THE LUNGS. 



'95 



surrounding the alveoli are in many places common to the opposed spaces 

 belonging to the same unit. These networks, the terminations of the pul- 

 monary artery and the beginnings of the pulmonary veins, possess exceed- 

 ingly small meshes, the distance between the capillaries often being less than 

 the diameter of the vessels. The latter, not confined to one plane but 

 sinuous, are excluded from the interior of the alveoli by practically only the 

 thin layer of respiratory epithelium, an arrangement manifestly advantageous 

 in effecting the interchange between the carbon dioxide of the venous blood 

 and the oxygen of the inspired air. 



Although preformed openings or stomata do not exist between the 

 alveolar epithelial elements, particles of foreign materials pass through 

 the wall of the air-spaces and eventually into the interlobular connective 



Smaller cells 

 Larger cells 



Epithelium 

 lining al- / 

 veoli V.\ 



FIG. 243. Section of lung, showing alveolar epithelium and collections of colored particles in connective 



tissue. X 140. 



tissue, where they accumulate as the more or less conspicuous collections of 

 pigment which aid in defining the outlines of the lobules. A certain amount 

 of such pigmentation is always found in adult lungs and may be regarded as 

 normal; when, however, individuals are subjected to an atmosphere unduly 

 laden with colored particles, as coal dust, the pulmonary tissues may be so 

 filled with pigment as to be in places almost black. It is probable that the 

 migratory leucocytes are an important means of transporting the colored 

 particles from the alveoli into the connective tissues. 



The blood-vessels of the lung, as those of the liver, include two sets, 

 one for the function of the organ, the other for the nutrition of its tissues. 

 The former are the branches of the pulmonary artery and veins; the latter 

 are the bronchial vessels. The nutrient arteries arise from the aorta, not 

 directly from the heart. The branches of the pulmonary artery follow 

 closely the ramifications of the air-tubes, entering the lobules near their 

 apices, along with the intralobular bronchioles, and finally breaking up into 

 the close capillary networks in the walls of the alveoli. From these networks 



