SEC. 7. THE NERVOUS MECHANISM OF RESPIRATION. 



361. 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 

 intervention 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 spinal bulb. 



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 

 carefully coordinated muscular contractions should be brought 

 about in any other way than by coordinate nervous impulses 

 descending along efferent nerves from a coordinating nervous 

 centre. By experiment we find this to be the case. 



When in a rabbit the trunk of a phrenic nerve is cut, the dia- 

 phragm on that 'side remains motionless, and respiration goes on 

 without it. When both nerves are cut, the whole diaphragm 

 remains quiescent, though the costal respiration becomes ex- 

 cessively laboured. 



