32 



The Form of Insects 



(which have large nuclei and are often branched) and 

 nerve-fibres connected with the cells. A nerve-centre 

 {ganglion) contains both cells and fibres, the former 

 mostly occurring in the outer parts of the mass, the 

 latter lying more deeply ; a nerve-cord, joining two 

 nerve-centres, consists almost entirely of fibres. Both 

 centres and cords are covered, and protected by a hard 

 sheath {neurilemma) formed of cells with elongate 

 nuclei (fig. 24) (l). 



In the Cockroach we find in the head, in front 

 of the gullet, a large nerve-centre, the brain {siipra- 



FiG. 24. — Cross-section of third thoracic ganglion of Cockroach, /r. air-tubes ; 

 neu. neurilemma ; 'jfc. nerve-cells. Magnified 70 times. A. nerve-cells 

 more highly magnified. From Miall & Denny. 



oesophageal ganglion), which gives off nerves to the 

 eyes and feelers. The brain is connected by a 

 thick nerve-ring with another nerve-centre {sub- 

 cesophageal ganglion) giving off nerves to the jaws, 

 and situated behind the gullet (fig. 25). From this 

 passes backwards a pair of nerve-cords which run 

 side by side to the hinder end of the body, con- 

 necting a series of nine nerve-centres, one in each 

 of the three segments of the fore-body, and one 

 in each of the first six segments of the hind-body 



