THE THOBACIC CAVITY. 1083 



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 78'4 

 (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. 



CAVUM THOKACIS. 



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 

 sssels which spring from its arch, the pulmonary artery, and the great veins in 

 the neighbourhood of the heart, the thyrnus or its remains, the trachea, oesophagus, 

 and thoracic duct, and the vagi and phrenic nerves. 



Cavum Pleurae. The pleural cavities, in which the two lungs lie, comprise 



much the larger part of the thoracic cavity. Each pleural cavity is bounded in- 



-riorly 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 



rtical 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 



tibs and the intercostal muscles as far as the costal angles ; dor sally, by the 



portions of the ribs, with the intervening intercostal muscles, which lie medial to 



costal angles ; and medially, by the bodies of the vertebrae and the medias- 

 tinal septum, which completely shuts off the one chamber from the other. 



The mediastinal septum is not uniformly median in position. Owing to the 



