SEC. 6. THE NERVOUS MECHANISM OF RESPIRATION. 



Breathing is an involuntary act. Though the diaphragm and 

 all the other muscles employed in respiration are voluntary muscles, 

 i.e. muscles which can be called into action by a direct effort of the 

 will, and though respiration may be modified within very wide 

 limits by the will, yet we habitually breathe without the interven- 

 tion of the will : the normal breathing may continue, not only in 

 the absence of consciousness, but even after the removal of all 

 the parts of the brain above the medulla oblongata. 



We have already seen how complicated is even a simple respira- 

 tory act. A very large number of muscles are called into play. 

 Many of these are very far apart from each other, such as the 

 diaphragm and the nasal muscles ; yet they act in harmonious 

 sequence in point of time. If the lower intercostal muscles con- 

 tracted before the scaleni, or if the diaphragm contracted alternately 

 with the other chest-muscles, the satisfactory entrance and exit of 

 air would be impossible. These muscles moreover are coordinated 

 also in respect of the amount of their several contractions ; a gentle 

 and ordinary contraction of the diaphragm is accompanied by gentle 

 and ordinary contractions of the intercostals, and these are preceded 

 by gentle and ordinary contractions of the scaleni. A forcible con- 

 traction of the scaleni, followed by simply a gentle contraction of 

 the intercostals, would perhaps hinder rather than assist inspiration, 

 and at all events would be waste of power. Further, the whole com- 

 plex inspiratory effort is often followed by a less marked but still 

 complex expiratory action. It is impossible that all these so 



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