192 APPLIED PHYSIOLOGY 



which prevents the two pleural surfaces from separating. 

 After the injury keep the wounded side as much as 

 possible in a state of expiration, and do not on any 

 account take a deep inspiration.' 



Eespiration is a process in which the whole body wall 

 participates. In textbooks of physiology it is usually 

 stated that inspiration alone is a muscular act, that 

 expiration takes place as the result of elastic recoil ; but, 

 as Keith has shown, all the muscles of respiration are 

 involved in both phases. During inspiration the in- 

 spiratory muscles overcome those of expiration ; during 

 expiration the reverse takes place. This view is in con- 

 formity with the well-recognized principle that in per- 

 forming any movement active relaxation of one set of 

 muscles has as much to do with it as the active con- 

 traction of their opponents. Inspiration, however, is an 

 opposed act, the opposing forces being the contractility 

 of the lungs, the elasticity of the chest wall, and, to 

 some extent, the force of gravity. Expiration is an 

 unopposed act. Hence the thorax is in a position of 

 expiration after fainting or death, and in great weakness 

 it is inspirations alone which are delayed. Expiration 

 follows inspiration at the usual interval. The opposition 

 to be overcome during inspiration is considerable, and 

 involves, it has been calculated, a muscular strain 

 equivalent to raising a weight of 100 pounds, not count- 

 ing the contractility of the lungs. This has to be raised 

 eighteen to twenty times per minute. Where inspiration 

 has to be repeated very frequently, as in many conditions 

 of disease, the expenditure of muscular energy may be- 

 come very great. It is inadvisable, therefore, to add to 



