EFFICIENCY OF UWC MECHANISM ;3l)9 



substance, but offering a greater resistance to distortion. P^ven 

 within this zone extensibiUty is not uniform. The stratum lying 

 immediately below the pleura is nuich more extensible than the 

 inner stratum. Inflation of a lung recently removed from the 

 body clearly demonstrates that certain parts of the surface are 

 inflated flrst and that the inflation of certain parts of the sub- 

 pleural stratum spreads from these points. 



(3) The middle zone, lying between the root and surface zones, 

 is intermediate to them in its elastic properties, containing as it 

 does highly elastic pulmonary tissue interspersed between the 

 rays of the bronchial and vascular systems. 



(6) That the elastic force of a material alters with the degree of 

 distension is a physical fact that has already been considered in 

 dealing with the force of the heart. Since the pressure of a gas 

 acts equally in all directions, the pressure caused by any given 

 tension of the walls of the hollow (spherical) vessel containing air 

 will increase with the diameter of the vessel. If we consider that 

 the diameter of each air sac is doubled during inspiration, then the 

 total pressure exerted by the walls will be increased four times, 

 i.e. distending force = resistance to distension = pressure of gas 

 multiplied by area of vessel. Moreover, with increasing distension, 

 the lung substance will become more attenuated. 



(c) The blood and lymph enmeshed in the pulmonary system 

 has to adjust its position to suit every alteration in the shape of 

 the lungs. These fluids are highly viscous, and as such resist 

 distortion roughly in proportion to their pressure and to the 

 area of the cross-section of their vessels. Further, the capillary 

 vessels are so narrow that the corpuscular component of the blood 

 viscosity becomes predominant. 



{d) In addition to these factors which may be deduced from 

 a study of lungs removed from the thorax one must take into 

 consideration the position of the lungs in the thorax. Certain 

 parts of the thoracic wall are stationary, and the surfaces of the 

 lungs in contact with these parts cannot directUj expand, viz. — 



(i.) the mediastinal surface in contact with the pericardium and 

 with the structures of the mediastinum, (ii.) the medial surface 

 lying close against the vertebral column and spinal portions of the 

 ribs and its anterior portion, in contact with the mediastinal pleura, 

 (iii.) The posterior part of the apical surface is bounded by Sibson's 

 fascia at the root of the neck. 



On the other hand the parts of the lungs in contact with (iv.) the 



diaphragm, (v.) the lower ribs (vcntro-lateral aspect) and (vi.) upper 



ribs (sternal asp(»ct) undergo direct expansion at each inspiration. 



VI, The efficiency of the lung mechanism, If figures could be 



