EESPIKATIOK. 



601 



deeper, while the pauses between the respiratory movements are more 

 prolonged. If both pneumogastric nerves are divided the respirations 

 are still further slowed, the pauses are prolonged, and each respiratory 

 movement is deeper than normal. If the quantity of air displaced in any 

 given number of respiratory movements before and after section of these 

 nerves be estimated, it will be found to be almost unchanged ; in other 

 words, the increased depth of the respiratory movements after section of 

 the pneumogastric compensates for their decrease in frequency. The 

 amount of carbon dioxide exhaled and oxygen absorbed from the pul- 

 monary surfaces, therefore, remains unchanged. 



If after section of the pneumogastrics the central end of one of the 

 divided nerves be irritated, the rapidity of respiration 

 will be increased. B}' carefully graduating the strength 

 of stimulation the normal rhythm of respiration may be 

 again almost exactly restored. If the irritation be 

 gradually increased, the movements of respiration will 

 increase in vigor until, finally, they 

 will apparently run into each other, 

 and respiration will then be arrested 

 with the diaphragm in a condition of 

 tetanic contraction. Respiration is 

 thus stopped in the phase of extreme 

 inspiration. 



If the central end of the superior 

 laryngeal nerve be stimulated with the 

 faradie current the respiratory move- 

 ments will be slowed, and with a 

 powerful irritation will be arrested, 

 the diaphragm being completely re- 

 laxed, and the thorax and lungs being 

 in the condition seen in forced expira- 

 tion. It would therefore appear that 

 the trunks of the pneumogastric 

 nerves are the paths of impulses trav- 

 eling from the lungs, which, reaching 

 the respiratory centre, tend to exalt its activity. The vagus may, 

 therefore, be regarded as a stimulating nerve for this centre. The laryn- 

 geal nerves, on the other hand, may be regarded as inhibitory nerves 

 of the respiratory centre, and, when stimulated, arrest respiration, the 

 diaphragm being in a state of relaxation and the thorax contracted 

 (Fig. 257). 



While the respiratory centre is thus capable" of being modified by 

 impulses coming from these nerves, its degree of activity is, above all, 



Fig. 257.— Scheme op the Chief Re- 

 spiratory Nerves, after Ruther- 

 ford. {Landois. ) 



INS, inspiratory, and EXP, expiratory centres- 

 motor nerves are in smooth lines ; expiratory motor 

 nerves to abdominal muscles, AB ; to muscles of back, 

 DO; inspiratory motor nerves, phrenics to the dia- 

 phragm, D; INT, intercostal nerves ; RL, recurrent 

 laryngeal; CX, pulmonary fibres of vagus that excite 

 inspiratory centre ; CX', pulmonary fibres that excite 

 expiratory centre; CX", fibres of superior laryngeal 

 that excite expiratory centre ; INH, fibres of superior 

 laryngeal that inhibit the inspiratory centre. 



