RESPIRATION. 97 



air through the windpipe to fill up the lungs as they expand 

 to occupy the increased space. We call this breathing in 

 air or an inspiration. When this act is completed, the 

 diaphragm at once ascends and the ribs are drawn down 

 by another set of oblique muscles, bringing the lungs back 

 to their ordinary size and forcing out the air. This is called 

 breathing out or expiration. Tinder ordinary conditions the 

 act of breathing is uniform and at the rate of about seven- 

 teen times to the minute, or about once to every four beats of 

 the heart. If the heart is made to beat more rapidly by work, 

 exercise or excitement, the breathing is also more rapid. 



Breathing may be varied in other ways. In sighing, for 

 instance, there is a prolonged inspiration followed by an 

 expiration more or less audible. Laughing and crying are 

 rapid, short contractions of the diaphragm. We distinguish 

 them by the appearance of the face amd the sound of the 

 voice. In coughing and sneezing there is a sudden and 

 forcible expulsion of the air intended to dislodge and carry 

 out some offending substance either through the mouth or 

 nose. Hiccough is a sudden spasm of the diaphragm, causing 

 the air to rush against the closed glottis, producing a char- 

 acteristic sound. Yawning is similar to sighing, but the air 

 is drawn in through the mouth and the jaw lowered in a 

 characteristic manner. 



8. The Change of Elements in the Lungs. The 

 impurities in the venous blood brought to the lungs are 

 carbonic acid in the form of a gas, water in the form of 

 vapor mixed with certain organic matter, and slight traces 

 of ammonia, also a gas. The air which reaches the lungs 

 contains two gases, oxygen and nitrogen. The former is 

 the life-giving element. Undiluted oxygen is so strong that 

 if we place an animal in a jar of this gas it is exhilarated 

 for a little while, and runs about with great animation, but 

 it very soon dies chemically burnt to death. Nature has 

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