INTERNAL ANATOMY OF INSECTS. 5 



great sympathetic, the intercostal, or trisplanchnic nerves*. 

 While the first of these two systems is the messenger of 

 the will, by means of the organs of the senses connects 

 us with the external world, and is subject to have its 

 agency interrupted by sleep or disease b ; the latter is 

 altogether independent of the will and of the intellect, 

 is confined to the internal organic life, its agency con- 

 tinues uninterrupted during sleep, and is subject to no 

 paralysis. While the former is the seat of the intellec- 

 tual powers, the latter has no relation to them, but is the 

 focus from whence instincts exclusively emanate : from it 

 proceed spontaneous impulses and sympathies, and those 

 passions and affections that excite the agent to acts in 

 which the will and the judgement have no concern c . 



It is probable, though the above appear to exhibit 

 the primary types of nervous systems, that others exist 

 of an intermediate nature, with which future investiga- 

 tors may render us better acquainted d : but as our bu- 

 siness is solely with that upon which insects in this re- 

 spect have been modelled, without expatiating further in 

 this interesting field, I shall therefore now confine myself 

 to them. 



We have before seen e that the nervous system of in- 

 sects belongs to the ganglionic type : but it requires a 

 more full description, and this is the place for it. It ori- 



3 They are called trisplanchnic because they render to the three 

 cavities of the viscera: viz. the thoracic, the abdominal and the 

 pelvic. N. Diet. d'Hist. Nat. xxii. 524. 527. 



b In Hemiplegia, &c. c N. Diet. d'Hist. Nat. xvi. 307. 



d Thus in the Molluscoe there must be a great difference in this 

 respect, since in some of these the brain or cerebral ganglion has 

 been cut off with the head, and another reproduced. Ibid. xvi. 306. 

 Comp. v. 391. VOL. III. p. 29. 



