RESERVE AIR. 390 



are exceedingly elaborate, and were made on an immense 

 number of subjects of both sexes, and of all ages and occupa- 

 tions. They are generally accepted by physiologists as the 

 most extended and accurate. 



Residual Air. Perhaps there is not one of the questions 

 under consideration more difficult to answer definitely than 

 that of the quantity of air which remains in the lungs after a 

 forced expiration ; but fortunately it is not one of any great 

 practical importance. The residual air remains in the lungs 

 as a physical necessity. The lungs are always, in health, in 

 contact with the walls of the thorax ; and when this cavity 

 is reduced to its smallest dimensions, it is impossible that 

 any more air should be expelled. The volume which thus 

 remains has been variously estimated at from 40 cubic inches 

 (Fontana) to 220 cubic inches (Jurin). Dr. Hutchinson, who 

 has carefully considered this point, estimates the residual 

 volume at about 100 cubic inches, but states that it varies 

 very considerably in different individuals. Taking every 

 thing into consideration, we may assume this estimate to be 

 as nearly correct as any. It is certain that the lungs of a 

 man of ordinary size, at their minimum of distention, contain 

 more than 40 cubic inches of air ; and from measurements 

 of the capacity of the thorax, deducting the estimated space 

 occupied by the heart and vessels and the parenchyma of 

 the lungs, it is shown that the residual air cannot amount to 

 any thing like 200 cubic inches. 1 



There is no special division of the function of res- 

 piration connected with the residual air. It remains in 

 the lungs merely as a physical necessity, and its volume 

 must not be taken into account in considering the volumes 



inson are contained in extenso in the Cyclopcedia of Anatomy and Physiology r , vol. 

 iv., part 1, article Thorax. 



1 Hutchinson found the mean absolute capacity of the thorax to be 312 cubic 

 inches. He allows 100 cubic inches for the heart and blood-vessels, and 100 for 

 the parenchyma of the lungs, leaving about 100 for the residual volume. Op. cit., 

 p. 1067. 



