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costal sinus, but even after the deepest breath it never reaches the extreme lower limit of this recess.
The diaphragmatic surfaces of the lungs establish important relations with certain of the viscera which occupy the costal zone of the abdominal cavity, the diaphragm alone intervening. Thus the diaphragmatic surface of the right lung rests upon the right lobe of the liver; whilst that of the left lung is in relation to the left lobe of the liver, the fundus of the stomach, the spleen, and in some cases to the left colic flexure.
The costal surface is extensive and convex. It is accurately adapted to that part of the wall of the pleural cavity which is formed by the costal arches and the intervening intercostal muscles, and it presents markings corresponding to these. Thus the imprint of the ribs appear as shallow oblique grooves, while the intercostal spaces show as elongated intervening bulgings.
The mediastinal surface presents a smaller area than the costal surface. It is applied to the mediastinal septum, and presents markings in accordance with the inequalities upon this (Figs. 870 and 871). Thus it is deeply hollowed out in adaptation to the pericardium upon which it fits. This pericardial concavity comprises the greater part of the mediastinal surface, and owing to the greater projection of the heart to the left side, it is much deeper and more extensive in the left lung than in the right lung. Above and dorsal to the pericardial hollow is the hilum of the lung. This is a wedge-shaped depressed FIG. 871. THE MEDIAL SURFACE OF A RIGHT LUNG HARDENED IN SITU. area, within which the blood-vessels, nerves, and lymph vessels, together with the bronchus, enter and leave the organ. Amidst these structures there are also some bronchial glands. The hilum is surrounded by the reflection of the pleura from the surface of the lung on to the pulmonary Dorsal to the hilum and pericardial area there is on each lung a narrow strip of the mediastinal surface of the lung which is in relation to the lateral wall of the dorsal mediastinum. On the right lung this part of the surface is depressed, and corresponds to the oesophagus; on the left lung it presents a broad longitudinal groove, which is produced by the contact of the lung with the thoracic aorta, and also, close to the base, a small flattened area ventral to this which is applied to the esophagus where it pierces the diaphragm.
The portion of the mediastinal surface of the lung which lies above the hilum and pericardial hollow is applied to the lateral aspect of the superior mediastinum. and the markings are accordingly somewhat different on the two sides: On the left lung a broad deep groove, produced by the aortic arch, curves dorsally above the hilum, and becomes continuous with the aortic groove on the dorsal part of the mediastinal surface. From the groove for the aortic arch a narrower, deeper, and
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much more sharply marked groove runs upwards, and laterally over the apex pulmonis a short distance from the summit. This is the sulcus subclavius, and it contains the left subclavian artery when the lung is in place. Ventral to the subclavian sulcus a shallow wide groove, also leading to the ventral aspect of the apex, corresponds to the left innominate vein. In the right lung the hilum is also circumscribed above by a curved groove, which is narrow and more distinctly curved than the aortic groove on the left side. It lodges the vena azygos as it turns ventrally to join the vena cava superior. From the ventral end of the azygos sulcus a wide shallow groove extends upward to the inferior part of the anterior aspect of the apex pulmonis. This is produced by the apposition of the lung with the vena cava superior and the right innominate vein. Close to the summit of the apex there is also, on its medial aspect, a sulcus for the superior end of the innominate artery.
In addition to the hilum, it must now be evident that the mediastinal surface of each lung presents three areas which correspond respectively with (1) the middle mediastinum (i.e. the pericardial hollow), (2) the dorsal mediastinum, and (3) the superior mediastinum; and that in each of these districts impressions corresponding to structures contained within these portions of the interpleural space may be noticed.
The dorsal part of the lung at the junction of the costal and mediastinal surfaces is thick, long, and rounded. It forms the most bulky part of the organ, and occupies the deep hollow in the thoracic cavity which is placed at the side of the vertebral column.
The ventral border or margo anterior of the lung is short, and exceedingly thin and sharp. It begins abruptly immediately below the groove on the apex for the innominate vein, and extends to the base, where it becomes continuous with the sharp inferior border. The thin ventral part of the lung is carried ventrally and medially, ventral to the pericardium, into the narrow pleural costomediastinal sinus, dorsal to the sternum and costal cartilages. The ventral border of the right lung fills up this recess completely, and in the upper part of the chest is separated from the corresponding border of the left lung only by the two layers of mediastinal pleura which are reflected from the sternum to the pericardium. The ventral border of the left lung, in its lower part, shows a marked deficiency or notch, the incisura cardiaca, corresponding to the apex of the heart, and where this exists the Jung margin leaves a considerable portion of the pericardium uncovered, and fails to fill up completely the costo-mediastinal sinus of the pleural cavity. During respiration the ventral margin of the left lung at the incisura cardiaca advances and retreats to a small extent in this pleural sinus, ventral to the pericardium.
Fissures and Lobes of the Lung.-The left lung is divided into two lobes by a long deep fissure, the incisura interlo baris, which penetrates its substance to within a short distance of the hilum. On the upper and lower sides of the hilum this fissure cuts right through the lung and appears on the mediastinal surface. Viewed from the costal surface, it begins dorsally about two and a half inches below the apex, about the level of the vertebral end of the third rib, and is continued downwards and ventrally in a somewhat spiral direction to the diaphragmatic surface of the lung, which it reaches a short distance from its ventral end. The lobus superior lies above and ventral to this cleft. It is conical in form, with an oblique base, and the apex and the whole of the ventral border of the lung belong to it. The lobus inferior lies below and dorsal to the fissure. It is the more bulky of the two, and includes almost the entire diaphragmatic surface and the greater part of the thick dorsal part of the lung.
In the right lung there are two incisuræ interlobares, which subdivide it into three lobes. One of the incisuræ interlobares is very similar in its position and relations to the fissure in the left lung. It is directed, however, rather more vertically, and ends somewhat farther from the median plane. It separates the lobus inferior from the lobus medius and lobus superior. The second incisura interlobaris begins in the main fissure at the dorsal part of the lung, and proceeds ventrally, to end at the ventral border of the lung at the level of the fourth costal
cartilage. The middle lobe of the right lung is triangular or wedge-shaped it outline.
Variations. Variations in the pulmonary fissures are fairly common. Thus, it sometimes happens that the middle lobe of the right lung is imperfectly cut off from the lobus superior Supernumerary fissures also are not infrequent, and in this way the left lung may be cut ink three lobes, and the right lung into four or even more lobes. The occurrence of the lobus azygos in the right lung is a variation of some interest, seeing that such a lobe is constant in certain mammals. It is a small accessory lobe, pyramidal in form, which makes its appearance on the lower part of the mediastinal surface of the right lung. In certain cases the vena azyg is enclosed within a fold of pleura, and is sunk so deeply in the pulmonary substance of the right lung that it marks off a small accessory lobe.
The term root of the lung is applied to a number of structures which enter and leave the lung at the hilum on its mediastinal surface. They are held together by an investment of pleura, and constitute a pedicle which attaches the lung to the mediastinal wall of the pleural cavity. The phrenic nerve passes downwards a short distance ventral to the pulmonary root, whilst the vagus nerve breaks up into the dorsal or posterior pulmonary plexus on its dorsal aspect under cover of the investing pleura. The delicate ventral or anterior pulmonary plexus is placed ventral to the root of the lung under cover of the pleura, whilst from the inferior border of the root of the lung the ligamentum pulmonale extends towards the diaphragm. These relations are common to the pulmonary root on both sides of the body, but there are others which are peculiar to each side. On the right side the vena cava superior lies ventral to the pulmonary root, whilst the vena azygos arches over its upper border. On the left side the aorta arches above the root of the lung, whilst the descending thoracic aorta passes dorsal to it.
Constituent Parts of the Pulmonary Root.-The large structures which enter into the formation of the pulmonary root are (1) the two pulmonary veins, (2 the pulmonary artery, (3) the bronchus. But in addition to these there are one or more small bronchial arteries and veins, the pulmonary nerves and the pulmonary lymph vessels, and some bronchial glands.
The pulmonary nerves come from the vagus nerve and also from the sympathetic system. They enter the lung and follow the air-tubes through the organ. The bronchial arteries are small vessels which carry blood for the supply of the lung-tissue. They arise from the aorta or from an intercostal artery, and vary in number from one to three for each lung. In the root of the lung they lie on the dorsal aspect of the bronchus, and they follow the air-tubes through the organ. Part of the blood conveyed to the lung by the bronchial arteries is returned by the pulmonary veins; the remainder is returned by special bronchial veins which open on the righ side into the vena azygos, and on the left side into the vena hemiazygos.
The lymph-vessels of the lungs are numerous and well developed, and are divided into two groups, superficial and deep.
The superficial lymph-vessels form a network on the surface of the lung and eventually terminate by four or five vessels in the broncho-pulmonary glands of the hilum. It is usually stated that the superficial lymph vessels communicate freely with the deep. This, however, is denied by Miller, who maintains that anastomoses between the two systems of vessels are very rare. A specimen of secondary carcinoma of the lung in the Pathology Museum of the University of Melbourne shows a direct continuation of the disease through the lung-substance from th surface to the tubes by way of the lymph vessels, and would thus tend to disprove Miller's
The deep lymph-vessels are subdivided into bronchial accompanying the bronchi and vascular accompanying the blood-vessels. Both systems communicate freely together, and at the level of the hilum terminate in the broncho-pulmonary glands.
The pulmonary or broncho-pulmonary lymph-glands, found at the hilum, are usually numerous and variable in size. They are situated either just outside the lung or within the lung-substance itself.
From these broncho-pulmonary glands the lymph-flow is continued onward from the lung partly directly into the thoracic duct, and partly by a more circuitous route as follows:
From the broncho-pulmonary lymph-glands vessels pass on to the tracheo-bronchial glandsituated at the angles produced by the bifurcation of the trachea into the two bronchi. Of these glands there are, therefore, three groups, an inferior and right and left superior. From these glands the lymph-flow is continued upwards through the tracheal lymph-glands lying on either side of the trachea into the deep cervical lymph-glands, and thence into the thoracic duct.
The bronchus in the root of the lung lies dorsal to the great pulmonary vessels
The pulmonary artery occupies a different position on the two sides, in relation to the main or undivided part of the bronchus. On the right side it is placed below it, whilst on the left side it crosses the bronchus and occupies a higher level in the pulmonary root. The two pulmonary veins, on both sides, lie at a lower level in the root of the lung than the pulmonary artery and bronchus, whilst the superior of the two veins occupies a plane ventral to the pulmonary artery (Figs. 870 and 871).
Distribution of the Bronchial Tubes within the Lungs.-The two lungs are not symmetrical; the right lung is subdivided into three lobes, and the left lung is cleft into two lobes. The bronchi exhibit a corresponding want of symmetry. The right bronchus, as it approaches the pulmonary hilum, gives off two branches for the superior and middle lobes of the right lung respectively, and then the main stem of the tube enters the inferior lobe. The left bronchus sends off a large branch to the superior lobe of the left lung, and then sinks into the inferior lobe. The first branch of the right bronchus, for the superior lobe, leaves the main stem about one inch from the trachea. The first branch of the left bronchus, on the other hand, takes origin about twice that distance from the trachea.
The relation of the pulmonary artery to the bronchial subdivisions is different on the two sides. On the right side it turns dorsally, to reach the dorsal aspect of the bronchus, inferior to the first, and superior to the second, bronchial branch. On the left side the pulmonary artery turns dorsally above the level of the first bronchial branch. On the right side, therefore, the
first bronchial branch is Eparterial bronchus -
placed above the pulmonary
others lie below the artery, and are termed hyparterial bronchial rami. On the left side there is no eparterial branch; they are all hyparterial.
Left pulmonary artery Hyparterial bronchus
Dorsal branch of bronchus
When the main stem of the bronchus is followed into the inferior lobe of each lung, it is seen to travel downwards and dorsally in the pulmonary substance until it reaches the thin dorsal part of the diaphragmatic surface of the lung which lies between the diaphragm and the thoracic wall, and there it ends. As it proceeds through the inferior lobe it gives off a series of large ventral and a series of smaller dorsal branches. As a rule these are three in number in each case, and the dorsal and ventral branches do not arise opposite to each other, but alternately, one from the dorsum, and then another, after a slight interval, from the ventral surface of the tube. The first hyparterial division on each side (i.e. the branch to the middle. lobe of the right lung and the branch to the superior lobe of the left side) is generally regarded as the first member of the ventral group.
FIG. 872.-DIAGRAM OF THE RELATIONS OF THE PULMONARY
It was Aeby who first recognised the existence in each lung of a main or stem bronchus giving off a ventral and dorsal series of branches, and who drew the distinction between the eparterial and hyparterial bronchial rami. A consideration of these relations led this author to conclude that the eparterial bronchus and the superior lobe of the right lung have no morphological equivalents on the left side of the body. In other words, he was led to believe that the middle lobe of the right lung is the homologue of the superior lobe of the left lung. Hasse, who has also investigated the subject, endorsed this view, with certain modifications and additions; and the hypothesis, either in its original state as presented by Aeby, or as subsequently modified by Hasse, has been, until lately, very generally accepted by anatomists. More recent research.
however, has seriously affected the stability of this conclusion. Narath contends that the distinction between the eparterial bronchus of the right side and the hyparterial bronchi of both sides is not one of fundamental importance, and that a branch which arises from the first hyparterial bronchus on the left side and turns upwards into the apex of the left lung is the direct equivalent of the eparterial bronchus of the right side. This he terms the apical bronchus, and he believes that it represents the first dorsal branch of the left stem-bronchus. Huntington, in a very convincing paper, strongly supports the contention of Narath, and holds that, except "for purposes of topography, we should abandon the distinction between eparterial and hyparterial bronchi." With Narath he regards the eparterial bronchus as a secondary branch which has migrated in an upward direction on the main stem. According to Huntington, therefore, Aeby's proposition should be amended as follows:
The cardiac lobe mentioned in this table is the occasional azygos lobe to which reference has already been made, and it is interesting to note that, whilst the lobe in question as a separate entity is rarely seen in the human lung, the bronchus which corresponds to it is always present in the pulmonary substance as an accessory branch, which proceeds from the main stem as it traverses the inferior lobe of the right side. It receives the name of the cardiac bronchus.
STRUCTURE OF THE LUNG.
The lung is constructed so that the blood which reaches it through the pulmonary artery is brought into the most intimate relation with the air which enters it through the trachea and bronchi. An interchange of materials between the blood and the air is thus rendered possible, and the object of respiration is attained. As a result of this interchange the dark, impure blood, which flows into the lung through the pulmonary artery, is rendered bright red and arterial.
Lobules of the Lung.-A thin layer of subpleural connective tissues lies subjacent to the continuous coating which the lung receives from the pulmonary pleura. From the deep surface of this subpleural layer fine septal processes penetrate into the substance of the lung, and those, with the connective tissue which enters at the hilum upon the vessels and bronchi, constitute a supporting framework for the organ. The lung is lobular, and on the surface the small polygonal areas which represent the lobules are indicated by the pigment present in the connective tissue septa which intervene between them. Although no pigment is present, the lobular character of the lung is particularly well marked in the fœtus, and with a little care the surface lobules in the foetal lung can be separated from each other by gently tearing through the intervening connective tissue. The lobules thus isolated are piriform or pyramidal in form. The broad bases of these lobules abut against the subpleural layer, whilst each of the deep narrow ends receives a minute division from the bronchial system of tubes. The lobules which lie more deeply in the substance of the organ are not so large, and are irregularly polygonal in form.
The Lung Unit. The unit of lung-structure is the lung-lobule. This comprises a terminal bronchus with its air-spaces, blood-vessels, lymph vessels, and nerves.
The terminal bronchus of the lung-unit is attained as follows:-The larger branches of the bronchi, as they traverse the lung, give off numerous divisions, which, by repeated branching, ultimately form a system of tubes which pervade the entire organ. At first the bronchial divisions come off at very acute angles, but as the finer ramifications are reached this character becomes much less apparent. The finer ramifications of the bronchi are termed bronchioles, which by subdivision give rise to the respiratory bronchiole of the lung-unit.
Within the lung unit the respiratory bronchiole gives off a series of terminal bronchi or alveolar ducts, each of which leads to a group of air-spaces termed atria. Each one of the atria communicates, in its turn, with a further and secondary series of air-spaces termed air-sacs or alveolar saccules, the walls of which are pouched out to form the very numerous alveoli or air-cells of the lung-unit.
Structure of the Bronchi.-When the large bronchi enter the lung they become cylindrical, and lose the flattening on the dorsal aspect which is characteristic of the primary bronchi outside the lung. They possess the same coats as are present in the case of the trachea and primary bronchi, but as the tubes become smaller by repeated divisions, these coats become correspondingly thinner and finer. Certain marked differences also in the manner in which the constituents of these coats are arranged become apparent.
In the external fibro-cartilaginous coat the cartilage is no longer present in the form of incomplete rings, but in irregular plates or flakes deposited at various points around