60 HUMBLE CREATTJRES. 



vided internally with a pair of lateral tubes ; in both 

 cases the main tracheae give out innumerable minor 

 branches, as already noticed, around which the blood 

 flows, becoming by that means thoroughly oxy- 

 genized. 



But if the nutritive and circulating systems of 

 the House-fly afford, through their advanced organi- 

 zation, a remarkable contrast with those of the 

 Worm, still more striking shall we find the indica- 

 tions of a comparatively high degree of development 

 presented by the nervous system of the insect. This 

 portion of its anatomical structure (PI. VI. fig. 1, b b) 

 consists — 1, of the brain, otherwise termed the 

 cephalic or supra-msophageal ganglion, which is com- 

 posed of two small masses of nervous matter situated 

 (as the foregoing expressions denote) in the head, 

 " above the oesophagus" or throat. Prom the sides 

 of this ganglion there proceed two large nerves to 

 the eyes, and in front a nerve to each antenna, and 

 some smaller ones to the parts of the proboscis. 

 3. From the lower portion of the brain a small 

 nervous chord passes downward on each side : these 

 surround the gullet and form below it, 3, the sub- 

 (Bsophageal ganglion; from which again, 4, the two 

 nervous chords, united into one, pass to the centre 

 of the thorax, where we find, 5, a large mass of 

 grey matter, called (from its position) the thoracic 

 ganglion. 



From this large nervous centre nerves are given off 

 in pairs (see PI. VI. fig. 1, 6 6) in all directions, viz. 



