926 THE RESPIRATORY SYSTEM. 
The first collateral branch arises from the right bronchus, much nearer the 
trachea than in» the ease of the left bronchus. It is this which determines the 
length of these primary divisions of the trachea, and although there is much varia- 
tion in this 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 
rings in the right and from nine to twelve rings 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: 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 hilus 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 
tendeney which foreign bodies exhibit, when introduced into the trac thea, to drop 
into the right in preference to the left bronchus. The av erage 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 mesial plane (Merkel). 
Relations of the Bronchi.—Arching forwards over the right bronchus is the 
vena azygos major, whilst arching backwards over the left bronchus there is 
the arch of the aorta. Occupying the interval between the bronchi there is a 
cluster of bronchial lymphatic glands, and an irregular chain of similar glands is 
earried along each tube towards the lung. On the posterior aspect of each bronchus 
the vagus nerve breaks up into the posterior pulmonary plexus, whilst the left 
bronchus, as it proceeds downwards and outwards, crosses in front of the cesophagus 
and the descending thoracic aorta. But perhaps the most interesting relation is 
that presented on each side by the corresponding pulmonary artery. On the left 
side the pulmonary artery crosses in front of the left bronchus above the level of its 
first collateral branch, and then turns round its outer side to gain its posterior aspect. 
All the left bronchial branches, therefore, are placed below the left pulmonary 
artery, and are in consequence termed hyparterial. The right pulmonary artery, 
on the other hand, crosses in front of the continuation of the right bronchus below 
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 THORACIC CAVITY. 
A central vertical partition, termed the mediastinum thoracis, which extends 
from the vertebral column behind to the anterior thoracic wall in front, subdivides 
the thoracic cavity into two large lateral chambers which contain the lungs. From 
the fact of each of these chambers being lined by an extensive and separate serous 
membrane called the pleura, they receive the name of the pleural cavities. 
The mediastinum or intervening partition is built up of several structures 
which lie in or in elose proximity to the mesial 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 gland or its remains, the trachea, 
cesophagus, and thoracic duct, and the pneumogastric and phrenic nerves. 
The pleural cavities in which the two lungs lie comprise much the largest part 
of the thoracic cavity. Each is bounded below by the corresponding cupola of the 
diaphragm ; and as the right cupola rises to a higher level than the left, the mght 
pleural cavity presents a smaller vertical depth than the left. Jn front, the wall 
of each pleural chamber is formed by the costal cartilages and the sternum, 
laterally, by the shafts of the ribs and the intercostal muscles as far back as the 
angles of the ribs, behind, by the portions of the ribs, with the intervening inter- 
costal muscles which lie internal to the costal angles, and internally, by the bodies 
of the vertebrae and the mediastinal partition which completely shut off the one 
