684 PHYSIOLOGY OF RESPIRATION. 



That is, are the stimuh discharged from it produced witliin its own 

 cells as a result of its own metabolism under the normal conditions 

 of circulation? Or, on the other hand, is it, hke most of the motor 

 nuclei of the central nervous system, only a reflex center, its motor 

 discharges being dependent upon impulses received from other 

 neurons by way of the sensory paths? Ob\'iously the only way to 

 answer such a question directly is to isolate the center from all 

 afferent paths and leave it connected with the respiratory muscles 

 only by motor nerves. If under such conditions the respiratory 

 rhythm continues the center may be regarded as essentially auto- 

 matic, however susceptible it may be to reflex influences. A close 

 approximation at least has been made to such an experiment. 

 Rosenthal finds that rhythmical respiratory movements continue 

 after the following operations : first, section of the brain at the cor- 

 pora quadrigepiina to cut off influences from the cerebrum, thala- 

 mus, and midbrain; second, section of the vagi, to shut off afferent 

 impulses from the viscera, especially from the lungs; tliird, section 

 of the cord at the seventh cervical vertebra to exclude sensory 

 influences through all the underlying posterior roots; and, fourth, 

 section of the posterior roots of the cervical spinal nerves. The 

 medulla with its respiratory center was thus isolated from all 

 afferent impulses except such as might enter through the fifth, 

 seventh, eighth, and ninth cranial nerves. Since under these con- 

 ditions the center continued to act rhythmically we may draw 

 the probable conclusion that it is essentially automatic, and that 

 it probably possesses an intrinsic rhythmical activity resembling 

 that of the heart. An interesting supplementary experiment 

 upon this point is reported by Winterstein.f The rhythmic 

 activity of the respiratory center has been referred by some authors 

 to reflex stimulation of the center by sensory impulses arising in 

 the respiratory muscles themselves. To exclude this pos.sibility 

 Winterstein curarized an animal to immobilize the respiratory 

 muscles, and then divided the phrenic nerve and connected its 

 central stump with a galvanometer. Under these conditions the 

 galvanometer recorded rhythmic action currents, which demon- 

 strated that the respiratory center was sending out rhythmic 

 discharges of nerve impulses. 



Reflex Stimulation of the Center.^ — According to the results 

 of numerous observers, stimulation of any of the sensory nerves 

 of the body may affect the rate or the amplitude of the respiratory 

 movements. This experimental result is confirmed by our own 

 experience, since every one must have noticed that the respiratory 

 movements are readily affected by strong stimulation of the cutane- 

 * Winterstein, "Pfluger's Archiv," 138, 159, 1911. 



