THE LUNGS 



1093 





into its two branches, the right and left pulmonary veins, the phrenic nerves, 

 and some bronchial lymph glands. 



The Posterior Mediastinum (Figs. 968, 969) is an irregular triangular space 

 running parallel with the vertebral column; it is bounded in front by the peri- 

 cardium above, and by the posterior surface of the diaphragm below, behind by 

 the vertebral column from the lower border of the fourth to the twelfth thoracic 

 vertebra, and on either side by the mediastinal pleura. It contains the thoracic 

 part of the descending aorta, the azygos and the two hemiazygos veins, the vagus 

 and splanchnic nerves, the esophagus, the thoracic duct, and some lymph glands. 



THE LUNGS (PULMONES). 



The lungs are the essential organs of respiration; they are two in number, placed 

 one on either side within the thorax, and separated from each other by the heart 

 and other contents of the mediastinum (Fig. 970). The substance of the lung is 

 of a light, porous, spongy texture; it floats in water, and crepitates when handled, 

 owing to the presence of air in the alveoli; it is also highly elastic; hence the retracted 

 state of these organs when they are removed from the closed cavity of the thorax. 

 , The surface is smooth, shining, and marked out into numerous polyhedral areas, 

 indicating the lobules of the organ: each of these areas is crossed by numerous 

 lighter lines. 



Cut edge of pericardium 



FIG. 970. Front view of heart and lungs. 





At birth the lungs are pinkish white in color; in adult life the color is a dark 

 slaty gray, mottled in patches; and as age advances, this mottling assumes a black 

 color. The coloring matter consists of granules of a carbonaceous substance 

 deposited in the areolar tissue near the surface of the organ. It increases in quan- 



