298 MECHANISM 01 THE RESPIRATORY MOVEMENTS. 



initial contraction and the subsequent tonic contraction of the diaphragm 

 are absent ; the respirations become at once deeper and less frequent, 

 each inspiration being followed by an expiratory pause which may be 

 accompanied with active contraction of the expiratory muscles. 



3. Effects produced by increasing the duration or intensity of 

 the normal action of the vagi.— The results of increasing the duration 

 of the normal vagal stimulation by closing the trachea at the end of 

 inspiration or of expiration, as observed by Hering and Breuer, have 

 been already described. The effects of increasing the extent of the 

 stimulation by artificial inflation or collapse of the lungs were also 

 studied by these observers. Since, however, certain new facts were 

 brought to light by Head's experiments, some further mention of them 

 is necessary. 



If the lungs be gently inflated, the first result is an immediate 

 cessation of the rhythmic inspiratory contractions, so that a pause is 

 produced which may last fifteen to twenty seconds, but is always, sooner 

 or later, broken by an inspiratory contraction. If any tonic contraction 

 of the diaphragm were previously present, the lever, after its first rapid 

 fall to the ordinary expiratory position, continues to sink slowly until, 

 if the pause last long enough, the muscles attain complete relaxation. 



If the inflation is carried out too violently, an inspiration may 



Fig. 173. — The right vagus has been divided, and the curve shows the effect produced by 

 complete collapse of the left lung. The left pleural cavity was opened at +, and the 

 resulting tonic contraction of the diaphragm lasted twenty-five seconds (only the be- 

 ginning and end of the pause are shown). — Head. 



precede the expiratory pause, due probably to the stimulation of nerves 

 in the body wall, — since it may be produced even after division of the 

 vagi. 



I have already mentioned that collapse of the lungs produces a 

 prolonged contraction of the diaphragm. Since the further course of 

 the experiment is complicated by the onset of dyspnoea, we may only 

 perforate one pleura. In this case, since the resulting inspiratory 

 dilatation of the sound lung tends to inhibit the inspiratory movement 

 which produces it, the vagus nerve must be divided previously on the 

 sound side. If, as is usually the case, each lung is supplied only by the 

 vagus nerve of its own side, the result of the unilateral collapse is to 

 cause a quick contraction of the diaphragm, which may continue in this 

 condition for over twenty seconds. The first incomplete expiratory 

 elongations then appear, but it is many minutes before the respiratory 

 movements in any way approach the normal type. Finally, the breath- 

 ing assumes a form different from the normal in the strength of the 

 inspirations and the presence of a certain amount of inspiratory tone, 

 very similar to that resulting from division of both vagi (Fig. 173). 



A similar result (inspiratory spasm and increased inspiratory tone) 

 may be produced by momentary suction applied to the tracheal tube. 



Mechanical apncea.— The increased inspiratory activity following 



