RESPIRATION. 239 



attached to their anterior extremities, are stretched and separated from 

 each other. In this way, the orifice of the glottis may be nearly doubled, 

 its area being increased from 0.94 to 1.69 square centimetre. 



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

 relaxed, the elasticity of the vocal chords replacing them in their 

 former position. 



The mechanism of respiration consists, therefore, of two sets of 

 movements, those of the chest and tho'se of the glottis. These move- 

 ments, in the normal condition, correspond with each other both in 

 time and intensity. It is at the same moment 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 Respiration. The movements of respiration in man 

 follow each other for the most part with great regularity, and, according 

 to the most extensive and varied observations, at the average rate of 20 

 inspirations per minute. This rate varies under different conditions, 

 one of the most important of which is age. As a rule, respiration is 

 more rapid in children than in adults. Quetelet has found the average 

 rate in the newly born infant 44 per minute, and at the age of 5 years 

 26 per minute, being reduced between the ages of fifteen and twenty 

 years, to the standard rapidity of 20 per minute. In the adult, according 

 to the same observer, a condition of rest or activity readily influences the 

 number of respirations ; which are less frequent during sleep than in 

 the waking condition. Even a difference in posture has a perceptible 

 effect, the number of respirations in one individual being 19 per minute 

 while lying down, and 22 per minute when standing up.* Any special 

 muscular activity, as rapid walking or running, at once increases the 

 frequency of respiration, which returns to its ordinary regularity soon 

 after the exertion has ceased. 



The movements of respiration are involuntary in character, and even 

 their acceleration or diminution is mainly 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 the breathing 

 or diminish its frequency, after a short interval the nervous impulse 

 becomes too strong to be controlled, and the movements recommence 

 as usual. If on the other hand we purposely accelerate respiration to 

 any great degree, the exertion soon becomes too fatiguing for contin- 

 uance, and the movements return to their normal standard. 



Quantity of Air used in Respiration. Like all quantitative esti- 

 mates connected with respiration, that of the inspired and expired air 

 varies considerably as given by different observers. The peculiarities 

 of individual constitution, as well as the conditions of rest and activity, 



* Milne-Edwards, Lefons sur la Physiologie. Paris, 1857, tome ii., p. 483. 



