NEEVUS VAGUS. 



329 



intimate composition of the nervous apparatus described above is by no 

 means so simple as it appears to ordinary observation ; and, since the 

 experiments of Sir Charles Bell and Magendie demonstrated the existence 

 of distinct columns or tracts in the spinal axis of vertebrate animals, 

 various anatomists have endeavoured to show that corresponding parts 

 may be pointed out in the ventral chain of articulated animals. There 

 can, indeed, be no doubt that this portion of the nervous system of an 

 insect corresponds in function with the medulla spinalis ; and if, in the 

 one case, the nerves which preside over the general muscular move- 

 ments arise from a different column to that whence the nerves that 

 correspond with the periphery of the body originate, while those which 

 regulate the motions of respiration emanate from a distinct tract, we 

 might reasonably suppose a similar arrangement to exist in the struc- 

 ture of the nervous system we are now examining. It has, in fact, 

 been well ascertained that the nerves given off to the muscular system 

 of the Homogangliata are not derived from the ganglionic masses them- 

 selves, but from the cords which connect them together, while the 

 nerves distributed to the integument and external parts of the body 

 communicate immediately with the ganglia. These different modes of 

 origin give presumptive evidence that at least two distinct tracts exist 

 in the central axis of insects ; but, from the extreme minuteness of the 

 different parts, it is not easy satisfactorily to demonstrate them sepa- 

 rately. In the larger ARTICTTLATA, however, as for example in the 

 CRUSTACEANS, two distinct columns 

 of nervous matter are readily de- 

 tected ; it will therefore be more 

 convenient to defer the investigation 

 of this interesting subject until we 

 have an opportunity of describing 

 these parts upon an enlarged scale ; 

 enough has been said at present to 

 enable the reader to compare the 

 nervous axis of an insect with that 

 of a lobster, and draw correct con- 

 clusions from the comparison. 



(859.) The last division of the 

 nervous apparatus, which we have 

 already mentioned as being the re- 

 presentative of the sympathetic sy- 

 stem, consists of two portions, one 

 corresponding, in distribution at 

 least, with the nervus vagus of VERTEBRATA, while the other represents, 

 apparently, the sympathetic ganglia. The nervus vagus, as we shall call 

 it, and which has been named by Swammerdam * and Cuvier the recurrent 



* Biblia Nature. 



Fig. 165. 



Nervus vagus and sympathetic system 

 of an Insect : a a, optic nerves ; d d, su- 

 pra-cesophageal ganglion; I, oesophagus; 

 b b, origins of the recurrent nerve ifk; 

 g g, nerves surrounding the oesophagus, 

 and communicating between the supra- 

 cesophageal ganglion, d d, and the first pair 

 of ventral ganglia, h; c c, 1 1, sympathetic 

 ganglia. 



