MECHANISM OF RESPIRATORY ACTIONS. 221 



act independently of each other. The position of the latter 

 is indicated in the diagram. 



Let us take 

 first the simple 

 act of sighing. 

 In this case 

 there is a rather 

 prolonged in- 

 spiratory effort 

 by the dia- 

 phragm and 

 other muscles 

 concerned in 

 inspiration ; 

 the air almost 

 noiselessly pas- 

 sing in through 

 the glottis, and 

 by the elastic 

 recoil of the 

 lungs and chest- 

 walls, and pro- 

 bably also of 

 the abdominal 

 walls, being 

 rather suddeoly 

 expelled again. 



Now, in the first, or inspiratory part of this act, the 

 descent of the diaphragm presses the abdominal viscera 

 downwards, and of course this pressure tends to evacuate 

 the contents of such as communicate with the exterior of 

 the body. Inasmuch, however, as their various openings 

 are guarded by sphincter muscles, in a state of constant 

 tonic contraction, there is no escape of their contents, 

 and air simply enters the lungs. In the second, or expira- 

 tory part of the act of sighing, there is also pressure made 



