28 MR. LUBBOCK ON THE DISTRIBUTION 



the most distant tubes, or the ultimate air-cells, of their contents. The bullc 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 respu-ation, 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 

 snialler 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 

 ordinary 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 is kept up throughout the whole 

 lungs. 



" There can be no doubt that much of this quantity occupies constantly and perma- 

 nently the most minute tubes and air-cells ; for it can scarcely be withdrawn by means of 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 destitute. This state of distension must be highly useful by 

 exposing surface ; and the law of diffusion enables us to account for it. The heavy car- 

 bonic acid which these minute cells may contain, is not merely exchanged for oxygen, 

 but for a larger volume of oxygen, in the proportion of the diffusion-volumes of carbonic 

 acid and oxygen ; namely, 81 carbonic acid are replaced by 96 oxygen. The resistance to 

 passage through the most minute tubes is overcome by the diffusion-action, as in the 

 case of the pores of the stucco plug ; and there follows a tendency to accumulation on the 

 side originally occupied by the carbonic acid. This accumulation is limited by the in- 

 creased facility with which the air-vessels can empty themselves mechanically of a portion 

 of their contents, from their distended state. 



" In the law of diflPasion of gases, we have, therefore, a singular provision for the full 

 and permanent inflation of the ultimate air-cells of the lungs. But it is in the respiration 

 of insects that the operation of this law will be most distinctly perceived. The minute 

 air-tubes accompanying the blood-vessels to every organ, and like them ramifying till 

 they cease to be visible under the most powerful microscope, are kept distended during 

 the most lively movements of the little animals, and the necessary gaseous circulation 

 maintained wliolly, we may presume, by the agency of diffusion." 



Certainly, however, in many insects the respiratory movements are as well-marked as 

 in any of the higher animals ; and even in Caterpillars and other insects* where they are 

 absent, still the interchange of gases must be assisted by the ordinary movements of the 

 body. Yet Prof. Graham is, no dotibt, quite correct in denying the existence of any 

 actual current of air in the smaller tracheae; and that under these circumstances the 

 supply of oxygen should be sufficient shows well how rapidly and forcibly the diffusion 

 of gases takes place f ; but I am still doubtful whether the superior quantity of oxygen 



* Ranatra has no respiratory movements. In this genus, according to Newport, the whole respiration is effected 

 through the caudal tuhe. It has, however, three pairs of spiracles on the under side of the abdomen ; and each spi- 

 racle receives a moderate-sized trachea. 



t The membrane of the spiracle well represents the plaster plug used by Prof. Graham in his experiments. 



