612 THE LUNGS. 



The lymphatics are abundant both on the surface and in 

 the substance of the lungs, and go to the bronchial glands. 



The nerves come from the pneumogastric and sympa- 

 thetic ; chiefly from the former. A plexus exists on the 

 front and back of the roots of the lungs, called the anterior 

 and posterior pulmonary plexuses. These nervous filaments 

 are traced along the bronchial tubes, forming anastomoses 

 around them, and are supposed to expand themselves upon 

 the mucous membrane and blood-vessels. 



All these different elements, constituting what is termed 

 the parenchyma of the lungs, are collected and run together 

 in the root, at which point their relation with one another 

 is as follows : after the pleura is removed from the anterior 

 part of the root, we see the pulmonary veins below but in 

 front of the pulmonary artery, this latter being above and 

 behind the veins, while the bronchial tube is above and 

 behind the artery. 



Function. The function of the lungs or of respiration, is 

 quite a complex act. This act consists in eliminating car- 

 bonic acid from the blood, and supplying its place with 

 oxygen, or, in other words, in converting venous into arte- 

 rial blood. To accomplish this object, a variety of organs 

 are employed. The ribs and intercostal muscles, the dia- 

 phragm, the scaleni, the great and superior serrati muscles, 

 with others, all concur in greater or less degree to enlarge 

 the diameters of the chest, thereby increasing its capacity 

 for the reception of air, and thus accomplishing the act of 

 inspiration. The abdominal muscles, and the posterior 

 inferior serrati, aided by the elasticity of the cartilages, 

 draw down the ribs, and the diaphragm at the same time 

 ascending, the diameters of the chest are diminished, 

 thereby expelling the air from the lungs, and in this way 

 accomplishing the second act of respiration, termed ex- 

 piration. 



The pneumogastric, intercostal, phrenic and sympa- 

 thetic nerves are essential elements in putting all this 

 machinery in motion, as well as in maintaining it in 

 action. 



