398 EXTERNAL RESPIRATION 



to be twisted and the muscles overlying the chest wall have to be 

 stretched. 



(iii.) The elasticity of the abdominal wall. 



(iv.) The elasticity of the vertebral column. During inspiration 

 the spinal colunni is lengthened by a stretching of the ligaments, 

 cartilages and articular processes, 



(v.) Gravity — weight of chest wall, etc. 



These loads may be resolved into one applied to the upper 

 surface of the ribs at their frontal tips. That is, we are dealing 

 with levers of the third order where power is applied between 

 load and fulcrum^ — giving speed at the expense of strength 

 (Chap. XXX.). 



(b) Expiration. 



The main resistance to expiration is the resistance to the outflow 

 of air from the lungs. We have seen that the principal force 

 causing expiration is the inspiratory load. Here then we have 

 a lever of the second class with the load between the power and 

 the fulcrum. During forced expiration, when every muscle that 

 can reduce the size of the thorax is brought into play, we have a 

 simple bellows action. The front of the thorax acts like the 

 movable side of a pair of bellows and is depressed towards the 

 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 clastic 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 t(j 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 part contains much fil)rous tissue and. tlierefore, 

 offers considerable resistance to distortion. Using physical terms 

 one may say that its elasticity is strong, but far from j^erfect (p. 206). 



(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 perfect! 1/ hut feeblfj elastic and the capillaries (em])ty) have 

 a modulus of about 0-04 X 10^^ — not quite so perfect as the lung 



