CHAPTER XXIII. 

 THE RESPIRATION (Cont'd). 



Nervous Control of Respiration. Under normal conditions 

 we breathe from 14 to 18 times a minute. According to the de- 

 mand of the tissues for oxygen, we breathe fast or slow, but the 

 respirations are rhythmic in time and under like conditions are 

 equal in volume. The respiratory movements, unlike those of 

 the heart, are entirely dependent upon impulses transmitted 

 from the central nervous system. These come from the so-called 

 respiratory centers in the medulla oblongata (p. 256). Anatomic- 

 ally these centers cannot be sharply localized, but destruction of 

 the portion of the medulla in which they exist causes an imme- 

 diate cessation of respiratory movements. The centers are con- 

 nected with the muscles of respiration, by the phrenic nerves 

 to the diaphragm, the intercostal nerves to the muscles of the 

 ribs, and by the nerves running to the larynx and nares. Like 

 all other nerve centers, the respiratory center is influenced by af- 

 ferent impulses, the chief ones of which come from the lungs by 

 way of the vagus, but there are many others. In fact all the sen- 

 sory nerves of the body, as well as the higher centers of the brain, 

 are able to influence the respiratory center. Disease of the phren- 

 ic nerves causes paralysis of the diaphragm, and impairs the ven- 

 tilation of the lungs. Likewise paralysis involving the spinal 

 cord below the exit of the phrenic nerves may paralyze the nerves 

 of the thoracic muscles, and throw the whole work of respiration 

 on the diaphragm. 



If the vagus nerves of a dog or cat are cut in the neck, the 

 respiration becomes deeper and slower, yet the volume of air re- 

 spiivd per minute is not greatly altered. This change is due to 

 the elimination of stimuli normally coming from the lungs by 

 way of the vagi to the respiratory center, which serve to control 

 the depth of respiration. It can be experimentally demonstrated 



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