612 PHYSIOLOGY OF RESPIRATION. 



the operation causes a prolonged inhibition of the underlying spinal centers. 

 They state that young animals, especially if made hyperirritable by the in- 

 jection of strychnin, may continue to breathe after section of the cord below 

 the medulla. This point of view, however, has not prevailed in physiology. 

 Other operations on the cord or brain are not attended by such profound 

 inhibition, and indeed Porter and Miihlberg have shown* that, if half of the 

 eord alone' is cut, the movements of the diaphragm on that side are permanently 

 paralyzed. It is entirely conceivable that under exceptional conditions 

 the lower neurons, the direct motor centers of the respiratory muscles, might 

 be made to act rhythmically, since during life they have been rhythmically 

 stimulated from the medullary center; but the evidence at present is alto- 

 gether against any distinct physiological independence on the part of these 

 neurons. 



The Automatic Activity of the Respiratory Center. The 



constant activity of the respiratory center throughout life suggests 

 the question as to its automaticity. Is it automatic like the heart? 

 That is, are the stimuli discharged from it produced within its own 

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

 of circulation? Or, on the other hand, is it, like 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? Obviously 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-, 

 ma tic, 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 quadrigemina 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; third, 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. 



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 



* " American Journal of Physiology, " 4, 334, 1900. 



