THE STRUCTURE AND FUNCTION OF THE ELEMENTARY LUNG 447 



dioxid is constantly liberated during the oxidations proves that its 

 partial pressure is higher inside the organism than in the medium, and 

 hence, the molecules of this gas must move outward, i.e., in a direction 

 opposite to that of the particles of oxygen. Brief mention should also 

 be made of the fact that nitrogen is a functionally inert gas, serving 

 merely as the medium in which the diffusion of the other two gases 

 takes place. The function of argon, krypton and neon is not under- 

 stood as yet, but it seems that they are of no importance in respiration. 



With the gradually increasing size and complexity of the organisms 

 this method of interchanging the gases becomes wholly insufficient, 

 because the diffusion-pressures are not high enough to drive the oxygen 

 directly into the innermost recesses of a multicellular body. In- 

 vaginations make their appearance which finally take the form of 

 small pouches suspended in the body cavity and communicating with 

 the outside through small openings. This is the beginning of the 

 lung, a specialized organ set aside for the purpose of bringing the air 

 into close relation even with those cellular units of the body which 

 under ordinary conditions could not be reached by direct diffusion. 

 This end is then attained in an indirect manner with the help of the 

 body fluids. To begin with, an interchange of the gases takes place 

 in the lungs, where the atmospheric air is brought into relation with 

 the blood. This process is known as external respiration. The freshly 

 aerated blood is then directed to the different parts of the body, where 

 it enters into a vivid interchange with the tissues through the inter- 

 vention of the lymph. This process is designated as internal respira- 

 tion. In all the higher animals, therefore, two centers for the diffusion 

 of the gases are in existence, namely, one in the lungs and one in the 

 tissues. 



The Elementary Lung. In its most elementary form the lung 

 consists of a pouch-like invagination of the body-surface, containing 

 air from which oxygen is constantly abstracted, while carbon dioxid 

 is passed into it. But if this air were perfectly stationary, an equaliza- 

 tion of the partial pressures would soon result, which in turn would 

 lead to a cessation of the diffusion. Obviously, therefore, it is im- 

 perative that the original partial pressures be maintained and this 

 end can only be accomplished by frequently renewing the air in this 

 pouch. If this is done at regular intervals, as the metabolism of the 

 body may demand, the diffusion will continue at its normal height 

 during the entire life of the animal. 



The question may now be asked, how is this renewal of the air 

 effected? Inasmuch as the lung is connected with the outside by 

 means of a relatively long and narrow tube, its contents are well pro- 

 tected against all movements of the atmospheric air. Consequently, 

 the intake as well as the outgo of the air must be accomplished by a 

 definite activity on the part of the body, in which, however, the lung 

 plays only a passive part. The lung as such does not possess the power 

 of increasing or decreasing its size, and hence, is quite unable to pro- 



