xii MECHANICS OF KESPIKAT1ON 421 



little change in its course and duration, abdominal respiration 

 becomes slower in the middle of its period. This slowing is the 

 proof that in quiet breathing the abdominal muscles do not in any 

 way function as expiratory factors. On the other hand, the 

 absence of any effects of increased expiratory resistance in the 

 pneumograms of the thorax proves that the thoracic expiratory 

 muscles come actively into play, so that they readily overcome 

 this resistance. Thus, then, " thoracic expiration, even when 

 accomplished under conditions of perfect rest, as is the case in 

 sleep, is an active process." 



VII. The respiratory movements, of which we have so .far 

 treated, directly determine the alternate filling and emptying 

 with air of the lungs inspiration and expiration. Besides these 

 respiratory movements in the strict sense of the word, there are 

 others, which affect the air-passages beyond the bronchial tubes and 

 the thorax, and indirectly favour pulmonary ventilation. These 



Fir,. 18G. Thoracic and abdominal pneumograms recorded (with two exploring button tam- 

 bours) during quiet sleep of an individual breathing with a mask through Miiller's water- 

 valve. (Aducco.) At T and A, the inspiratory and expiratory valves offer least resistance. 

 At T' and A' the expiratory valve offers higher resistance owing to addition of a little water. 

 The effect of this greater resistance is seen in the descending expiratory line of the abdominal 

 tracing. 



are known as accessory or concomitant respiratory movements. 

 Some of them are purely passive in character, i.e. they represent 

 simple secondary effects of the respiratory movements proper: 

 such are the movements of the larynx and trachea, which in 

 inspiration are drawn down by the expansion of the lungs and fall 

 of the diaphragm, rising again with the succeeding expiration. 

 Others, however, and it is these which must now be mentioned 

 briefly, are of an active character, due to the contraction of certain 

 special muscles. 



In the lirst place, we must consider the respiratory movements 

 of the laryngeal muscles proper, which produce inspiratory dilata- 

 tion and expiratory constriction of the glottis. On some animals, 

 particularly dogs, these movements may be regularly observed, 

 while in men their presence in quiet breathing is much disputed, 

 since there seems rather to be a permanent widening of the glottis. 

 In a certain percentage of cases (16 per cent, according to Semon) 

 these movements are, however, perceptible in man during quiet 

 breathing. 



