MODIFIED RESPIRATORY MOVEMENTS. 349 



centre. No impulse is discharged until the venosity of the 

 blood in the centre arrives at a certain point, and then the accu- 

 mulated 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 distention 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- 

 mogastrics. 



MODIFIED MOVEMENTS OF THE RESPIRATORY MUSCLES. 



Besides the ordinary respiratory motions and the voluntary 

 modifications made use of in speaking and singing, etc., the mus- 

 cles of respiration perform a series of movements of an involun- 

 tary reflex nature indicative of certain emotions and mental 

 states. 



They will be seen to resemble each other in the mechanism of 

 their production, though differing essentially in expression. The 

 following are the more important : 



Coughing is caused by a stimulus applied to certain parts of 

 the air passages, but more particularly to the larynx ; the stimulus 

 passing along the superior laryngeal branch of the pneumogas- 

 tric. It consists in a deep inspiration, closure of the glottis, and 

 then a more or less violent expiratory effort, accompanied by 

 two, three or more sudden openings and closures of the glottis, 



