THE RESPIRATORY CENTRE. 391 



the medulla oblongata only, the rest of the brain being un- 

 developed, and yet they breathe perfectly well. If, on the 

 other hand, the spinal cord be divided immediately below 

 the medulla of an animal all breathing movements of 

 the chest cease at once. We conclude, therefore, that the 

 nervous impulses calling forth contractions of the respira- 

 tory muscles arise in the medulla oblongata, and travel 

 down the spinal cord and thence out along the phrenic and 

 intercostal nerves. This is confirmed by the fact that if 

 the spinal cord be cut across below the origin of the fourth 

 pair of cervical spinal nerves (from which the phrenics 

 mainly arise) but above the first dorsal spinal nerves, 

 the respiratory movements of the diaphragm continue but 

 those of the intercostal muscles cease; this phenomenon 

 has sometimes been observed in men stabbed in the back, so 

 -as to divide the spinal cord in the region indicated. Finally, 

 that the nervous impulses exciting the inspiratory muscles 

 originate in the medulla, is proved by the fact that if a 

 -small portion of that organ, the so-called vital point, be 

 destroyed, all the respiratory movements cease at once and 

 iorever, although all the rest of the brain and spinal cord 

 may be left uninjured. This part of the medulla is known 

 as the respiratory centre. 



In the above statements, for the sake of simplicity, atten- 

 tion has been chiefly confined to the diaphragm and the 

 intercostal muscles; but what is said of them is true of the 

 Tespiratory innervation of all other breathing muscles, 

 whether expiratory or inspiratory, normal or extraordinary; 

 in all cases the impulse giving rise to a respiratory move- 

 ment starts from the centre placed in the medulla oblon- 



Is the Respiratory Centre Reflex? Since this centre 

 goes on working independently of the will we have next to 

 inquire is it a reflex centre or not; are the efferent dis- 

 charges it sends along the respiratory nerves due to afferent 

 impulses reaching it by centripetal nerve-fibres; or does it 

 originate efferent nervous impulses independently of excita- 

 tion through afferent nerves? 



We know, in the first place, that the respiratory centre is 



