EXPANSION OF THE LUNG 113 



In the lower figure, rs represents an internal intercostal 

 muscle. When this contracts the ribs are depressed, the 

 area of the parallelogram becoming diminished. 



But in the upper conical part of the chest the above 

 considerations do not apply, for the costal movement con- 

 sists during inspiration in a closing up of the upper six 

 ribs towards the first, which is fixed. This is brought 

 about by simultaneous contraction of the external and 

 internal intercostals. As each rib is an arc of a wider 

 circle than the one above it, this movement causes an 

 increase in the capacity of this part of the chest. 



The capacity of the chest is further increased in inspiration 

 by extension of the spine. 



Expiration is due not to a passive recoil, but to a co- 

 ordinated muscular movement the reverse of that which 

 causes inspiration. 



Expansion of the Lung 



The movement of the lung takes place fro7n the apex 

 downwards, forwards and outwards. In this movement 

 the root of the lung participates. Indeed it is only owing 

 to this movement of the root that any expansion can occur 

 in that part of the lung which lies between the root and the 

 posterior wall of the thorax. 



During expansion and retraction the posterior part of 

 the apex remains practically stationary. From the com- 

 parative disuse of this part of the lung arises its great 

 hability to tuberculous infection. 



The degree of expansion increases from the apex where 

 it is shght, to the base where it is considerable. 



In the upper conical part of the chest there is no relative 

 movement between the chest wall and the lungs, but in 

 the lower part the lungs glide up and down beneath the ribs. 



The whole of the lung tissue does not expand equally. 



The root and the tissues in its neighbourhood expand least, 



whilst the greatest expansion occurs in the infundibula 



into which the alveoh open. The amount of air breathed 



8 



