EFFECTS OF RESPIRATION ON THE AIR. 521 



is justly remarked by Dr. Reid, "furnishes an excellent illustration of the 

 numerous difficulties with which the physiologist has to contend, from the 

 impossibility of insulating any individual organ from its mutual actions and re- 

 actions, when he wishes to examine the order and dependence of its phenomena." 

 In such investigations, no useful inference can be drawn from one or two ex- 

 periments only; in order to avoid all sources of fallacy, a large number must 

 be made ; the points in which all agree must be separated from others in which 

 there is a variation of results ; and it must be then inquired, to what the latter 

 is due. 1 



2. Effects of Respiration on the Air. 



557. The total amount of air which can be drawn into the Lungs by the 

 deepest possible inspiratory movement, by no means affords a measure of the 

 quantity which they ordinarily contain. It is in fact composed, as was first 

 pointed out by Mr. Julius Jeffreys, 2 of several different quantities, which may 

 be distinguished as follows : 



1. Residual Air ; that which cannot be displaced by the most powerful expi- 

 ration, which always remains in the thorax so long as the lungs retain their 

 natural structure, and over which, therefore, we have no control. 



2. Supplemental Air ; that portion which remains in the chest after the 

 ordinary gentle expiration, but which may be displaced at will. 



3. Breathing or Tidal Air; that volume which is displaced by the constant 

 gentle inspiration and expiration. 



4. Complemental Air ; the quantity which can be inhaled by the deepest 

 possible inspiration, over and above that which is introduced in ordinary 

 breathing. 



The amount which can be expelled by the most forcible expiration after the 

 fullest inspiration, and which is consequently the sum of the 2d, 3d, and 4th of 

 these quantities, is designated by Dr. Hutchinson 3 as the Vital Capacity, being 

 that volume of air which can be displaced by living movements. This "vital 

 capacity" is less dependent than might have been supposed, upon the absolute 

 dimensions of the thoracic cavity, being yet more influenced by its mobility. 

 Thus of two sets of men of the same height, one measuring 35 inches around 

 the chest, and the other 38 inches, the average vital capacity of the first was 

 found to be 235 inches, and that of the second only 226 inches; for notwith- 

 standing the greater absolute capacity indicated by the larger circumference of 

 the latter, the inferior mobility of their chests caused more "residual air" to 

 remain behind after the deepest expiration. By taking the average of nearly 

 5000 observations, Dr. Hutchinson has arrived at the very remarkable conclu- 

 sion (Op. cit., p. 1072), that of all the elements whose variation might be sup- 

 posed to affect the "vital capacity," Height alone seems to have any constant 

 relation to it ; and that this relation is capable of being expressed in a simple 

 numerical form. The following table represents the "vital capacity" regarded 



1 On the important subject of the Mechanism of Respiration, the following Memoirs may 

 be consulted in addition to those already referred to : Dr. J. Reid's Art. " Respiration" in 

 "Cyclop, of Anat. and Physiol."; Dr. Hutchinson in" Med.-Chir. Trans.," vol. xxix. ; Dr. 

 Sibsonin "Phil. Trans.," 1846, "Med. Gaz.," vol. xli., " Med.-Chir. Trans.," vol. xxxi., 

 and "Trans, of Prov. Med. Assoc.," 1850; Beau and Maissiat in "Archiv. Gen.," 1842 ; 

 Mendelssohn, "Der Mechanismus der Respiration und Circulation," Berlin, 1845; and 

 Simon, "Ueber die menge der ausgeathmeten Luft bei verscheidenen Meuschen," Giessen, 

 1848. 



2 "Statics of the Human Chest," 1843. 



3 " Cyclop, of Anat. and Physiol," Art. " Thorax." 



