344 MANUAL OF PHYSIOLOGY. 



mal. If the stimulus be very strong, respiratory spasm can be 

 produced. On the other hand, if the central end of the superior 

 laryngeal branch of the vagus be stimulated, breathing becomes 

 slow, and can be made to cease while the thorax is in the position 

 of ordinary expiration, a spasm of the laryngeal and expiratory 

 muscles is caused. 



So that in the pueumogastric nerve, fibres exist which convey 

 impulses of two kinds to the respiratory centre, the one increas- 

 ing its excitability and causing more rapid discharges of inspira- 

 tory impulses, the other decreasing its irritability and causing a 

 slowing of the respiratory movements. The marked change 

 which has just been described as occurring when the two pneu- 

 mogastrics are cut proves that these afferent influences are con- 

 stantly at work modifying the respiratory rhythm. We may 

 assume that the slow, deep respirations which follow section of 

 the vagi are caused by the unregulated automatic action of the 

 centre. No impulse is discharged until the venosity of the blood 

 in the centre arrives at a certain point, and then the accumulated 

 energy is sent to the respiratory muscles, and a deep gasping 

 inspiration occurs, and thus each respiratory act is called forth 

 by the blood becoming so venous as to act as a powerful stimulus. 



So long, however, as the centre enjoys the regulating influence 

 of the vagi this venous condition is not allowed to occur, and the 

 intense excitation of the centre is thereby prevented, and the 

 necessary movements are performed with a minimum of muscle 

 energy. 



The exact mode of stimulation of the pulmonary terminals of 

 the afferent fibres of the pneumogastric is not certain. It has 

 been suggested that distension or retraction of the lungs may act 

 as a mechanical stimulus to fibres inhibiting and exciting re- 

 spectively the inspiratory centre. Each expansion of the lungs 

 calls forth the ensuing relaxation, and the relaxed state, in its 

 turn, induces a new inspiration, and thus the lungs themselves 

 are able to guide the thoracic movements by means of the pneu- 

 mogastric* 



