GENERAL STATEMENT OF THE KESPIEATOEY ACT. 165 



coming from the pulmonary vesicles, diffusion steadily takes place, tend- 

 ing to remove the cell gas into the atmosphere; but this gas is not 

 brought from the vesicles by diffusion alone, which could not act with 

 sufficient speed, but by the contraction of the circular organic muscles of 

 the bronchial tubelets and of the cells, the different bronchial trees not 

 acting simultaneously, but successively. As soon as contraction is over, 

 the tubes expand by their elasticity, and the air is drawn into the cells, 

 each bronchial tree, by its contraction, aiding the expansion of the adja- 

 cent ones. The lungs are therefore not altogether passive during respi- 

 ration, as is sometimes said. The exchange between the gas in the cells 

 and that in the blood does not take place through simple diffusion, or in 

 quantities proportional to the diffusion volumes of oxygen and carbonic 

 acid. It is a complex diffusion, in which the disturbances arise from the 

 gases in the blood being either dissolved or combined, and through sev- 

 eral intervening membranes, that of the air-cells, that of the pulmonary 

 artery, and that of the blood disc, all of which exert a condensing action, 

 of the result of which it is impossible to furnish any numerical estimate. 

 The process ends by the expulsion of the foul air which has accumulated 

 in the larger bronchi and trachea, by the diminution which takes place 

 in the capacity of the chest during expiration, occasioned by the contrac- 

 tion of the expiratory muscles, the elasticity of the walls of the chest, 

 and of the lungs themselves. 



Such is the arrangement by which fresh air is constantly presented to 

 the blood, and the gases and vapors exhaling from it are removed. The 

 degree of exhaustion occurring in the chest scarcely justifies the ex- 

 pression sometimes used, "a tendency to a vacuum," since it is rarely 

 more than competent to raise water a single inch. This may be readily 

 proved by dipping a glass tube, open at both ends, and half an inch in 

 diameter, into a cup of water, and placing the projecting extremity be- 

 tween the lips, taking care to keep the muscles of the mouth at complete 

 rest. It will then be seen that at each inspiration the water rises about 

 an inch, and at each expiration is depressed to a similar extent. Its 

 movements indicate the degree of rarefaction or compression occurring in 

 the chest. 



It has been found convenient to consider the gaseous contents of the 

 lungs under several different titles : 1st. The residual air is Divisions of 

 that portion which can not be removed by the most power- content, e( of S tiie 

 fill expiration ; 2d. The supplemental air remains after tran- lungs. 

 quil respiration, but can be removed at will ; 3d. The breathing or tidal 

 air is that portion which changes by tranquil inspiration and expiration ; 

 4th. The complemental air is that which can be inhaled by the deepest 

 inspiration, over and above that introduced by ordinary breathing. 

 These are terms introduced by Mr. Jeffreys. 



