENED I SITU.

line; whilst dorsally

it proceeds medially long a straight horizontal line so as to reach the vertebral column at the level f the extremity of the spine of the tenth thoracic vertebra. During respiration he thin inferior border moves freely in a vertical direction within the phrenico

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