MOVEMENTS OF RESPIRATION. 279 



separated from each other. In this way, the orifice of the glottis may 

 be nearly doubled in size, being increased from 0.94 to 1.69 square cen- 

 timetre. 



At the time of expiration, the posterior crico-arytenoid muscles are 

 relaxed, and the elasticity of the vocal chords brings them back to their 

 former position. 



The motions of respiration consist, therefore, of two sets of move- 

 ments, namely, those of the chest and those of the glottis. These move- 

 ments, in the natural condition, correspond with each other both in time 

 and intensity. It is at the same time and by the same nervous influence, 

 that the chest expands to inhale the air, while the glottis opens to admit 

 it ; and in expiration, the muscles of both chest and glottis are relaxed, 

 while the elasticity of the tissues restores the parts to their original 

 condition. 



Rapidity of the Movements of Respiration. The movements of res- 

 piration in the human subject follow each other in general with great 

 regularity, and, according to the results obtained from the most exten- 

 sive and varied observations, are performed on the average with a 

 rapidity of 20 inspirations per minute. This rate varies considerably 

 under the influence of different conditions, one of the most important of 

 which is age. It is well known that respiration, as a rule, is more rapid 

 in young children than in the adult, and Quetelet has found, as the 

 average of a large number of observations, that in the newly born infant 

 the rate is 44 per minute, and at the age of 5 years 26 per minute ; be- 

 coming reduced to the standard rapidity of 20 per minute between the 

 ages of fifteen and twenty years. Even in the adult, a condition of rest 

 or activity readily influences the number of respirations ; as, according 

 to the same observer, they are less frequent during sleep than in the 

 waking condition. Even a difference in position has been found to have 

 a perceptible effect, the number of respirations being, in the same indi- 

 vidual, 19 per minute while lying down, and 22 per minute when standing 

 up. 1 Any especial muscular activity, as the rapid motion of walking 

 or running, at once increases the frequency of respiration, which returns 

 to its ordinary regularity soon after the exertion has ceased. 



In all cases the movements of respiration are involuntary in character, 

 and even their acceleration or diminution is regulated by influences 

 beyond our control. It is possible for a short time to increase or retard 

 the rate of respiration, within certain limits, by voluntary effort ; but 

 this cannot be done continuously. If we intentionally arrest or diminish 

 the respiratory movements, after a short interval the nervous impulse 

 becomes too strong to be controlled, and the movements necessarily 

 resume their regular frequency. If on the other hand we endeavor to 

 breathe much more rapidly than twenty times per minute, the exertion 

 soon becomes too fatiguing to be continued, and the rate of movement 

 returns to its normal standard. The movements of respiration, accord- 



1 Milne-Edwards, Legons sur la Physiologic. Paris, 1857, tome ii. p. 483. 



