370 RESPIRATION. 



seen in the intercostal muscles of the corresponding space, and when the 

 spinal cord is divided below the origin of the seventh 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 vigor. 

 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 we 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 

 vasomotor centre in the immediate neighborhood 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. 



305. The nature of this centre must be exceedingly complex ; for while 

 ,even in ordinary respiration it gives rise to a whole group of coordinate ner- 

 vous impulses of inspiration followed in due sequence by a smaller but still 

 coordinate group of expiratory impulses of an antagonistic nature, in labored 

 respiration fresh and larger impulses are generated, though still in coordina- 

 tion 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 contrac- 

 tions. 



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 motor nerves, are con- 

 nected, just as they are about to issue from the spinal cord, with a nervous 

 machinery in which nerve-cells play a part a point which we shall consider 

 more fully in treating of the spinal cord ; we have reason to think that the 

 respiratory impulses starting from the respiratory centre pass into and are 

 modified by secondary spinal nervous mechanisms before they issue along the 

 motor nerve roots. Indeed, observations show that under particular condi- 

 tions, and especially in young animals, respiratory movements may be carried 

 out in the entire absence of the medulla oblongatn. Thus if, in a kitten or 

 puppy or young rabbit, after division of the spinal cord below the medulla 

 artificial respiration be kept up, and then pauses be made in the artificial 

 respiration, during these pauses not only may what appear to be respiratory 

 movements be induced in a reflex manner, by pinching or by blowing on the 

 skin, but, especially if the excitability of the spinal cord be heightened by 

 small doses of strychnine, even spontaneous eflbrts of breathing may occa- 

 sionally be observed. These are the exceptional instances mentioned above. 



