xin KESPIEATOEY EHYTHM 459 



conspicuous changes in the amplitude of respiration, or degree 

 of pulmonary ventilation in the time-unit. According to Gad, 

 after freezing the vagi the amplitude of respiration undergoes a 

 slight diminution ; according to Lindhagen, on the contrary (in 

 agreement with our own observations), it remains almost un- 

 altered ; this means that the increased depth of the respirations 

 almost perfectly compensates for the diminution in frequency. 



From these facts we may deduce the important conclusion 

 that the vagi reflexly exert a marked regulatory influence upon 

 the respiratory centres. The respiratory type witnessed after 

 suppression of this influence is, as justly remarked by Gad, very 

 ill-adapted for its purpose, since the respiratory effort is con- 

 siderably greater, while its utility, as represented by the respiratory 

 volume, is not increased, and may even be diminished. It is in 

 fact from the ratio between force and effective utility that we 

 must judge of the degree of adaptation. If with intact vagi the 



FIG. 212. Effect of freezing the vagus on respiratory rhythm of rabbit. (Lindhagen.) The rabbit 

 breathes from receiver of Fig. 187 (p. 422). The vertical line marks the moment at which the 

 vagi were frozen. The lower tracing marks seconds. 



play of the respiratory muscles is modified, this obviously means 

 that they reflexly regulate the respiratory rhythm in such a way 

 that the same effect is obtained with far less effort, and with 

 minimum expenditure of energy. 



In 1868 Hering and Breuer, in an important series of experi- 

 ments, attempted to elucidate the mechanism of this regulation of 

 the respiratory processes by the vagi. They found on animals 

 that any dilatation of the lungs, produced no matter by what 

 means, checked the inspiratory and promoted the expiratory act ; 

 whatever, on the contrary, caused contraction of the lungs, 

 inhibited expiration and determined inspiration. After vagotomy, 

 these effects ceased altogether; the respiratory rhythm assumed 

 the type described above, which undergoes no modification with 

 reference to the state of contraction or dilatation into which the 

 lungs are artificially thrown. The results of Hering and Breuer 

 may be recapitulated the better to define their effects, and bring 

 out their importance : 



