488 ROOT OP LUNG. 



Each lung is retained in its place by its root, which is formed by 

 the pulmonary artery, pulmonary veins and bronchial tubes, together 

 with the bronchial vessels and pulmonary plexuses of nerves The 

 large vessels of the root of each lung are arranged in a similar 

 order from before, backwards, on both sides, viz. 



Pulmonary veins, 

 Pulmonary artery, 

 Bronchus. 



From above, downwards, on the right side this order is exactly 

 reversed ; but on the left side the bronchus has to stoop beneath the 

 arch of the aorta, which alters its position to the vessels. They 

 are thus disposed on the two sides : 



Right. Left 



Bronchus, Artery, 



Artery, Bronchus, 



Veins. Veins. 



Structure. The lungs are composed of the ramifications of the 

 bronchial tubes which terminate in bronchial cells (air cells), of the 

 ramifications of the pulmonary artery and veins, bronchial arteries 

 and veins, lymphatics and nerves ; the whole of these structures 

 being held together by cellular tissue, which constitutes the paren- 

 chyma. The parenchyma of the lungs, when examined on the sur- 

 face or by means of a section, is seen to consist of small polygonal 

 divisions, or lobules which are connected to each other by an inter- 

 lobular cellular tissue. These lobules again consist of smaller 

 lobules, and the latter are formed by a cluster of air cells, in the 

 parietes of which the capillaries of the pulmonary artery and pul- 

 monary veins are distributed.* 



Bronchial tubes. The two bronchi proceed from the bifurcation 

 of the trachea to their corresponding lungs. The right takes its 

 course nearly at right angles with the trachea, and enters the upper 

 part of the right lung, while the left, longer and smaller than the 

 right, passes obliquely beneath the arch of the aorta, and enters the 

 lung at about the middle of its root. Upon entering the lungs they 

 divide into two branches, and each of these divides and subdivides 

 dichotomously to their ultimate termination in small dilated sacs, 

 the bronchial or pulmonary cells. 



The fibre-cartilaginous rings which are observed in the trachea 

 become incomplete and irregular in shape in the bronchi, and in the 

 smaller bronchial tubes are lost altogether. At the termination of 

 these tubes the fibrous and muscular coats become extremely thin, 



* The walls of the air cells are so imperfect that all the cells of any lobule commu- 

 nicate freely with each other, whilst the contiguous lobules, are separated by the paren- 

 chyma. Dr. Horner's dissections exhibit this in a beautiful manner. See Homer's 

 Special Anatomy, 3d edition. G. 



