INTERNAL ANATOMY OF INSECTS. 



ginates in a small brain placed in the head, and consist- 

 ing almost universally of two lobes, sometimes extremely 

 distinct. It is placed over or upon the oesophagus or gul- 

 let, and from its posterior part proceeds a double ner- 

 vous chord, which embracing that organ as a collar dips 

 below the intestines, and proceeds towards the anus, form- 

 ing knots or ganglions at intervals, in many cases cor- 

 responding in number with the segments of the body, 

 and sending forth nerves in pairs, the ramifications of 

 which are distributed to every part of the frame. In the 

 perfect insect the bilobed ganglion of the head or the 

 brain is usually of greater volume than in the larva, and 

 the ganglions of the spinal chord are fewer, which gives 

 a more decided character of centricity to the whole ner- 

 vous system a . This may be considered more particularly 

 with respect to its substance and colour ; its tunics ; and 

 parts. 



I. Substance and Colour. The nervous apparatus of 

 insects is stated by those who have examined it most 

 narrowly, though consisting of a cortical and medullary 

 part, the latter more delicate and transparent than the 

 former, to be less tender and less easy to separate than 

 the human brain b . It has a degree of tenacity, and 

 does not break without considerable tension; in general, it 

 is clammy and flabby, and under a microscope a number 

 of minute grains are discoverable in it, and when left to 

 dry upon glass, it appears to contain a good deal of oil, 

 which does not dry with the rest c . That of the gan- 

 glions differs from the substance of the rest of the spinal 



* Comp. PLATE XXX, FIG. 1. and 6. and Cams. Introd. to Cowp. 

 Anat. \. 64. 

 b Lybnet Analom. 100. c Ibid. 101. 



