274 RESPIRATION. 



vesicles." They have, according to Kolliker, an average diameter of 

 about 0.25 millimetre ; but owing to the elasticity of their walls, each 

 vesicle is capable of dilating to double or triple its former size, and 

 returning to its original dimensions when the distending force is re- 

 moved. There is every reason to believe that during life they are alter- 

 nately enlarged and diminished in size, as the lungs are filled and emptied 

 with the movements of respiration. 



Fig. 93. Fig. 94. 



SINGLE LOBULE OF HUMVN LUNO. NKTWOKK OF CAPILLARY BLOOD- 

 a. Ultimate bronchial tube. b. Cavity of VESSELS in the Pulmonary Vesicles of the 

 lobule. c,c,c. Pulmonary vesicles. Horse. (Frey.) 



Each pulmonary vesicle is covered upon its exterior with a 'close net- 

 work of capillary bloodvessels, which penetrate into the septa between it 

 and the adjacent cavities, and which are thus exposed on both sides to 

 the influence of the atmospheric air. In the walls of the vesicles, and 

 also in the interspaces between the lobules, there is an abundance of 

 elastic tissue, which gives to the pulmonary structure its property of 

 resiliency. The thin layer of pavement epithelium lining the ultimate 

 bronchial tubes extends into the cavities of the lobules and vesicles, 

 forming, according to the observations of Kolliker, a continuous invest- 

 ment of their internal surface. 



The abundant involution of the respiratory membrane, effected by the 

 subdivision of the bronchial tubes and the multiplication of the vascular 

 septa between the vesicles, existing in the lungs of man and the mam- 

 malians, evidently increases to an extraordinary degree the functional 

 activity of the organs of respiration. The entire extent of the respira- 

 tory surface in the human lungs has been estimated at 130 square 

 metres, which is probably not an exaggeration. The blood, accordingly, 

 in the pulmonary capillaries, distributed in thin layers over so large a 

 surface, in immediate proximity to the air in the cavity of the vesicles, 

 is placed under the most favorable conditions for its rapid and complete 

 arterialization. 



