THE NERVOUS SYSTEM. 
169 
two cords witli a ventral chain of ganglia—one pair, ap¬ 
parently a single one, for each segment. In the lower 
Arthropods, snch 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 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 
membranes. Both consist of gray 
and white nervous matter; but in 
the brain the gray is on the out¬ 
side, and the white within; while 
the white 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, one fifth natural size: a, 
great longitudinal fissure; b, an¬ 
terior lobe; c, middle lobe; d, 
medulla oblongata; e, cerebel¬ 
lum; /.first spinal nerve; g, 
brachial plexus of nerves supply¬ 
ing the arms; h , dorsal nerves; i, 
lumbar nerves; k, sacral plexus 
of nerves for the limbs; l, canda 
equina: the figures indicate the 
twelve pairs of cranial nerves, of 
which 1 is olfactory, 2 optic, and 
8 auditory. 
