1/6 NORMAL HISTOLOGY. 



and in the interlobular connective tissue to the air- 

 vesicles, where they break up into the rich capillary 

 net-work which is spread over their walls. The 

 capillaries wind over the free edges of the alveolar 

 walls, the vessels often projecting somewhat into 

 their cavities. The net-work on the alveolar walls is 

 quite dense, and the vessels are very broad, leaving 

 only small oval or rounded spaces between them. 

 A single net-work frequently supplies the walls of 

 adjacent vesicles, which in such cases are merged 

 into one. The blood passes from the vesicles into 

 the pulmonary veins in the interlobular connective 

 tissue, and then into larger trunks which, passing 

 inward, follow the course of the other large vessels. 



The surface of the lungs is invested with a thin 

 layer of connective tissue, the pulmonary pleura, 

 which contains numerous blood- and lymph-vessels, 

 and is covered with endothelium. Here and there, 

 beneath the pleura, as well as elsewhere in the 

 lungs, between the lobules and around the alveolar 

 passages and small bronchi, are small irregular 

 nodules of lymphoid tissue. 



The epithelial cells of the alveoli often contain 

 brown or black pigment, and pigment deposits are 

 of the most frequent occurrence, in the adult lung, 

 in the interlobular connective tissue and in the con- 

 nective tissue and lymph nodes at the base of the 

 lungs, as well as in the above-mentioned lymphoid 

 nodules. The greater part of this pigment is prol> 



