QUANTITY OF AIR RESPIRED. 2OI 



Quantity of Air Respired. 



The quantity of air that is changed in the lungs in each 

 .act of ordinary tranquil breathing is variable, and is very 

 difficult to estimate, because it is hardly possible to breathe 

 naturally while, as in an experiment, one is attending to 

 the process. Probably 30 to 35 cubic inches are a fair 

 .average in the case of Iiealtny^ young and middle-aged^ - 

 men ; but Bourgery is perhaps right in saying that old /^ / 

 people, even in health, habitually breathe more deeply, 

 and change in each respiration a larger quantity of air 

 than younger persons do. | 



The total quantity of air which passes into and out of 

 the lungs of an adult, at rest, in 24 hours, has been esti- 

 mated by Dr. E. Smith at about 686,000 cubic inches. 

 This quantity, however, is largely increased by exertion ; 

 and the same observer has computed the average amount 

 for a hard-working labourer in the same time, at 1,568,390 

 oubic inches. 



The quantity which is habitually and almost uniformly 

 changed in each act of breathing, is called by Mr. Hutchin- 

 son breathing air. The quantity over and above this which 

 a man can draw into the lungs in the deepest inspiration, 

 he names cornplemental air: its amount is various, as will 

 be presently shown. After ordinary expiration, such as 

 that which expels the breathing air, a certain quantity of 

 air remains in the lungs, which may be expelled by a 

 forcible and deeper expiration : this lie terms reserve air. 

 But, even after the most violent expiratory effort, the 

 lungs are not completely emptied; a certain quantity 

 always remains in them, over which there is no voluntary 

 control, and which may be called residual air. Its amount 

 depends in great measure on the absolute size of the chest, 

 and has been variously estimated at from forty to two 

 hundred and sixty cubic inches. 



The greatest respiratory capacity of the chest is indi- 



