CHAP, ii.] RESPIRATION. 585 



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

 nu'iits are seen in the intercostal muscles of the corresponding 

 -pace, and when the spinal cord is divided below the origin of the 

 -eventh cervical spinal nerve, that is below the exits of the roots of 

 the phrenic nerves, costal respiration ceases, though the diaphragm 

 continues to act, and that with increased vigour. When 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 

 nerves are divided. We have already stated that after removal of 

 the brain above the medulla, respiration still continues very much 

 as usual, the modifications which ensue from the 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, developed in the 

 central nervous system and starting in the first instance in the 

 medulla, find their way along the several efferent nerves. The 

 proof is completed by the fact that the removal of or extensive 

 injury to the medulla alone is, save in exceptional cases which 

 \\e will discuss presently, at once followed by the cessation of 

 all respiratory movements, even though the rest of the nervous 

 system including every muscle and every nerve concerned be 

 left intact. Nay more, if only a small portion of the medulla, a 

 tract whose limits have not been clearly defined, but which may 

 be described as lying below the vaso-motor centre in the im- 

 mediate neighbourhood of the nuclei of the vagus nerves, 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, ' nceud vital.' We shall speak of 

 it as the respiratory centre. 



362. 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 of 

 an antagonistic nature, 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 dyspnoea and asphyxia impulses overflow, so 

 to speak, from it in all directions, though only gradually losing 

 their coordination, 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 

 several efferent nerve-fibres to their several muscular destinations. 

 On the contrary we have reason to think that the respiratory motor 

 nerves, like other 7iiotor nerves, are connected, just as they are 

 about to issue from the spinal cord, with a nervous machinery, in 



