184 HANDBOOK OF PHYSIOLOGY, 



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 dilation of the nostrils is well seen, 

 although under ordinary conditions it may not be noticeable. The open- 

 ing at the upper part of the larynx, however, or rima glottidis (Fig. 

 143), is dilated at each inspiration, for the more ready passage of air, and 

 becomes smaller at each expiration; its condition, therefore, correspond- 

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

 further likeness between the two acts in that, under ordinary circum- 

 stances, 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, in the latter, considerable mus- 

 cular power exercised. 



Terms used to express Quantity of Air breathed. a. Breathing 

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

 formly changed in each act of breathing. In a healthy adult man it is 

 about 30 cubic inches. 



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

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

 as will be presently shown. 



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

 the breathing or tidal air, a certain quantity of air remains in the lungs, 

 which may be expelled by a forcible and deeper expiration. This is 

 termed reserve air. 



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

 the most violent expiratory effort. Its amount depends in great measure 

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

 inches. 



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 that he can 

 make; it expresses the power which a person has of breathing in the 

 emergencies of active exercise, violence, and disease. The average 

 capacity of an adult (at 60 F. or 15.4 C.) is about 225 cubic inches. 



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

 is usually measured by a modified gasometer (spirometer of Hutchinson), 

 into which the experimenter breathes making the most prolonged ex- 

 piration possible after the deepest possible inspiration. The quantity 

 of air which is thus expelled from the lungs is indicated by the height 

 to which the air chamber of the spirometer rises; and by means of a 



