354 NERVOUS MECHANISM. [BOOK n. 



carefully coordinated muscular contractions should be brought 

 about in any other way than by coordinate nervous impulses 

 descending along efferent nerves from a coordinating 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 respiration becomes excessively 

 laboured. 



When an intercostal nerve is cut, no active respiratory move- 

 ment is seen in that space, and when the spinal cord is divided 

 below the origin of the seventh cervical spinal nerve, costal 

 respiration ceases, though the diaphragm continues to act and 

 that with increased vigour. W T hen the cord is divided just below 

 the medulla, all thoracic movements cease, but the respiratory 

 actions of the nostrils and glottis still continue. These however 

 disappear when the facial and recurrent laryngeal are divided. We 

 have already stated that after removal of the brain above the 

 medulla, respiration still continues very much as usual, the modifi- 

 cations which ensue from loss of the brain being unessential. 

 Hence, putting all these facts together, it is clear that the 

 respiratory movements are, as we suggested, brought about by 

 coordinated impulses which, originated in the medulla, find their 

 way thence along the several efferent nerves. The proof is completed 

 by the fact that the removal or extensive injury of the medulla 

 alone is, save in exceptional cases, at once followed by the cessation 

 of all respiratory movements, even though every muscle and every 

 nerve concerned be left intact. Nay more, if only a small portion 

 of the medulla, a tract whose limts are not as yet exactly fixed, 

 but which lies below the vaso-motor centre, between it and the 

 calamus scriptorius, be removed or injured, respiration ceases, 

 and death at once ensues. Hence this portion of the nervous 

 system was called by Flourens the vital knot, or ganglion of life, 

 nosud vital. We shall speak of it as the respiratory centre. 



The nature of this centre must be exceedingly complex ; for 

 while even in ordinary respiration it gives rise to a whole group of 

 coordinate nervous impulses of inspiration followed in due sequence 

 by a smaller but still coordinate group of expiratory impulses, in 

 laboured respiration fresh and larger impulses are generated, 

 though still in coordination with the normal ones, the expiratory 

 events being especially augmented; and in the cases of more 

 extreme dyspnrea and asphyxia impulses overflow, so to speak, 

 from it in all directions, though only gradually losing their co- 

 ordination, until almost every muscle in the body is thrown into 

 contractions. 



We must not however conceive of this centre as one of such a 

 kind that the impulses leave it fully coordinated and equipped so 

 that nothing remains for them but to travel, unchanged, along the 



