THE NERVOUS SYSTEM. 
169 
two cords with a ventral chain of ganglia—one pair, appar¬ 
ently a single ganglion, for each segment. In the lower 
Arthropods, such as Crustacea, Centipedes, and larval In¬ 
sects, the arrangement is substan¬ 
tially the same. In higher Insects 
and Crustacea, many of the gan¬ 
glia are fused together in the head 
and thorax, indicating a concen¬ 
tration of organs for sensation and 
locomotion. 
In Yertebrates, the nervous 
system is more highly developed, 
more complex, and more concen¬ 
trated than in the lower forms. 
In fact, there are some parts, as the 
brain, to which we find nothing 
homologous in the Invertebrates; 
and while the actions of the lat¬ 
ter are mainly, if not wholly, au¬ 
tomatic, those of backboned ani¬ 
mals are largely voluntary. Its 
position, moreover, is peculiar, 
the great mass of the nervous 
matter being accumulated on the 
dorsal side, and enclosed by the 
neural arches of the skeleton. 
The brain and spinal cord lie in 
the cavity of the skull and spinal 
column, wrapped in three mem¬ 
branes. Each consists of gray 
and white nervous matter; but in 
the brain the gray is on the out¬ 
side, and the white within; while 
the w T hite of the spinal cord is external, and the gray in¬ 
ternal. Both are double, a deep fissure running from the 
Fig. 137.—Human Brain and Spinal 
Cord, about one tenth natural 
size; a, great longitudinal fissure; 
b, anterior lobe; c, middle lobe; 
d, medulla oblongata; e, cerebel¬ 
lum ; /, first spinal nerve; g, bra¬ 
chial plexus of nerves supplying 
the arms; h , dorsal nerves; i, 
lumbar nerves; k , sacral plexus 
of nerves for the limbs; l, cauda 
equiua: the figures indicate the 
twelve pairs of cranial nerves, of 
which 1 is olfactory, 2 optic, and 
8 auditory. 
