136 KESPIRATION. 



made on an immense number of subjects of both sexes and of all ages and occupations. 

 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 it fortunately 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 forty cubic inches (Fontana) to 

 two hundred and twenty cubic inches (Jurin). Dr. Hutchinson, who has carefully con- 

 sidered this point, estimates the residual volume at about one hundred cubic inches, but he 

 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 forty 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 two hun- 

 dred cubic inches. 



There is no special division of the function of respiration 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 which are changed in any of the opera- 

 tions connected with breathing. 



Reserve Air. This name is appropriately given to the volume of air which may be 

 expelled and changed by a voluntary effort, but which remains in the lungs, added to the 

 residual air, after an ordinary act of expiration. It may be estimated, without any 

 reference to the residual air, by forcibly expelling air from the lungs, after an ordinary 

 expiration. The average volume is one hundred cubic inches. 



The reserve air is more or less changed whenever we experience a necessity for a 

 more complete renovation of the contents of the lungs than ordinary. It is encroached 

 upon in the unusually profound inspiration and expiration which occur every five or six 

 acts. It is used in certain prolonged vocal efforts, in blowing, etc. Added to the residual 

 air, it constitutes the minimum capacity of the lungs in ordinary respiration. As it is 

 continually receiving watery vapor and carbonic acid, it is always more or less vitiated, 

 and, when reenforced by the breathing air, which enters with inspiration, is continually 

 in circulation, in obedience to the law of the diffusion of gases. Those who are in the 

 habit of arresting respiration for a time, as pearl-divers, learn to change the reserve air 

 as completely as possible by several forcible acts and then fill the lungs with fresh air. 

 In this way they are enabled to suspend the respiratory acts for from one to two minutes 

 without inconvenience. The introduction of fresh air with each inspiration, and the 

 constant diffusion which is going on and by which the proper quantity of oxygen finds 

 its way to the air-cells, give, in ordinary breathing, a composition to the air in the 

 deepest portions of the lungs which insures a constant aeration of the blood. 



Tidal, or Ordinary Breathing Air. The volume of air which is changed in the 

 ordinary acts of respiration is subject to immense physiological variations, and the 

 respiratory movements, as regards intensity, are so easily influenced by the mind, that 

 great care is necessary to avoid error in estimating the volume of ordinary breathing air. 

 The estimates of Herbst and of Hutchinson are the results of very extended observations 

 made with great care and are generally acknowledged to be as nearly accurate as pos- 

 sible. As a mean of these observations, it has been found that the average volume of 

 breathing air, in a man of ordinary stature, is twenty cubic inches. According to Hutch- 



