364 THE HUMAN BODY 



oblongata, and travel down the spinal cord and thence out along 

 the phr^fiic^andlintercQstal nerves. This is confirmed by the fact 



-i S.T "* ._,___ 



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 

 movements 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 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 for- 

 ever, although all the rest of the brain and spinal cord may be left 

 uninjured. This part of the medulla is known as the respiratory 

 center. 



Is the Respiratory Center Reflex? Since this center goes on 

 working independently of the will, we have next to inquire, Is it a 

 reflex center or not? Are the efferent discharges it sends along the 

 respiratory nerves due to afferent impulses reaching it by centrip- 

 etal nerve-fibers? Or does it originate efferent nervous impulses 

 independently of excitation through afferent nerves? 



We know, in the first place, that the respiratory center is largely 

 under reflex p.ontrpJ : a dash of cold water on the skin, the irritation 

 of the nasal mucous membrane by snuff, or of the larynx by a 

 foreign body, will each cause a modification in the respiratory 

 movements a long indrawn breath, a sneeze, or a cough. But, 

 although thus very subject to influences reaching it by afferent 

 nerves, the respiratory center seems essentially independent of 

 such. In many animals, as rabbits (and in some men), marked 

 breathing movements take place in the nostrils, which dilate during 

 inspiration ; and when the spinal cord of a rabbit is cut close to the 

 medulla, thus cutting off all afferent nervous impulses to the re- 

 spiratory center except such as may reach it through cranial 

 nerves, the respiratory movements of the nostrils still continue 

 until death. The movements of the ribs and diaphragm of course 

 cease, and so the animal dies very soon unless artificial respiration 

 be maintained. Moreover, if after cutting the spinal cord as above 

 described, the chief sensory cranial nerves be divided, so as to cut 

 off the respiratory center from almost all possible afferent nervous 



