

QUANTITY OF AIR RESPIRED. 211 



air into the chest, those which guard the opening through 

 which it enters are not passive. In hurried breathing the 

 instinctive dilatation of the nostrils is well seen, although 

 under ordinary conditions it may not be noticeable. The 

 opening at the upper part of the larynx, however, or rima 

 glottidis (fig. 65), is dilated at each inspiration, for the 

 more ready passage of air, and collapses somewhat at 

 each expiration, its condition, therefore, corresponding 

 during respiration with that of the walls of the chest. 

 There is a further likeness between the two acts in that, 

 under ordinary circumstances, the dilatation of the rima 

 glottidis is a muscular act, and its contraction chiefly an 

 elastic recoil ; although, under various conditions, to be 

 hereafter mentioned, there may be, as in expiration, con- 

 siderable muscular power exercised. 



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 healthy 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 

 cubic inches. 



The quantity which is habitually and almost uniformly 

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



