$ 329, 330. THE INSECTA. 409 
The ventral ganglia sre usually small, co¢qual, and give off no nerves 
except to the segments of the abdomen. The last ventral ganglion, alone, 
is larger, for it furnishes, in addition, nervous filaments to the rectum, and 
to the excretory ducts of the genital organs. 
Usually, the nerves arise in the ganglia by two or three principal roots. 
Some nerves, however, arise from the interganglionic cord. In those 
species where the ventral ganglia are entirely fused together, the nerves 
arise close together, but immediately diverge in different directions. 
§ 829. 
As to the Intimate Structure of the nervous system of the Insecta, 
both the nerves and the ganglia are always surrounded by a fibrous neu- 
rolemma, and, according to carefully-made researches, are never wanting in 
‘the two usual anatomical components. Between the extremely tenuous 
primitive fibres, are interposed, in the ganglia, very small globules. These 
last, nucleolated, usually contain also a finely-granular substance, colorless, 
though sometimes reddish or brownish.” 
There are, with the Insecta, as with the Crustacea, two modes of the dis- 
position of the fibres in the ganglia. The first, which form nervous, infe- 
rior cords, are disseminated in the. ganglia ; while the others, which belong 
to the superior nerves, simply pass through or over these ganglia. These 
two kinds of fibres give off, laterally, nervous filaments, which, uniting, 
form peripheric nerves of a mixed character. The superior cords corre- 
spond, probably, to the motor nerves, and the inferior to the sensitive nerves 
of the Vertebrata. 
§ 330. 
The Disposition of the nervous system in the various orders of Insecta 
presents the following differences : 
Among the Aptera, the ventral cord of the Pediculidae is composed of 
three contiguous ganglia situated in the thorax. The prothoracic gan- 
glion connects with the brain, and the metathoracic sends nerves to the ab- 
domen.” The nervous system of the Poduridae differs from this, in their 
three thoracic ganglia being separated, and their interganglionic longitudi- 
nal commissures being wide apart.© More widely different still, is the 
ventral cord of the Lepismidae; it is composed of eleven ganglia connected 
by double longitudinal commissures. 
With the Hemiptera, this system is limited to two thoracic ganglia, of 
which the anterior is the smaller. With Pentatoma, and Cicada, these 
two ganglia are not separated except by a constriction; while, with Nepa, 
p. 389, Pl. XIII.-XVII. and Cyclopaed. loc. cit. p. 
1 It was undoubtedly from their extreme deli- 
i 946). Hagen (Entom. Zeit. 1844, p.364) has since 
that these li globules were over- 
looked by Treviranus (Beitrég. zur Aufklar. d, 
Erscheinung. u. Gesetze d. organisch. Lebens. I. 
Hft. 2, p. 62). They have been distinctly seen by 
Ehrenberg (Unerk. Struct. &c. p. 56, Taf. VI. fig. 
6 (Geotrupes)), Pappenheim (Die specielle Gewebe- 
lehre d. Gehdrorg. p. 51), He/mholtz (De fabric. 
Syst. nat. &c. p. 21), Hannover (Recherch. 
microscop. &c. p. 71, Pl. VI. fig. 81, 82 (Aeschna)) 
and Will (Miiller’s Arch. 1844, p. 81). 
2 This difference of the nervous cords was first 
pointed out by Newport with the pupa and imago 
of Sphing ligustri (Philos. Trans. 1834, part II. 
35 
observed it with Aeschna grandis and Gryllo- 
talpa vulgaris. 
1 Various and special accounts of the general 
disposition of the nervous system of insects may 
be found ir Cuvier, Lecons, &c., ITI. 1845, p. 334. 
2 Swammerdamm, Bib. der Natur. p. 36, Taf. 
II. fig. 7. 5 
8 Nicolet, loc. cit. p. 44, Pl. IV. fig. 1 (Smyn- 
thurus). 
4 Treviranus, Verm. Schrift. II. Ift. 1, p. 17, 
Taf. IV. fig. 3. 
