MECHANISM AND CHEMISTRY OF RESPIRATION 195 



Head 



Brain 



Neck! 



'esp/rafory 

 Centre 



Phrenic 



Nerve to 

 Diaphragm 



intercostal 



Wuscte 



region (Fig 103); each set passes out of the cord by three 

 roots, i.e. by nerves between the second and third, third and 

 fourth, fourth and fifth vertebrae. These roots unite to 

 form a single nerve on each side which passes down through 

 the thorax behind the lungs and then spreads out in the 

 muscles of the diaphragm. Messages go over these phrenic 

 nerves from the respiratory center 

 to the diaphragm, but none go in 

 the opposite direction; Fig. 103. 

 The second set of fibres from the 

 respiratory center, the intercostal 

 nerves, go farther down the cord to 

 the region of the ribs to emerge 

 between the vertebrae and pass at 

 once into the intercostal muscles; 

 Fig. 103. 



If these nerves which carry these 

 impulses from the respiratory center 

 to the various muscles concerned in 

 breathing are cut, or if that center 

 itself is destroyed, breathing stops 

 at once and death follows. As long 

 as the respiratory center is active, it 

 sends out stimuli to the breathing 

 muscles with perfect regularity. The 

 action of this center is partly in- 

 voluntary, i. e. goes on without any 

 action of the will, as is shown by the 

 fact that breathing continues while 

 one is asleep. At the same time, that 

 the center is partly voluntary is 

 demonstrated by the fact that one 



can at any time breathe fast or slowly as he wishes. 

 During normal life the average rate at which respiratory 

 impulses are sent to the muscles concerned is fifteen to 



FIG. 103. SHOWING THE 

 ORIGIN OF THE NERVES 

 CONTROLLING BREATHING 



