xin EESPIEATOEY RHYTHM 461 



the external air, the other was connected with a receiver containing 

 more or less condensed or rarefied air. A turn of the tap was 

 sufficient instantly to change the connection of the lungs from 

 atmospheric to rarefied or condensed air. 



The results obtained by this method do not seem substantially 

 to contradict those of Hering and Breuer. The transition from 

 free to compressed air provokes a short or prolonged expiration, 

 according as the rise of pressure causes marked or slight expansion 

 of the lungs. Transition from free to condensed air induces an 

 inspiration more or less deep or prolonged, according as the 

 diminution of pressure evokes a slight or pronounced contraction 

 of the lungs. 



On the strength of the facts enumerated above, Hering and 

 Breuer propounded the so-called theory of the automatic regulation 

 of respiration, which consists in the assumption that the respiratory 

 movements comprise a respiratory mechanism in themselves, 

 regulated by the centripetal fibres of the pulmonary vagi, since 

 these excite the inspiratory centres when the lungs contract, and 

 the expiratory centres when the lungs expand. In this way, the 

 inspiratory state of the lungs reflexly cuts off inspiration and 

 promotes expiration, and the expiratory state reflexly inhibits 

 expiration and effects inspiration. Two different kinds of afferent 

 fibres must be distinguished in the pulmonary vagi those excited 

 by the dilated and those excited by the contracted state of the 

 lungs ; the former are in relation with the expiratory centres, the 

 latter with the inspiratory. 



The explanation offered by Stefani is somewhat different. 



He holds that the inspiratory fibres are stimulated, not by 

 pulmonary retraction but by the fall of pressure in the alveoli, 

 and that the expiratory fibres are excited, not by pulmonary 

 expansion, but by rise of pressure in the alveoli. These small 

 modifications explain why in cases of stenosis of the air-passages 

 the breaths are deep and infrequent (dyspnoea), and why in cases 

 of restriction of the respiratory tract there are frequent and 

 superficial respirations (tachypnoea). For in the first case, both 

 inspiratory depression and expiratory rise of intrapulmonary 

 pressure must be greater, owing to the greater difficulty 

 encountered by the air in penetrating or leaving the respiratory 

 passages. In the second series of cases the conditions are 

 exactly opposite, so that the excitation of both kinds of vagus 

 fibres is shorter and weaker. 



Confirmation for this theory of automatic regulation by the 

 afferent fibres of the pulmonary vagus has been sought in the 

 study of the phenomena consequent on exciting the central ends 

 of the divided vagi. Owing, however, to the presence of the two 

 antagonistic kinds of fibres in the vagus, the effects of its central 

 excitation are not constant, but vary with the nature and intensity 



