184 THE MECHANISM OF 



to necessitate a considerable alteration in our current teaching 

 of the mechanism of respiration. 



THE EXTENSIBILITY AND ELASTICITY OF THE LUNG 



It is usually presumed that the lungs are equally extensile 

 throughout, but an examination of their structure shows that this 

 cannot be so. From an anatomical point of view the lung may be 

 divided into three zones : (1) A root zone containing the bronchus, 

 artery, and vein, and their main divisions, with lymphatic glands 

 and vessels, and much fibrous tissue, all structures offering great 

 resistance to a distending force. (2) An intermediate zone in which 

 vascular and bronchial ramifications radiate towards the surface 

 of the lung with pulmonary tissue implanted between the rays. 

 It is a zone containing structures of varying degrees of extensi- 

 bility, the veins being the least extensile and the pulmonary tissue 

 the most. (3) An outer zone, estimated roughly at 25 to 30 mm. 

 in depth, which expands much more freely and equally than the 

 intermediate zone. Perhaps the subpleural stratum of the outer 

 zone should be distinguished, for if a lung, which has been re- 

 moved from the body, be gradually inflated, it will be found that 

 the subpleural stratum is at first elevated at certain points into 

 plateaux about 2 mm. above the surface of. the lung, and from 

 these elevated points the process of distension of the subpleural 

 stratum spreads out in all directions. This at first appeared to 

 be due to a more complete collapse of the subpleural air sacs, but 

 microscopic sections show that in the collapsed condition these 

 sacs are still as large as the deeper, so it may be concluded that, 

 when distended, they are larger than the deeper sacs, and collapse 

 to a greater degree. Seeing that the lung is intersected with 

 radiating bronchio- vascular rays, of a much lower degree of 

 extensibility than the pulmonary tissue between them, one must 

 suppose that these rays during the inspiratory expansion of the 

 lung must move apart so as to permit the pulmonary tissue lying 

 between them to expand. A consideration of the anatomy of the 

 lung and of its movement during inspiration shows that the 

 expansion of the lung is not the simple dilatation it was believed 

 to be ; its expansion is a regulated act resembling more the 

 opening of a Japanese fan than the distension of a simple elastic 

 sac. Among recent writers, Tendeloo is the only one to emphasise 



