398 RESPIRATION. 



of all the inspiratoiy muscles in a forced inspiration, a sup- 

 plemental quantity of air may be introduced into the lungs, 

 which then contain much more than they ever do in ordi- 

 nary respiration. For convenience, many physiologists have 

 adopted the following names, which are applied to these 

 various volumes of air : 



1. Residual Air that which is not, and cannot be, ex- 

 pelled by a forced expiration. 



2. Reserve Air / that which remains after an ordinary 

 expiration, deducting the residual air. 



3. Tidal, or ordinary Breathing Air that which is 

 changed by the ordinary acts of inspiration and expiration. 



4. Complemental Air the excess over the ordinary 

 breathing air, which may be introduced by a forcible inspi- 

 ration. 



The questions relating to the above divisions of the re- 

 spired air have been made the subject of numerous investiga- 

 tions ; but though at first it might seem easy to determine all 

 of them by a sufficient number of experiments, the necessary 

 observations are attended with considerable difficulty, and the 

 sources of error are numerous. In measuring the air changed 

 in ordinary breathing, it has been found that the acts of res- 

 piration are so easily influenced by the mind, and it is so 

 difficult to experiment on any individual without his knowl- 

 edge, that the results of many good observers are not to be 

 relied upon. This is one of the most important of the ques- 

 tions under consideration. The difficulties in the way of 

 estimating with accuracy the residual, reserve, or com pie- 

 mental volumes, will readily suggest themselves. The ob- 

 servations on these points, which may be taken as the most 

 definite and exact, are those of Herbst of Gottingen, and 

 Hutchinson of England. 1 Those of the last-named observer 



1 A summary of the observations of Herbst, made in 1828, is to be found in the 

 Archives Generates de Nedecine, tome xxi., p. 412. The observations of Hutch- 



