MECHANICS OF THORAX 309 



(ii) The external intercostal muscles may be regarded as a 

 triangular sheet of muscle having its origin in the dorsal part of 

 the lid and being inserted into the upper surfaces of the ribs. 

 It pulls upwards. 



(b) Expiration. 



(i) The power causing collapse of the chest wall is mainly the 

 elastic recoil of the lungs together with the weight and elasticity 

 of the chest wall. 



(ii) The abdominal muscles, especially the external oblique, play 

 a part in expiration in pulling down the ribs. The fixed basis 

 from which they act is the pelvis, and they act as if attached 

 to the lower margin of the ribs exactly opposite the external 

 intercostals. 



(3) Load. This too is different in inspiration and expiration. 

 ' (a) Inspiration. 



The resistance to be overcome is : 



(i) The elasticity of the lungs a variable load, as the greater 

 the expansion of an elastic body, the greater is the resistance 

 that it offers to further expansion. This factor, therefore, is 

 numerically greater towards the end of inspiration than at the 

 beginning. 



(ii) The elasticity of the chest wall the costal cartilages have 

 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 column 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. 



(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 



