THE STORY OF THE LUNGS, SKIN AND KIDNEYS 89 



bloodvessels into the blood, while in the reverse 

 direction the carbonic acid gas and other waste 

 matters pass from the blood into the air cells and are 

 thus breathed out. This is the essential feature of 

 the act of breathing, an interchange of gases taking 

 place in the air cells of the lung. The lung might in 

 fact be well compared to a market place or exchange 

 where business is conducted by two merchants 

 represented by the air and blood respectively. The 

 transaction is one of barter, the air proposing to 

 give to the blood its oxygen, whilst the blood 

 exchanges for this necessary item, carbonic acid gas, 

 heat and water, which are given forth in expiration. 

 How WE BREATHE. The one movement of breath- 

 ing, inspiration, differs very materially in its nature 

 from its neighbour, that of expiration. We may de- 

 termine this fact if we pay attention to our own 

 breathing movements. Breathing in is essentially a 

 muscular act which chiefly depends on the action of 

 a very large muscle forming a moveable floor to the 

 chest, this muscle being known as the diaphragm or 

 midriff (Fig- 23). This muscle, when it acts, descends 

 and becomes less convex in shape, with the result, 

 that on account of its attachment to the chest walls 

 it enlarges that cavity, making it broader and longer. 

 At the same time the breast bone is pushed forward 

 and upward chiefly by the action of the little mftecles 

 which exist between the ribs, so that the chest in this 

 way is also enlarged in depth. The lungs, which are 

 highly elastic bodies, follow every movement of the 

 chest, hence, pressed as they have been to a certain 

 degree before the act of inspiration begins, they ex- 

 pand in virture of their elasticity, and thus admit the 

 supply of air which is drawn in by the act of inspir- 

 ation. When this action comes to an end, it is 



