THE LUNGS AND RESPIRATION. 



211 



who is not conscious that his breathing movements are 

 being observed, the rate is from fifteen to twenty per 

 minute. It is greater in children, and in infants is on an 



Fig. 92. DIAGRAM TO SHOW THE OPERATION OF THE EXTERNAL AND INTERNAL INTER- 

 COSTAL MUSCLES. 

 R, R", r, r", ribs in elevated position; R, R', r, r', ribs in depressed condition; a, 6. 



external intercostals; a', &', same contracted elevating the ribs; d', c', internal intercos- 



tals; d, c, same contracted depressing ribs. 



average as high as forty-four. From this it gradually sinks 

 during childhood until the adult rate of fifteen to twenty 

 just given is reached. This rate, however, may be much 

 influenced by emotions, by muscular exercise and by tem- 

 perature. 



3. The Capacity of the Lungs. The capacity of the 

 lungs no less than the rate varies with different classes of 

 people, and even with individuals of the same class. On 

 an average, however, in a properly exercised person the 

 following figures do not come very far from the actual con- 

 dition of things : 



If a person breathe out as much air as he can possibly 

 do, or, in other words, if he reduce the capacity of his 

 lungs as far as he is able to do, he still has in his lungs a 

 considerable amount of air. During life it is impossible, of 

 course, to get the lung- perfectly air free, and so the amount 

 of air which is left in the lung after the most forced ex- 

 piration possible, can be measured only after death. Ex- 

 periments of this kind give about 1,640 centimeters, or 



