THE VITAL CAPACITY. 819 



sometimes the supplemental air. The quantity inspired and expired 

 at each ordinary respiratory act is called the breathing or tidal air; 

 and the still further quantity, which can be drawn in by a forcible in- 

 spiration, is termed the complemental air. (Julius Jeffreys.) The total 

 quantity which, after the deepest inspiration, can be expelled by the 

 fullest expiration, is considered the measure of the so-called vital ca- 

 pacity of the chest or of the individual, because it shows the volume of 

 air which is commanded by the vital movements of the thoracic walls. 

 It is the extreme differential capacity of the chest, minus the space 

 occupied by the residual air, which cannot be expelled ; it represents 

 the total difference between the fullest inspiration and the fullest 

 expiration. This vital capacity in any individual is of great import- 

 ance as indicating the extreme power of breathing in exercise or effort ; 

 and it furnishes highly significant information in certain diseases, es- 

 pecially in those of the lungs themselves. (Hutchinson.) 



The determination of the actual quantities of air which are the 

 measures of the residual, reserve, breathing, and complemental air, 

 and that of the total respiratory power or vital capacity of the chest, is 

 extremely difficult. The elaborate and successful researches of Hutch- 

 inson were made by means of his so-called spirometer. This apparatus 

 is really a miniature gasometer ; it consists of an inner cylinder, closed 

 at its upper end, but open below, where it dips into water contained in 

 an outer larger cylinder, which is closed below and open above ; and 

 it has a scale by which its ascent and descent can be measured. The 

 inner cylinder is accurately balanced by weights attached to cords 

 passing over pulleys affixed to the outer cylinder. The inner cylinder 

 being depressed, and allowed to fill with water, the person experimented 

 on blows air into it by a tube which passes beneath its open mouth. 

 The cylinder is raised, and when the expiratory effort is complete, the 

 tube is closed by a stop-cock, so as to retain the air in the spirometer, 

 and its quantity is read off upon the scale. 



The residual air has been variously estimated at from 40 to 260 

 cubic inches ; but, according to Hutchinson, it ranges from 75 to 100 

 cubic inches. It is most difficult to measure ; for, after death, the 

 lungs are not so empty of air, as they are after forced expiration 

 during life, and it is not easy to estimate the difference. Besides, al- 

 though the amount of residual air corresponds generally with the size 

 of the chest, it is influenced also by the relative mobility or stiffness 

 of the walls, so that age, imparting rigidity to the costal cartilages, in- 

 creases the residual, at the expense of the reserve or supplemental air. 

 The residual and reserve air together are taken, in the adult male, to 

 be from 150 to 200 cubic inches. (Hutchinson.) 



Accurate estimates of the so-called breathing air and vital capacity, 

 are of great importance. The breathing air has been differently cal- 

 culated, from 10 to 92 cubic inches ; but, according to the most recent 

 observers, it ranges, in the adult male, from 16 to 20 cubic inches 

 (Hutchinson), from 17 to 33 (Vierordt), from 16 to 25 (Coathupe), and 

 from 30 to 39 cubic inches, in persons whose stature varies from 5 feet 

 7J inches to 6 feet. (Dr. E. Smith.) A fair estimate for a person of 



