166 RESPIRATION. 



and the beginning of the next inspiration. The respiratory 

 rhythm may be thus expressed: 



Inspiration, ........ 6 



Expiration, 7 or 8 



A very slight pause. 



Respiratory Movements of the Glottis. 



During the action of the muscles which directly draw 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 di- 

 lated at each inspiration, for the more ready passage of air, 

 and collapses somewhat at each expiration, its condition, there- 

 fore, 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 here- 

 after mentioned, there may be, in the contraction of the glottis, 

 considerable 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 per- 

 haps right in saying, that old people, even in health, habitu- 

 ally 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 estimated by 

 Dr. E. Smith at about 686,000 cubic inches. This quantity, 

 however, is largely increased by exertion ; and the same ob- 

 server has computed the average amount for a hard-working 

 laborer 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. Hutchinson 

 breathing air. The quantity over and above this which a man 

 can draw into the lungs in the deepest inspiration, he names 

 complemental air : its amount is various, as will be presently 



