328 RESPIRATION 



vagotomy, views are very divergent : Gad asserts that it no longer reaches its 

 former level, whereas Lindhagen has found that the relaxation of the in- 

 spiratory muscles is diminished little or none at all, and Boruttau remarks 

 that sooner or later expiration reaches the same height as before freezing the 

 vagi. According to Gad and Lindhagen, the expiratory pause is nearly always 

 shortened, according to Boruttau it is the same as before or even a little 

 longer. These statements all apply to the rabbit. In the dog, after freezing 

 of the vagi, the expiratory muscles fall into a state of almost regular activity 

 (Boruttau). The breath volume also, according to Gad, becomes smaller after 

 freezing the vagi ; according to Lindhagen, it remains on the whole unchanged. 



FIG. 129. The respiratory curve of a rabbit recorded with the apparatus shown in Fig. 122, 

 after Lindhagen. To be read from left to right. The downstroke representing inspiration. 

 The lower tracing is a time record in seconds. At the vertical line the two vagi were 

 "blocked" by freezing. 



These discrepancies depend in part at least upon the species of animal 

 used in the experiment, in part upon the depth of narcosis and upon other 

 circumstances not yet fully understood. We shall soon see, however, that 

 they can be explained without great difficulty (cf. page 330). 



This much, at all events, is well established by the experiment of discon- 

 necting the vagi by rendering them nonirritable, that the control exercised by 

 those nerves is such as to induce respiratory acts of greater frequency and of 

 less depth than otherwise, and that the inspiratory pause is thereby prevented. 

 Since this inspiratory pause is not of the least service in the ventilation of 

 the lungs, and the contraction of the muscles of inspiration maintaining it 

 is therefore of no use whatever, it is evident that even when the breath volume 

 before and after disconnection of the vagi is the same, respiration afterwards 

 is carried on with greater effort than normally. The result achieved by the 

 vagus reflex, therefore, is that respiration takes place with less expenditure 

 of energy. 



The investigations of Hering and Breuer have yielded some very valuable 

 results as to the way in which that regulation is accomplished. Artificial 

 inflation of the lungs inhibits the inspiratory muscles and induces an act of 

 expiration ; collapse of the lungs calls out an act of inspiration. Self -regula- 

 tion of the respiratory movements would thus seem to be afforded by the vagi 

 i. e., when the lungs have expanded to a certain- extent inspiration is re- 

 flexly interrupted, and when they are afterwards emptied to a certain extent, 

 expiration is interrupted and an act of inspiration is reflexly induced. Both 

 phases of the regulation are lost when the vagi are sectioned. 



