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These differences in the calibre of the tube are determined by the surroundings of the trachea. The cervical part is narrowed owing to its being clasped by the thyreoid gland. Further, a short distance above its bifurcation, an impression, sometimes strongly marked, is usually seen on the left side of the trachea. It is due to the close contact of the aortic arch as it passes dorsally against that part of the tube. It is evident therefore that the second slight diminution in calibre which is described by Braune and Stahel is pro
duced by the proximity of the aorta. Lejars gives the average ventro-dorsal diameter of the trachea in the living person as 11 mm., and the transverse diameter as 12.5 mm. In the dead subject the lumen of the tube is considerably greater.
The trachea adheres rigorously to the median plane except towards its termination, where it deviates very slightly to the right. As it passes downwards it recedes rapidly from the surface. This is due to its following the curvature of the vertebral column, from which it is separated by the œsophagus alone.
Relations of the Trachea. In the study of the relations of the trachea it is convenient to consider it in the two stages of cervical and thoracic.
When the chin is held so that the face looks directly forwards the cervical part of the trachea measures from 2 to 2 inches in length; but when the head is thrown dorsally or backwards the length is considerably increased. It is clasped by the thyreoid gland, the isthmus of which is applied to its ventral surface, and covers the second, third, and fourth rings; while on each side the corresponding lobe of the thyreoid gland is applied to the side of the trachea and extends downwards to the fifth or sixth ring. On
either side of the cervical part of the trachea is the common carotid artery, whilst the recurrent nerve passes upwards in the groove between the trachea and the oesophagus. Dorsally the trachea is in relation to the oesophagus, which intervenes between it and the bodies of the vertebræ and deviates somewhat to the left as it passes downwards.
In addition to the isthmus of the thyreoid gland two thin muscular strata, composed of the sterno-hyoid and the sterno-thyreoid muscles, as well as the deep.
cervical fascia and integument, separate the cervical part of the trachea from the surface. In the median plane of the neck there is a narrow diamond-shaped space between the medial margins of these muscles, within which the trachea is covered merely by the integuments and fascia. It is important to note that in the inferior
part of the neck the cervical fascia is in two layers-viz, a strong stratum applied to the ventral surface of the sterno-hyoid and sterno-thyreoid muscles, and a weaker superficial layer stretching across between the two sterno-cleido-mastoid muscles. Dorsal to these muscular and fascial layers the inferior thyreoid veins pass downwards on the ventral surface of the trachea, and sometimes the occasional thyreoidea ima artery passes upwards on the ventral aspect of the tube. At the superior border of the manubrium sterni the innominate artery may be seen crossing the trachea obliquely.
The thoracic part of the trachea is situated in the dorsal part of the superior mediastinum, being separated from the bodies of the ver tebræ by the œsophagus alone Immediately above its bifurcation the deep cardiac plexus of nerves is placed on the ventral and latera aspects of the trachea. At the level of the fourth thoracic vertebra the aortic arch very intimately related to it lying first on the ventral side of the tube, and thereafter on its left side. The three great vessels which spring from the aortic arch are also placed in close proximity to the traches. The innominate and the left common carotid arteries lie at first ventral to the trachea. and then gradually diverging
FIG. 859.-TRANSVERSE SECTIONS through the trachea and its imme- as they proceed upwards.
diate surroundings at the level of each of the upper five thoracic vertebræ,
come to lie on either side of it- the innominate to the right, and the left common carotid to the left. Ventral to these vessels are the left innominate vein and the remains of the thymus. On the right side the thoracic part of the trachea is in relation to the right vagus nerve, and is clothed by the right mediastinal pleura; on the left side are the left subclavian artery and the left recurrent nerve.
Structure of the Wall of the Trachea.-The walls of the trachea and bronchi are composed of (1) a fibro-elastic membrane in which the cartilaginous bars are I embedded; (2) within this, and on the dorsal aspect of the tube, a layer of muscular tissue, termed the musculus trachealis; and (3) the lining mucous membrane.
The fibro-elastic membrane is strong and dense. It passes round the whole circumference of the tube, and becomes continuous, above, with the perichondrium which invests the cricoid cartilage. Embedded in its substance are the series of cartilaginous bars. These vary in number from 15 to 20, and are composed of hyaline cartilage. They are horseshoe-shaped, the dorsal fourth of the circumference being deficient, so that dorsally each bar ends in two rounded extremities. The outer surface of each tracheal bar is flat and even, and does not project much beyond the level of the membrane in which
FIG. 860. 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.
it is embedded; the inner surface, however, is convex in the vertical direction, and consequently it bulges slightly into the lumen of the tube. The intervals between the bars are somewhat narrower than the bars themselves, and neighbouring bars frequently show a more or less complete fusion, whilst others present other irregularities, such as a tendency to bifurcate. The lowermost bar is specially adapted to the tracheal bifurcation. In the median plane, ventrally, it inclines downwards, and from this median peak a cartilaginous strip is carried backwards in the fork between the two bronchi.
The m. trachealis is a continuous layer of involuntary muscular tissue, placed in the dorsal part of the wall of the trachea internal to the fibro-elastic membrane. The muscular bundles are arranged transversely, and are attached to the extremities of the bars, and also to the inner surfaces of the bars for a short distance beyond their extremities. In the intervals between the bars the transverse muscular bundles are attached to the fibro-elastic membrane. It is evident that, by its contraction, this muscle will reduce, in a marked degree, the lumen of the tube.
The mucous membrane is laid smoothly over the interior of the tube upon a layer of submucous areolar tissue. Lymph tissue enters largely into the composition of the tracheal mucous membrane, and its inner surface is lined with columnar ciliated epithelial cells. The action of the cilia exercises an important influence in producing an upward movement of the mucus which is present on the surface of the mucous membrane.
Numerous longitudinal bundles of elastic tissue are present in the dorsal wall of the trachea, more particularly in its inferior part, between the mucous membrane and the trachealis muscle.
In connexion with the mucous membrane there is a plentiful supply of acinous mucous glands. These are placed in the submucous tissue, and also, on the dorsal aspect of the tube, on the exterior of the trachealis as well as amidst its muscular bundles. They send their ducts to the surface of the mucous membrane, where they open by trumpetshaped mouths.
The two chief bronchi proceed obliquely downwards and laterally from the ter mination of the trachea, each towards the hilum of the corresponding lung. Like the
FIG. 861.-DRAWING OF A STEREOSCOPIC SKIAGRAPH OF THE TRACHEA AND BRONCHI INJECTED
trachea, they are kept permanently patent by the presence of cartilaginous bars in their walls. These bars are deficient dorsally, so that each bronchus exhibits & flattened dorsal surface, in every respect similar to the trachea. The two bronchi differ from each other, not only in the relations which they present to surrounding structures, but also in length, in width, and in the direction which they pursue (Fig. 858, p. 1079).
The first collateral branch of the right bronchus (ramus bronchialis eparterialis) arises much nearer the trachea than the first branch of the left bronchus. This difference determines the length of the primary divisions of the trachea, and, although there is much variation in the matter, it may be said that, as a rule, the
left bronchus is at least twice as long as the right bronchus. According to Henle there are from six to eight bars in the right, and from nine to twelve bars in the left, bronchus. A marked difference is also noticeable in the calibre of the two tubes. The right bronchus is wider than the left in the proportion of 100 to 784 (Braune and Stahel), and this asymmetry is clearly due to the fact that the right lung is more bulky than the left. The right bronchus, as it passes towards the hilum of the right lung, takes a more vertical course than the left bronchus. It therefore lies more in the line of the trachea, and to this, as well as to its greater width, is due the greater tendency which foreign bodies exhibit, when introduced into the trachea, to drop into the right bronchus, in preference to the left. The average angle which the right bronchus forms with the median plane is 24-8°, whilst the angle formed by the left bronchus with the median plane is 45-6°. The more horizontal course of the left bronchus is probably determined by the marked projection of the heart to the left side of the median plane (Merkel).
Relations of the Bronchi.—Arching above the right bronchus, from behind forwards, is the vena azygos, whilst arching above the left bronchus, from before backwards, there is the arch of the aorta. Occupying the interval between the bronchi there is a cluster of bronchial lymph glands, and an irregular chain of similar glands is carried along each tube towards the hilum of the lung. On the dorsal aspect of each bronchus the vagus nerve breaks up into the plexus pulmonalis posterior, whilst the left bronchus, as it proceeds downwards and laterally, crosses the ventral surfaces of the oesophagus and descending thoracic aorta. The most interesting relation is, however, that presented on each side by the corresponding pulmonary artery. On the left side the pulmonary artery crosses ventral or anterior to the left bronchus on the superior side of its first collateral branch, and then turns round its lateral side to gain its dorsal aspect. All the left bronchial branches, therefore, are placed inferior to the left pulmonary artery, and are in consequence termed hyparterial. The right pulmonary artery, on the other hand, crosses ventral to the continuation of the right bronchus, inferior to its first collateral branch. This branch is therefore termed the eparterial bronchus, whilst all the others are classified as hyparterial.
Structure of the Walls of the Bronchi.-The walls of the bronchi present a structure similar to that seen in the trachea.
The cavity of the thorax is divided into two large lateral chambers, which contain the lungs, by a median partition termed the mediastinal septum, which extends from the vertebral column to the ventral thoracic wall. From the fact that each of these chambers is lined by an extensive and separate serous membrane called the pleura, they receive the name of the pleural cavities.
Septum Mediastinale. The mediastinal septum is built up of several structures which lie in, or in close proximity to, the median plane. The more important of these are the heart, enveloped in its pericardium, the thoracic aorta, with the great vessels which spring from its arch, the pulmonary artery, and the great veins in the neighbourhood of the heart, the thymus or its remains, the trachea, œsophagus, and thoracic duct, and the vagi and phrenic nerves.
Cavum Pleuræ. The pleural cavities, in which the two lungs lie, comprise much the larger part of the thoracic cavity. Each pleural cavity is bounded inferiorly by the corresponding cupola of the diaphragm; and as the right cupola. rises to a higher level than the left, the right pleural cavity presents a smaller vertical depth than the left. Ventrally, the wall of each pleural chamber is formed by the costal cartilages and the sternum; laterally, by the bodies of the ribs and the intercostal muscles as far as the costal angles; dorsally, by the portions of the ribs, with the intervening intercostal muscles, which lie medial to the costal angles; and medially, by the bodies of the vertebræ and the mediastinal septum, which completely shuts off the one chamber from the other.
The mediastinal septum is not uniformly median in position. Owing to the