938 THE RESPIRATORY SYSTEM. 
Fissures and Lobes of the Lungs.—The J/eft /uwng is divided into two lobes 
by a long deep fissure which penetrates its substance to within a short distance of 
the hilus. Above and below the hilus this fissure cuts right through the lung 
and appears upon the mediastinal or inner surface. Viewed from the outer aspect 
of the organ, it begins at the posterior border about two and a-half inches below the 
summit of the apex, about the level of the vertebral end of the third rib, and is 
continued downwards and forwards in a somewhat spiral direction to the base of the 
lung, which it reaches a short distance behind its anterior end. The upper lobe of 
the lung (lobus superior) lies above and in front of this cleft. It 1s conical in 
form, with an oblique base, and the apex and the whole of the anterior border of 
the lung belong to it. The lower lobe (lobus inferior) lies below and behind the 
fissure. It is the more bulky of the two, and includes almost the entire base and 
the greater part of the thick posterior border. 
In the right lung there are two fissures subdividing it into three lobes. One 
of these fissures is very similar in its position and relations to the fissure in the left 
lung. It is, however, rather more vertical in its direction, and ends below somewhat 
further outwards. It separates the lower lobe from the upper and middle lobes. 
The second cleft begins in the main fissure at the posterior border of the lung, and 
proceeds horizontally forwards, to end at the anterior border of the lung at the 
level of the fourth costal cartilage. The middle or intermediate lobe of the right 
lung is triangular or wedge-shaped in 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 upper lobe. 
Supernumerary fissures also are not infrequent, and in this way the left lung may be cut into 
three lobes, and the right lung into four or even more lobes. The occurrence of a lobus azygos 
in the right lung is a variation of some interest, seeing that such a lobe is constant in certain 
inammals. It is a small accessory lobe, pyramidal in form, which makes its appearance on the 
lower part of the inner aspect of the right lung. In certain cases the vena azygos major 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. 
ROOT OF THE LUNG. 
The term root of the lung (radix pulmonis) is applied to a number of structures 
which enter and leave the lung at the hilus on its inner 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 descends a 
short distance in front of the pulmonary root, whilst the vagus nerve breaks up 
into the posterior pulmonary plexus on its posterior aspect under cover of the 
investing pleura. The delicate anterior pulmonary plexus is placed in front of the 
root of the lung beneath the pleura, whilst from the lower border of the root of the 
lung the ligamentum latum pulmonis extends downwards. These are the relations 
which 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 superior vena cava 
lies in front of the pulmonary root, whilst the vena azygos major arches over its 
upper aspect. On the left side the aorta arches over the root of the lung, whilst 
the descending thoracic aorta passes down behind it. 
Constituent Parts of the Pulmonary Root.—The most important 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 lymphatic 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 posterior aspect of the bronchus, and they follow the air-tubes through 
the organ. Part of the blood conveyed to the ling by the bronchial arteries is returned by the 
pulmonary veins; the remainder is returned by special bronchial veins which open on the right 
side into the vena azygos major, and on the left side into the vena azygos minor superior. 
The lymphatic vessels of the lung, as they emerge from the hilus, unite into a small number 
of trunks, which, placed behind the large pulmonary vessels, open into the bronchial glands. 
