194 RESPIRATION 



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

 on unconsciously and without exertion on our part, it is, never- 

 theless, 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 inspira- 

 tions,, the time may be extended to one and a half or even two 

 minutes. This should be remembered, and acted upon, before 

 passing through a burning building, or any place where the air 

 is, ye-ry foul. The arrest of the respiration may be still further 

 prolonged by training and habit. It is said that the pearl- 

 fishers of India can remain three o* four minutes under water 

 without being compelled to breathe. 



18. Capacity of the Lungs. The lungs ape 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 breathing 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 a minute ; and by the gradual introduction of the 

 external air, its temperature is considerably elevated before it 

 reaches the delicate capillaries that surround the air-cells. In 

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

 is. called into exercise, leaving a reserve capacity of about one 

 hundred and twenty cubic inches, equivalent to three and one- 

 half pints. This provision is indispensable to the continua- 

 tion of life; otherwise, a slight interference with respiration 

 by an ordinary cold, for instance would suffice to cut 



is insoluble in water. After the air has been blown through for a few 

 minutes, the water becomes turbid, and after a longer time, milky. Upon 

 standing, this milkiness subsides to the bottom in the form of a fine pow- 

 der, carbonate of lime. 



11. Kespira'tion 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 tran- 

 quil respiration ? Importance of the provision ? 



