4: INTERNAL ANATOMY OF INSECTS. 
consists of a series of ganglions connected by nervous 
threads or a medullary chord, placed, except the first 
ganglion, below the intestines, from which proceed nerves 
to the various parts of the body*. This prevails in the 
Classes Insecta, Crustacea, Arachnida, Mollusca, Annelida, 
&ec. In the third, the cerebro-spinal, the nervous tree 
may be said to be double, or to consist of two systems— 
the first taking its origin in a brain formed of two hemi- 
spheres contained in the cavity of the head, from. which 
posteriorly proceeds a spinal marrow, included in a dor- 
sal vertebral column. These send forth numerous nerves 
to the organs of the senses and the muscles of the limbs. 
Thesecond consists of two principal ventral chords, which 
by their ganglions, but without any direct communica- 
tion, anastomose with the spinal nerves and some of 
those of the brain, and run one on each side from the 
base of the skull to the extremity of the sacrum. ‘This 
system consists of an assemblage of nervous filaments 
bearing numerous ganglions, from which nervous threads 
are distributed to the organs of nutrition and reproduc- 
tion’. Its chords are called the 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? the latter is altogether independent of 
a N. Dict. @ Hist. Nat. xvi. 306. 
_ > Ibid. 307. The great sympathetic nerves in fishes are said to 
have no ganglions. Cuv. wbi supr. 297. 
¢ They are called trisplanchnic because they render to the three 
cavities of the viscera :—viz. the thoracic, the abdominal and the 
pelvic. N. Dict. d’ Hist. Nat. xxii. 524. 527. 
¢ In Hemiplegia, &c. 
