468 PHYSIOLOGY CHAP. 



which, although it provides for a degree of pulmonary ventilation 

 sufficient to maintain life, must yet be termed dyspnoeic, since it 

 is not obtained without useless expenditure of muscular energy. 

 Under these conditions it seems to us probable that a vicarious 

 self -regulation comes into play, due to the rhythmical and 

 alternate excitation of the sensory paths to the inspiratory and 

 expiratory muscles. 



(d) The dyspnoeic respiration, consequent on section of the vagi, 

 is largely maintained by the active intervention of the cerebral 

 respiratory centres which tend to compensate the deficiency of the 

 vagus. When, indeed, the influence of the descending cerebral 

 tracts is also cut off, respiration becomes far more highly 

 dyspnoeic, and is inadequate for the needs of existence, although 

 rhythm, i.e. the alternation of inspiratory and expiratory acts, still 

 persists. 



(e) All the other centripetal nerves, which are capable of 

 reflexly influencing the respiratory mechanism, are normally 

 inactive, since their occlusion produces no apparent change in 

 respiration, and they are inadequate, after the vagi and afferent 

 cerebral paths to the bulb have been cut out, to compensate the 

 deficiency and substitute their own functions. 



It would be a mistake to conclude from these facts as a whole 

 that the rhythmically alternating impulses which emanate from 

 the inspiratory and expiratory centres localised in the spinal bulb 

 are merely reflex acts determined by stimulation of the said 

 afferent nerve tracts. As a matter of fact, we have seen that 

 respiratory rhythm, even when the spinal bulb is cut off from the 

 brain and section of the vagi, persists in a highly energetic form, 

 although it is inadequate for physiological requirements. If, after 

 these two operations, we proceed to a third, in which the cervical 

 cord is bisected at the level of the exit of the fourth pair of 

 cervical nerves, from which the fibres of the phrenic emerge, the 

 thoracic abdominal respiratory rhythm persists, though it is 

 represented almost exclusively by the energetic rhythmical con- 

 tractions of the diaphragm (Eosenthal). Lastly, if the spinal bulb 

 is suddenly and completely isolated by another transverse cut 

 below the tip of the calamus scriptorius all thoracic movement 

 ceases, but the facial, nasal, and laryngeal movements that 

 accompany the movements of respiration continue. That is to 

 say, the respiratory centres persist in their rhythmical functions, 

 although these can only find expression in the few motor paths 

 that remain (Eosenthal). 



Since, however, we know that the sensory tracts which are 

 still connected with the bulb under these conditions of isolation 

 are able reflexly to provoke rhythmic and alternate excitation, it 

 would be rash to conclude from these data that the respiratory 

 rhythm of the bulbar centres is automatic in character, i.e. entirely 



