302 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



increase in the strength of the current produces the inspiratory effects 

 described by Eosenthal. The inspiratory tetanus in the latter case is 

 brought about chiefly by a decrease in the completeness of the expiratory 

 relaxation of the diaphragm, which finally enters into a state of tonic 

 contraction. A further increase in the strength of the exciting current 

 leads to active expiratory contractions (possibly due to escape of current 

 to the superior laryngeal nerves ?). 



Negative after-action. Head has described an important after- 

 effect of changes in the volume of the lung on the respiratory centre, 

 under the term " negative after-action." The nature of this effect may 

 be best explained by two or three illustrations. 



If the lungs be inflated for a short time, the return of these organs 

 to their normal volume is often followed by a slow inspiratory contraction 

 of great duration but of comparatively small strength, and the elongation 

 after the next two or three inspirations is incomplete. Here the 

 positive effect of the inflation is an abolition of inspiratory tone. The 

 negative after-effect is increased inspiratory tone. 



FIG. 178. Respiratory tracing showing negative after-eii'ect. The left 

 vagus was stimulated electrically with currents just insufficient to 

 cause Rosen thal's 'tetanus.' The period of stimulation is marked 

 by the rise of the line above the seconds time-tracing. During this 

 time both lungs were inflated for a period marked by the rise of the 

 tracheal manometer curve (dotted line). This inflation caused instant 

 elongation of the inspiratory muscles. As soon as the lungs were 

 allowed to return to their normal volume, the muscles again contracted 

 under the influence of the vagal stimulation, and fell into a condition 

 of complete tonic contraction. Head. 



During the normal breathing of a rabbit, with partial pneumothorax 

 on one side, the diaphragm is in a condition of slight tonic contraction. 

 If we wait until the tone has disappeared and then inflate the lungs, we 

 notice, on the resumption of the movements after the arrest immediately 

 following the inflation, that the tone has reappeared. It disappears 

 again after a little time, but can be again recalled by another inflation. 



If the central end of the divided vagus on one side be stimulated by 

 means of an interrupted current, it is possible so to graduate the 

 strength of the current that it is just insufficient to cause a complete 

 ' tetanus ' of the diaphragm. If the lungs be now inflated, the dia- 

 phragm at once elongates. As soon, however, as the lungs return to 

 the normal volume, the diaphragm contracts powerfully and completely, 

 the stimulus which was previously ineffectual being now able to evoke 

 a complete tetanus, since it is reinforced by the negative after-action due 

 to the inflation (Fig. 178). 



What light do these observations throw on the functions of the 

 vagus in normal respiration ? 



We have seen that the immediate effect of section of both vagi is to 



