158 THE COCKROACH : 



The injection of air by muscular pressure into a system of 

 very fine tubes may, however, appear to the reader, as it 

 formerly did to ourselves, extremely difficult or even impossible. 

 Can any pressure be applied to tubes within the body of an 

 Insect which will force air along the passages of (say) '0001 in. 

 diameter ? It may well seem that no pressure would suffice to 

 distend these minute tubules, in which the actual replacement 

 of carbonic acid by oxygen takes place, but that the air would 

 either contract to a smaller volume or burst the tissues. 



If we question the physical possibility of Landois' explana- 

 tion, an alternative is still open to us. The late Prof. Graham 

 has applied the principle of Diffusion to the respiration of 

 animals, and has shown how by a diffusion-process the carbonic 

 acid produced in the remote cavities would be moved along the 

 smaller tubes, and emptied into wider tubes, from which it 

 could be expelled by muscular action. The carbonic acid is 

 not merely exchanged for oxygen, but for a larger volume of 

 oxygen (0 95 : C 2 81) ; and there is consequently a tendency 

 to accumulation within the tubes, which is counteracted by the 

 elasticity of the air vessels, as well as by special muscular 

 contractions.* 



Whether diffusion or injection by muscular pressure is 

 the chief means of effecting the interchange of gases between 

 the outer air and the inner tissues of the Insect, is a question 

 to be dealt with by physical enquiry. 



If we suppose two reservoirs of different gases at slightly 

 different pressures to be connected by a capillary tube of 

 moderate dimensions, such as one of the larger tracheae of the 

 Cockroach, transference by the molecular movements of diffu- 

 sion would be small compared with that effected bv the flow 



* *j 



of the gas in mass. But if the single tube were replaced 

 by a number of others, of the same total area, but of the 

 fineness (say) of the pores in graphite, the flow of the gas 

 would be stopped, and the transference would be effected by 

 diffusion only. We may next consider tubes of intermediate 

 fineness, say a tracheal tubule of the Cockroach at the point 



* Phil. Mag., 1833. Reprinted in "Researches," p. 44. Graham expressly applies 

 the law of diffusion of gases to explain the respiration of Insects. Sir John Lubbock 

 quotes and comments upon the passage in his paper on the Distribution of the 

 Tracheae in Insects. (Linn. Trans. Vol. XXIII.) 



