128 THE LUNGS AND RESPIRATION. 



expulsion of air from the lungs, the former process being 

 a little shorter than the latter. A pause follows each 

 expiration before there is another inspiration. At 

 birth the normal rate of respiration is 42, but it grows 

 slower as the child grows older, being 26 at the age of 

 five or six, while in the adult it averages 17 to 20 times 

 a minute. It is slower during sleep and more rapid 

 during physical activity. The average amount of air 

 taken in with every inspiration is 30 cubic inches and 

 the minimum air space per individual should be 3000 

 cubic feet per hour. 



Breathing is of two kinds, diaphragmatic or abdom- 

 inal and chest or rib breathing, the former usually being 

 more pronounced in men than in women, probably be- 

 cause of centuries of tight dressing on the part of the 

 latter. As a rule, however, both diaphragm and ribs 

 come into play; for in inspiration, which is an active 

 movement, the thorax becomes enlarged from before 

 backward, laterally, and vertically. The ribs are raised 

 by the external intercostals chiefly, though the internal 

 intercostals aid somewhat, and swinging out upon the 

 vertebrae, widen the chest as well as deepen it. The 

 diaphragm, which is dome-like when relaxed, becomes 

 flattened in contraction and so increases the size of 

 the chest from above downward. As the chest enlarges, 

 the lungs expand, the air in them becomes rarefied, and 

 more air rushes in. When the lungs are full they re- 

 lax and the muscles relax after their contraction, so that 

 expiration is a passive movement, due largely to the 

 elastic relaxation of lungs and muscles, the air being 

 driven out by the lessened capacity of the lungs. 



Difficult Breathing. In heart and lung troubles, 

 where too little oxygen is carried to the tissues, dys- 

 pnoea or difficult breathing results and may even ad- 

 vance to asphyxia, a condition in which no air is ob- 

 tained. In difficult or labored respiration the pectoral 

 muscles are used in inspiration and the scaleni, which 

 pass from the vertebrae of the neck to the sternum, 



