310 EXTERNAL RESPIRATION 



other side by the abdominal muscles. This is also a lever action 

 of the second order. 



V. Elasticity of the lungs. The work done by the respiratory 

 musculature cannot be treated as a simple problem in hydraulics. 

 The dynamics of the ordinary force pump cannot be applied to 

 this question. Not only are the walls of the pump elastic and 

 complex but (a) they are not equally extensible throughout and 



(b) their elastic force varies with the degree of extension. Further, 



(c) the fluid enmeshed in the pulmonary capillaries has to change 

 its position to be accommodated at every alteration in the exten- 

 sion of the lungs. 



(a) Examination of the structure of the lungs shows that they 

 cannot be equally extensile throughout. Anatomists divide each 

 lung into three zones. 



(1) Root zone containing bronchus, artery, vein, lymphatic 

 vessels, etc. This apical part contains much fibrous tissue and, 

 therefore, offers considerable resistance to distortion. Using 

 physical terms one may say that its elasticity is strong but far 

 from perfect (p. 168). 



(2) Outer zone, estimated as extending for about 30 mm. from 

 the pleura containing very little fibrous tissue and made up mostly 

 of small capillaries and pulmonary tissue. Of these the pulmonary 

 tissue is perfectly but feebly elastic and the capillaries (empty) have 

 a modulus of about 0-04 xlO 6 not quite so perfect as the lung 

 substance but offering a greater resistance to distortion. Even 

 within this zone extensibility is not uniform. The stratum lying 

 immediately below the pleura is much 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 first and that the inflation of certain parts of the sub- 

 pleural stratum spreads from these points. 



(3) The middle zone, lying between the apical 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. 



(b) 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 



