28 
MR. LUBBOCK ON THE DISTRIBUTION 
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 
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nently the most minute tubes and air-cells ; for it can scarcely be withdrawn by means of 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 95 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 diffusion 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 wholly, we may presume, by the agency of diffusion." 
Certainly, however, in many insects the respiratory movements are as well-marked as 
m 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 doubt, quite correct in denying the existence of any 
actual current of air in the smaller trachese ; 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 
respiratory 
In 
through the caudal tube. „ „ w , uuwerer, tnree pairs 
racle receives a moderate-sized trachea. 
t The membrane of the sniraolp «r*n .*»»_*«**. ^^ . 
