SEC. 6. THE NERVOUS MECHANISM OF RESPIRATION. 



292. Breathing is an involuntary act. Though the dia- 

 phragm 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 breath- 

 ing 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 (medulla oblongata). 



We have already seen how complicated is even a simple 

 respiratory 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 contracted before the scaleni, or if the diaphragm con- 

 tracted 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 ordi- 

 nary contractions of the scaleni. A forcible contraction of the 

 scaleni, followed by simply a gentle contraction of the inter- 

 costals, would perhaps hinder rather than assist inspiration, and 

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

 complex 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 

 diaphragm 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 exces- 

 sively laboured. 



472 



