MECHANICS OF THORAX 397 



apices of the lungs is a factor favouring the development of the 

 disease, is as yet an unsolved problem. On the whole the evidence 

 tends to show that ossification of the costal cartilages in question 

 is a consequence rather than a cause of a limited expansion of the 

 apices of the lungs. 



Posteriorly the lid is articulated to the vertebral colunm by a 

 joint which is set more transversely and is wider in the extent of 

 its attachment than any other of the costal arcs. 



IV. Mechanics of Thorax. The ribs are a series of bent levers. 



(1) The fulcra or hinges on which the levers work have been 

 mentioned when dealing with the ribs of the various thoracic 

 segments. 



(2) The power applied differs according to whether inspiration 

 or expiration is being performed (p. 423). 



(«) Inspiration. 



(i.) The lid or operculum is raised by the action of a flat tri- 

 angular muscle {.scaleni). The scalenus anticus is inserted into 

 the inner border of the first rib and passes almost vertically to 

 the transverse processes of the third, fourth, fifth and sixth 

 cervical vertebrae. The scalenus medius lies posteriorly to the 

 anticus and passes to the transverse processes of the lower six 

 cervical vertebrae. 



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

 triangular sheet of muscle having its origin in the posterior part 

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

 It pulls upwards. 



(6) 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 towai-ds the end of insj^irntion than at the 

 begiiming. 



(ii.) The elasticity of the chest wall — the costal cartilages ha\e 



