256 Mr GRAHAM on the Law of 



tents of the second-sized tubes will advance at the same time 

 into the largest tubes, but no further, and will recede again into 

 their original depositories on the next inspiration, which will fill 

 the larger tubes with fresh air ; which identical quantity will 

 again be expelled in the next expiration. This illustration is 

 perhaps too strongly stated ; but it is evident, that, in ordinary 

 respiration, the slight mechanical compression will have little or 

 no effect in emptying the most distant tubes, or the ultimate air- 

 cells, of their contents. The bulk of the air, also, is not altered 

 during respiration, although, for a quantity of oxygen, carbonic 

 acid gas is substituted. This substitution, which is the great end 

 of respiration, undoubtedly takes place most abundantly in the 

 minute and distant air-cells, which present the largest surface to 

 the blood ; and the carbonic acid there produced, must be moved 

 along the smaller tubes by the diffusion process, (which we know 

 to be extremely energetic, and also inevitable), till it is thrown 

 into the larger tubes, from which it can be expelled by the ordi- 

 nary action of respiration. But the action of diffusion is always 

 twofold : at the same time that carbonic acid is being carried 

 outward from the air-cells, oxygen is carried inward in exchange, 

 and thus the necessary circulation kept up throughout the whole 

 lungs. 



Farther, by a forced expiration, from 160 to 178 cubic inches 

 may be expelled, after which, there still remain in the lungs 

 about 120 cubic inches, which are not under the control of the 

 respiratory action. 



There can be no doubt that much of this quantity occupies 

 constantly and permanently the most minute tubes and air-cells, 

 for it can scarcely be withdrawn by means of the air-pump. 

 Now, the question has arisen, how these ultimate tubes and air- 

 cells are so powerfully inflated ; for they are not distended by 

 the action of muscular fibre, of which they are known to be des- 

 titute. This state of distention must be highly useful, by expos- 

 ing surface ; and the law of diffusion enables us to account for it. 



