

BREATHING.] PHYSIOLOGY, lOf 



which makes us breathe. We breathe often without 

 knowing it; we breathe in our sleep when our will 

 is dead ; we breathe whether we will or no, because 

 we cannot help it. We can quicken our breathing, 

 we can take a short or deep breath as we please, 

 we can change our breathing by the force of our 

 will ; but the breathing itself goes on without,' and in 

 spite of, our will. It is an involuntary act. 



Though breathing is not an effort of the will, it 

 is an effort of the brain ; an effort, too, of one par- 

 ticular part of the brain, that part where the brain 

 joins on to the spinal cord. Nerves run from the 

 diaphragm and the intercostal and other muscles 

 through the spinal cord) to this part of the brain. 

 And seventeen times a minute a message comes dowr 

 along these nerves, from the brain, bidding them con- 

 tract ; they obey, and you breathe. Why and how 

 that message comes, you will learn at some future 

 time. When your head is cut off, or when that part 

 of the brain which joins on to the spinal cord is 

 injured by accident or made powerless by disease, 

 the message ceases to be sent, and you cease to 

 breathe. 



44. At every breath, then, a certain quantity of air 

 goes in and out of the chest; but only a small quantity. 

 You must not think the lungs are quite 

 emptied and quite filled at each breath. On 

 the contrary, you only take in each time a mere hand- 

 ful of air, which reaches about as far as the large 

 branches of the windpipe, and does not itself go into | 

 the air-cells at all. This is often called tidal air; \ 

 and the rest of the air in the lungs, which does not j 

 move, is often called the stationary air (see Fig. 13). | 



■MUMMia 



