NERVOUS SYSTEM. 
68 
cords and conduct the nervous force from the ganglions, 
where it originates, and the impressions derived from 
without through the medium of the senses to these 
medullary centres. Each nervous 
fibril is quite distinct from one ex- 
tremity to the other, and to this is 
ascribed the distinctness of sensa- 
tions, and referring them to the peri- 
pheral extremities (Klein, 79). 
The ganglia are formed principally 
of cells, called ganglionic cells (Fig. 
30, a), intimately connected with 
fibres, and they originate the nerves 
of sensation and motion, which pass 
into every part of the body and 
regulate the animal life of the insect. 
The fibres and cells are enclosed in 
a double case (Fig. 30, a, b, and c), 
the interior lining of which is thick 
and hard, and coated with a granular 
layer called the stratum.^ the outer 
coating being a soft and thin mem- 
brane, into which the minute ends of 
the tracheae enter. 
The cells (Fig. 30, a) are oblong or globular, 
nucleated, and the largest are found in the abdominal 
ganglia. 
The fibres which connect different parts of the 
body with the ganglionic chain and brain are called 
commissures.^ or commissural fibres, and they also com- 
municate with both sides, so as to connect the 
Fig. 29. 
Nervous wSystem. 
