TEE RESPIRATORY MECHANISM. 415 



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

 ous impulses calling forth contractions of the respiratory 

 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 thoracic spinal nerves, the respiratory move- 

 ments of the diaphragm continue, but those of the intercostal 

 muscles cease; this phenomenon has sometimes been observed 

 on men so stabbed in the back as to divide the spinal cord in 

 the region indicated. - Finally, that the nervous impulses ex- 

 citing 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 move- 

 ments cease at once and forever, 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. The im- 

 pulses proceeding from it probably do not pass directly to 

 the motor nerve-fibres concerned, but first to subsidiary 

 centres in the cord, from which properly co-ordinated impulses 

 are sent to the muscles concerned. Occasionally in young 

 animals, especially after a small dose of strychnia has been 

 administered, a few respiratory movements are seen after 

 section of the cord high up in the neck. But the broad 

 general fact remains, that in. the normal working of the Body 

 the spinal respiratory centres only send out respiration-caus- 

 ing impulses when excited by impulses descending to them 

 from the main respiratory centre in the medulla. 



In the above statements, attention has been chiefly con- 

 fined to the diaphragm and the intercostal muscles; but 

 what is said of them is true of the respiratory innervation of 

 all other breathing muscles, whether expiratory or inspira- 

 tory, normal or extraordinary. 



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 discharges 

 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 excitation through 

 afferent nerves? 



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



