130 RESPIRATION. 



distress from the want of air. This takes place when we 

 run until we are " out of breath. 7 ' At the end of every 

 fifth or sixth breath, the inspiration is generally longer 

 than usual, the effect being to change more completely the 

 air of the lungs. 



11. Although, as a general rule, the work of respiration 

 goes on unconsciously and without exertion on our part, 

 it is nevertheless under the control of the will. We can 

 increase or diminish the frequency of its acts at pleasure, 

 and we can "hold the breath," or arrest it altogether 

 for a short time. From twenty to thirty seconds is 

 ordinarily the longest period in which the breath can be 

 held ; but if we first expel all the impure air from the 

 lungs, by taking several very deep inspirations, the time 

 may be extended to one and a half or even two minutes. 

 This should be remembered, and acted upon, before pass- 

 ing through a burning building, or any place where the 

 air is very foul. The arrest of the respiration may be still 

 further prolonged by training and habit ; thus it is said, 

 the pearl-fishers of India can remain three or four minutes 

 under water without being compelled to breathe. 



12. Capacity of the Lungs. The lungs are not 

 filled and emptied by each respiration. For while their 

 full capacity, in the adult, is three hundred and twenty 

 cubic inches, or more than a gallon, the ordinary breath- 

 ing air is only one-sixteenth part of that volume, or twenty 

 cubic inches, being two-thirds of a pint. Accordingly, a 

 complete renovation, or rotation, of the air of the lungs 

 does not take place more frequently than about once in a 

 minute ; and by the gradual introduction of the external 

 air, its temperature is considerably elevated before it 

 reaches the delicate pulmonary capillaries. In tranquil 

 respiration, less than two-thirds of the breathing power is 



1 1 . Respiration controlled by the will ? Advantage of the knowledge to us ? 



12. Capacity of the lungs ? Time required to renovate the air in the lungs ? 

 In tranquil respiration ? Importance of the provision ? 



