84 THE COCKROACH [CH. Ill 



of the tracheal tubes, of which these are the external 

 apertures, the air is distributed to all parts of the body 

 direct. The main tubes, previous to branching, in each 

 segment are connected by longitudinal trunks with those 

 of the other segments, so that in the event of one spiracle 

 becoming accidentally blocked the segment normally 

 supplied through that spiracle is able to obtain fresh 

 supplies of air through the neighbouring apertures. The 

 animal is able voluntarily to close the spiracles. In order 

 that inspiration and expiration may be effected certain 

 more or less rhythmic movements of the abdomen are 

 executed. Of these the most conspicuous is the flattening 

 of the abdomen, the dorsal and ventral surfaces being 

 brought closer together by the contraction of the dorso- 

 ventral muscles, which are able to reduce the depth of 

 the' body by as much as one-eighth ; at the same time 

 a lateral compression also takes place. By this mechanism 

 the tracheal tubes are squeezed and the air driven out at 

 the spiracles. When the muscles relax the skeletal parts 

 return to their original positions, the tracheal tubes widen 

 out again in virtue of their elasticity due to the spiral 

 thickening of their walls and, the pressure of air within 

 being thus less than that of the atmosphere without, fresh 

 air rushes in. In the ultimate ramifications of the tracheal 

 tubes the air is renewed by diffusion rather than by a 

 direct inrush, in much the same way as the " tidal air " 

 inspired by us does not by any means fill the lungs with 

 entirely fresh air but mixes with and diffuses into that 

 which remains in the lungs and which cannot be driven 

 out by any effort on our part. 



