THE ORGANS OF CIRCULATION AND RESPIRATION. 157 



long supposed that, when the abdomen contracted, air was 

 expelled from the body, and the air passages emptied ; that 

 when the abdomen expanded again by its own elasticity, the 

 air passages were refilled, and that no other mechanism was 

 needed. Landois pointed oat, however, that this was not 

 enough. Air must be forced into the furthest recesses of the 



o 



tracheal system, where the exchange of oxygen and carbonic 

 acid is effected more readily than in tubes lined by a dense 



J t/ 



intima. But in these fine and intricate passages the resistance 

 to the passage of air is considerable, and the renewal of the air 

 could, to all appearance, hardly be effected at all if the inlets 

 remained open. Landois accordingly searched for some means 

 of closing the outlets, and found an elastic ring or spiral, which 

 surrounds the tracheal tube within the spiracle. By means of 

 a special muscle, this can be made to compress the tube, like 

 a spring clip upon a flexible gas pipe. When the muscle 

 contracts, the passage is closed, and the abdominal muscles can 

 then, it is supposed, bring any needful pressure to bear upon 

 the tracheal tubes, much in the same way as with ourselves, 

 when we close the mouth and nostrils, and then, by forcible 

 contraction of the diaphragm and abdominal walls, distend the 

 cheeks or pharynx. Landois describes the occluding apparatus 

 of the Cockroach as completely united with the spiracle. It 

 consists, according to him, of two curved rods, the " bow " and 

 the " band/' one of which forms each lip of the orifice. From 

 the middle of the band projects a blunt process for the attach- 

 ment of the occlusor muscle, which passes thence to the 

 extremity of the bow. The concave side of each rod is fringed 

 with setoo, and turned towards the opening, which lies between 

 the two. Upon this description of the spiracles of the Cock- 

 roach we have to remark that there is no occluding apparatus 

 at all in the thoracic spiracles, which are provided with 

 external valves. In the abdominal spiracles the bow is per- 

 fectly distinct, but the " band ' of Landois has no separate 

 existence. Though the actual mechanism in this Insect does 

 not altogether agree with Landois' description, it is capable 

 of performing the physiological office upon which he justly 

 lays so much stress viz., the closing of the outlets of the 

 tracheal system, in order that pressure may be brought upon 

 the contained air. 



