196 . ADVANCED PHYSIOLOGY 



twenty per minute. In case of sickness, especially in fevers, 

 this rate may be much more rapid. 



Thus, although the stimuli for breathing all come from the 

 same general center, they go to the muscles by different 

 courses. Since all breathing impulses start from the brain 

 it is evident that if the neck be broken so as to cut off all con- 

 nection of these muscles with the brain, breathing will stop 

 at once. If, however, the neck should be broken below the 

 place where the phrenic nerves leave the cord, the diaphragm 

 would still be connected with the respiratory center and might 

 still be capable of making respiratory movements. Under 

 such circumstances persons have lived for years, although the 

 lower part of the body was cut off from the brain and of course 

 paralyzed. 



The intercostal and phrenic nerves carry messages away 

 from the brain; probably never toward it. 



One pair of nerves, however, carries messages from the 

 lungs to the brain; these are the respiratory branches of the 

 vagus nerves; Fig. 77, p. 144. The messages which go over 

 these respiratory branches are not for arousing movements, 

 but for informing the respiratory center of conditions in the 

 lungs, thus affecting the messages going to the muscles of the 

 diaphragm and ribs which are then modified to meet the 

 circumstances. 



The rate and nature of breathing are not only affected by 

 direct messages from the lungs, but by stimuli from the nose 

 lining. Fumes from ammonia or sulfur will practically stop 

 breathing processes for a short time. Moreover, sudden pain 

 in the abdomen may be followed by a cessation of breathing. 

 Dashes of cold water will set up more rapid breathing at first, 

 though later it may be followed by a slower respiration. None 

 of these influences on the body surface affects the breathing 

 muscles directly; sensations go from the surface to the 

 brain and then act indirectly through the respiratory 

 center. 



