MECHANICAL PHENOMENA OF RESPIRATION. 289 



summit, and also enlarging the other dimensions of the cone 

 by separating its walls and pulling out the surface of the 

 base. This produces a difference between the pressure of 

 the exterior air and that in the respiratory cone, and also 

 between that of the different layers of air in this cone, caus- 

 ing the interior and exterior gases to mingle more closely 

 together. 



This dilatation of the pulmonary cone takes place by 

 means of the cage of the thorax, of which the diameter is 

 increased by the contraction of the muscles and by the 

 working of the bony levers of which it is formed. The wall 

 of the thorax is composed in front and at the sides of the 

 sternum and the ribs, and of the diaphragm below. 



The ribs are bony arches, sloping from top to bottom, from 

 back to front, and from within to without ; so that when they 

 rise, having as a fixed point their posterior extremity (costo- 

 vertebral articulation), their an- 

 terior extremity is thrown for- 

 ward, and their external convex- 

 ity thrown outwards, causing an 

 increase in the antero-posterior 

 and transverse diameter of the 

 lung: the Fig. 77 will better 

 illustrate this mechanism than 

 any explanation. The sternum 

 must obviously move freely 

 away from the vertebral col- 

 umn : the sternum and the 

 vertebral column, being joined 

 by the ribs, form, as it were, the 

 two supports of a ladder with 

 oblique rounds, and as these 

 rounds become horizontal, the 

 distance between the two sup- 

 ports increases; the forcible 

 dilator of the urethra employed 

 by surgeons constitutes a simi- 

 lar apparatus. Finally, the in- 



r. j i i -i Pig. 77. Thoracic cage.* 



clined plane formed by the rib 



sloping downwards and outwards, turns as it rises, about an 



oblique axis extending from the sternum to the vertebral 



* Vertebral column, with the ribs attached (dorsal region). These ribs ex- 

 tend to the front, where they ioin the sternum (directly, in the case of the seven 

 upper ribs). 



19 



