RESPIRATORY AND NERVOUS SYSTEMS. 181 



or stigmata on the sides of the thorax and abdomen, and 

 ramify through all parts of the body. Expiration is effected 

 by contraction of the muscles of the body, which compress 

 the tracheae ; inspiration, by dilatation of the tracheae through 

 the elasticity of their cuticular lining. 



The readiness with which air is carried to all parts of the 

 body, through the great perfection of the respiratory system, 

 accounts for the extreme imperfection of the circulatory 

 system in an animal which is otherwise very highly organised. 



1. The tracheal tubes when full of air are silvery white. 

 Remove a large trachea, and examine it microscopically in 



a. The cuticular lining has a very strong spiral 



thickening, which gives great elasticity to the 

 tube. 



b. The cells which form the organised wall of the tube, 



and secrete the chitinous lining, are thin and not 

 easily seen till stained. 



Stain with carmine, and note the nuclei of the cells. 



Remove small portions of muscle from the leg, of the fat- 

 body, of the salivary gland, etc., and note and examine the fine 

 tracheal tubes ramifying through them. Stain as above. 



D. The Nervous System. 



The nervous system consists of supra-oesophageal and 

 sub-oesophageal ganglia, and para-cesophageal connectives in 

 the head ; a double ventral chain in the thorax and abdomen ; 

 and a visceral nervous system in relation with the alimentary 

 canal (fig. 40, p. 176). 



1. The cephalic nervous system. 



If the buccal cavity has already been exposed, a second 

 specimen will probably be necessary for the dissection of the 

 nervous system of the head. 



Fix the head, with the anterior surface upwards, by means 

 of a fine pin through the upper part of the epicranium and 

 another between the mandibles. 



