RESPIRATION 



141 



Figure 82. Highly simplified dia- 

 gram of the structure of the lung 

 of terrestrial animals. The bron- 

 chiole surrounded by cartilage {top) 

 branches into ever finer bronchioles 

 devoid of cartilaginous support, 

 finally to pouch out into the alveolar 

 sacs. The septa between the alveoli 

 are shaded in, and are much thinner 

 than they are shown on the 

 diagram. 



neighbouring alveoli. The capillary wall, just like the wall of the alveoli, 

 is only one layer thick. Moreover, while the cells between capillaries are 

 normal cells whose nuclei can be seen under an ordinary microscope, they 

 are connected to each other by flimsy platelets covering the capillary 

 wall. These platelets can only be seen with an electron microscope. 

 Clearly, the thinnest possible wall divides the air in the alveoli from the 

 blood in the capillaries. 



The septa generally widen towards the top, wheie we find elastic 

 fibres and smooth muscle. Since the smooth muscle is so arranged that it 

 surrounds the opening of the alveolus, it can close this opening by contract- 

 ing. In the tissue of the septa and also in the alveoli themselves there are 

 large numbers of special cells whose function it is to collect what particles 

 of foreign bodies may have reached them from the outside. In many town- 

 dwellers and in all coal-workers these 'dust cells' are black. 



Though scientists first turned their attention to the microscopic 

 structure of Cetacean lungs during the nineteenth century, the first 

 important results were not published until 1914, by Barbosa. In 19 16 the 

 German scientist Fiebiger published a further paper, and then came 

 contributions from Neuville, Lacoste and Baudrimont (France), Laurie 

 (Britain), Wislocki and Engel (U.S.A.), Belanger (Canada) and Murata 

 (Japan). It is largely to them that we owe our knowledge of the fine 



