Because the lungs consist of millions of hollow tubes, and cells, which, having been emptied by throwing off carbonic acid gas and nitrogen, become compressed, and the atmospheric air flowing into these millions of spaces, and filling the lungs, just as water fills and swells a sponge, causes them to expand, and occupy greater room.
"All the while my breath is in me, and the spirit of God is in my nostrils. My lips shall not speak wickedness, nor my tongue utter deceit."—Job xxvii.
890. How does the blood communicate with the air in the lungs?
Through the sides of very minute vessels, of which, perhaps, a fine hair gives us the best conception. But these vessels are twisted and wound round each other in such a curious manner, that they form millions of cells, and by being twisted and wound, a much greater surface of air and blood are brought to act upon each other, than could otherwise be accomplished.
891. Why does the blood which is thus formed, impart vitality to the parts to which it is sent?
Because the blood is itself vitalised—is, in fact, alive, and capable of diffusing life and vitality to the organisation of which it forms a part.
This is a very wonderful fact, but no less true than wonderful, that dead matter which, but a little while ago, was being ground by the teeth, softened by the saliva, and solved by the gastric juice and bile, has now acquired life. Nobody can tell the precise stage or moment when it began to live. But somewhere between the stomach and the lungs, melted by the gastric juice, softened by the secretion of the pancreas, separated by the bile of the liver, macerated by the muscular fibres of the bowels, taken up by the absorbents, warmed by the heat of the body, and ærated in the lungs, it has by one, or by all of these processes combined, been changed from the dead to the living state, and now forms part of the vital fluid of the system.