When before you made the hole in the diaphragm you pulled the diaphragm down, you =still further lessened the pressure on the outside of the lungs=; hence the pressure inside the lungs caused them to swell up and follow the diaphragm. But this put the lungs still more on the stretch, so that when you let go the diaphragm and ceased to pull on it, the lungs went back again to their former size, emptying themselves of part of their air and pulling the diaphragm up with them. When there is a hole in the chest wall, pulling the diaphragm down does not make any difference to the pressure outside the lungs. They are then always pressed upon by the same atmospheric pressure inside and outside, and so remain perfectly quiet. =When in an air-tight chest the diaphragm is pulled down, the pressure of the atmosphere drives air into the lungs through the windpipe and swells them up. When the diaphragm is let go, the stretched lungs return to their former size, emptying themselves of the extra quantity of air which they had received.= Suppose now the diaphragm were pulled down and let go again regularly every few seconds: what would happen? Why, every time the diaphragm went down a certain quantity of air would enter into the lungs, and every time it was let go that quantity of air would come out of the lungs again. [Illustration: FIG. 12.--_The Diaphragm of a Dog viewed from the Lower or Abdominal Side._ _V.C.I._ the vena cava inferior; _O._ the œsophagus; _Ao._ the aorta; the broad white tendinous middle (_B_) is easily distinguished from the radiating muscular fibres (_A_) which pass down to the ribs and into the pillars (_C_ _D_) in front of the vertebræ. ] This is what does take place in breathing or respiration. Every few seconds, about seventeen times a minute, the diaphragm does descend, and a quantity of air rushes into the lungs through the windpipe. This is called =inspiration=. As soon as that has taken place, the diaphragm ceases to pull downwards, the stretched lungs return to their former size, carrying the diaphragm up with them, and squeeze out the extra quantity of air. This is called =expiration=. As the diaphragm descends it presses down on the abdomen; when it ceases to descend, the contents of the abdomen help to press it up. If you place your hand on your stomach, you can feel the abdomen bulging out each time the diaphragm descends in inspiration, and going in again each time the diaphragm returns to its place in expiration. =41.= But what causes the diaphragm to descend?