100 PHYSIOLOGY. 



of breathing by nerve impulses sent to the muscles of 

 respiration. 



The Control of the Diaphragm. The diaphragm is 

 under the control of the phrenic nerves, which arise from 

 the third, fourth, and fifth cervical nerves. If the neck is 

 broken above the point where these nerves are given off, 

 death almost always immediately follows, because the con- 

 nection of the respiratory center and the diaphragm is 

 broken. 



Composition of Dry Air (by volume) : 



Oxygen 21.00 



Nitrogen . 79.00 



Carbon Dioxid *- . . .04 



100.04 



Experiments illustrating the Chemistry of Respiration. EX- 

 PERIMENT i . If a piece of phosphorus be burned under a fruit jar 

 inverted and with the mouth under water (for directions consult any 

 chemistry), the oxygen will be consumed and water will enter part way 

 to take its place. The remainder is nitrogen. 



EXPERIMENT 2. If a burning taper be lowered into this nitrogen, 

 the flame will be extinguished. 



EXPERIMENT 3. If a chemical laboratory is at hand, some carbon 

 dioxid should be generated and tested to show that it extinguishes 

 flame. 



EXPERIMENT 4. Lime water is the test of carbon dioxid, and may 

 easily be prepared by putting a piece of quicklime the size of a hen's 

 egg into a quart of water. 



EXPERIMENT 5. Pour a little clear lime water into a jar contain- 

 ing carbon dioxid, and on shaking the contents the lime water will be 

 rendered milky. 



EXPERIMENT 6. By means of a tube (a straw will serve) breathe 

 through a small quantity of lime water to show that there is carbon 

 dioxid in the expired breath. 



EXPERIMENT 7. If a jar be inverted over a lighted taper, the flame 

 will soon be extinguished. Test the gas with lime water to 'see that 

 carbon dioxid is produced by a burning candle- 



