268 HANDBOOK OF PHYSIOLOGY. 



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, is 

 dilated at each inspiration for the more ready passage of air, and be- 

 comes smaller at each expiration; its condition, therefore, corresponding 

 during respiration with that of the walls of the chest. There is a fur- 

 ther likeness between the two acts in that, under ordinary circumstan- 

 ces, the dilatation of the rima glottidis is a muscular act and its contrac- 

 tion chiefly an elastic recoil; although, under various conditions to be 

 hereafter mentioned, there may be in the latter considerable muscular 

 power exercised. 



Terms used to express Quantity of Air breathed. a. Breath- 

 ing or tidal air, is the quantity of air which is habitually and almost 

 uniformly changed in each act of breathing. In a healthy adult man 

 it is about 30 cubic inches, or about 500 ccm., or half a litre. 



b. Complemented air, is the quantity over and above this which can 

 be drawn into the lungs in the deepest inspiration; its amount varies, 

 but may be reckonded as 100 cubic inches, or about 1,600 ccm. 



c. Reserve air. After ordinary expiration, such as that which expels 

 the breathing or tidal air, a certain quantity of air, about 100 cubic 

 inches (1,600 ccm.) remains in the lungs, which may be expelled by a 

 forcible and deeper expiration. This is termed reserve or supplemental 

 air. 



d. Residual air is the quantity which still remains in the lungs after 

 the most violent expiratory effort. Its amount depends in great meas- 

 ure on the absolute size of the chest, but may be estimated at about 100 

 cubic inches, or about 1,600 ccm. to 2,000 ccm. 



The total quantity of air which passes into and out of the lungs of 

 an adult, at rest, in 24 hours, is about 686,000 cubic inches. This quan- 

 tity, however, is largely increased by exertion; the average amount for 

 a hard-working laborer in the same time being 1,568,390 cubic inches. 



e. Respiratory Capacity. The greatest respiratory capacity of the 

 chest is indicated by the quantity of air which a person can expel from 

 his lungs by a forcible expiration after the deepest inspiration possible; 

 it expresses the power which a person has of breathing in the emergen- 

 cies of active exercise, violence, and disease. The average capacity of 

 an adult, at 15.4 C. (60 F.), is about 225 to 250 cubic inches, or 3,500 

 to 4,000 ccm. 



The respiratory capacity, or as John Hutchinson called it, vital capacity, 

 is usually measured by a modified gasometer or spirometer, into which the 

 experimenter breathes, making the most prolonged expiration possible after 

 the deepest possible inspiration. The quantity of air which is thus expelled 



