RESPIRATION. 189 



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 further 

 likeness between the two acts in that, under ordinary circumstances, the 

 dilatation of the rima glottidis is a muscular act, and itc contraction 

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

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

 able muscular power exercised. 



Terms used to express Quantity of Air breathed. 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. 



Complemented 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. 



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. 



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 quantity, 

 however, is largely increased by exertion; the average amount for a hard- 

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



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 scale 

 placed in connection with this, the number of cubic inches is read off. 



In healthy men, the respiratory capacity varies chiefly with the stature, 

 weight, and age. 



It was found by Hutchinson, from whom most of our information on 



