438 



ANIMAL RESPIRATION 



tubes which ramify on and within them. Looked at under the 

 microscope these present a very striking appearance. The 

 breathing-pores are valvular, and provided with minute muscles 

 by which their size is regulated, it being possible to close them 

 altogether. The contents of the air-tubes are expelled by the 



contraction of muscular bands that stretch 

 from the upper to the lower side of the body, 

 which is therefore flattened when they come 

 into action. On these muscles ceasing to 

 contract, the body resumes its former shape 

 as a result of elasticity, and air is conse- 

 quently drawn into the breathing-tubes. But 

 if the breathing-pores kept fully open all the 

 time air would only be renewed in the larger 

 air -tubes, while the minute branches would 

 have to rely upon diffusion for the purification 

 of their air. This, however, is not the case, 

 for competent authorities state that the breath- 

 ing-pores are closed for part of the time 

 the breathing-muscles are acting, so that air 

 is forced into many of the small air-tubes, 

 the contents of which are thus directly re- 

 newed. This is strikingly in contrast to 



Fig. 557 -Dissection of Cock- 



roach (Periplaneta orientalis , i -i '.11 r 1 i 



from above, to show chief air-tubes what happens in the lung of a human being 



(enlarged); r- 



segments of abdo- 



Qr oter ger vertebrate, wllCFC the Smallest 



air-spaces are of such a delicate nature that 



they would be liable to injury if currents of air constantly passed in 

 and out of them, and which therefore get rid of their carbonic acid 

 gas and keep up their supply of oxygen by gaseous diffusion. As 

 even the minutest branches of the insect's air-tubes are furnished 

 with a firm elastic lining they are not so liable to injury by a 

 tidal movement of air, such as must take place when air is 

 forced into them, and later on squeezed out again. The view 

 is here taken that air-tubes serve for getting rid of carbonic 

 acid gas as well as for the introduction of fresh oxygen, but 

 some zoologists maintain that only the latter purpose is served 

 by them. It must, indeed, be confessed that our knowledge of 

 the breathing of insects is very imperfect, and in this, as in so 

 many other directions, there is abundant scope for research upon 

 the physiology of lower forms. 



