THE CEREBRO-SPINAL SYSTEM OF NERVES, 637 
of the gray matter of the spinal cord, another division has 
been proposed, viz., a division of nerve-fibers and their centers 
of origin in the gray matter for the supply of the internal 
and the external parts of the body—i. e., into splanchnic and 
somatic nerves. The centers of origin of the splanchnic nerves 
are referred to groups of cells in the gray matter of the cord 
around the central canal; while the somatic nerves spring 
from the gray cornua and supply the integument and the 
ordinary muscles of locomotion, etc. The splanchnic nerves 
supply certain muscles of respiration and deglutition, derived 
from the embryonic lateral plates of the mesoblast ; the somatic 
nerves, muscles formed from the muscle-plates of the same 
region. 
It is assumed that the segmentation of the vertebrate and 
invertebrate animal is related; and that segmentation is pre- 
served in the cranial region of the vertebrate, as shown by the 
nerves themselves. 
The afferent fibers of both splanchnic and somatic nerves 
pass into the spinal ganglion, situated in the nerve-root, which 
may be regarded as stationary. 
It is different with the anterior roots. Some of the fibers 
are not connected with ganglia at all; others with ganglia not 
fixed in position, but occurring at variable distances from the 
central nervous system (these being the so-called sympathetic 
ganglia): thus, the anterior root-fibers are divisible into two 
groups, both of which are efferent, viz., ganglionated and non- 
ganglionated. The ganglionated belong to the splanchnic sys- 
tem, and have relatively small fibers; the non-ganglionated in- 
cludes both somatic and splanchnic nerves, composing the 
ordinary nerve-fibers of the voluntary striped muscles of respi- 
ration, deglutition, and locomotion. 
It would appear that these now isolated ganglia have been 
themselves derived from a primitive ganglion mass situated on 
the spinal nerves; so that the distinction usually made of gan- 
glionated and non-ganglionated roots is not fundamental. 
A spinal nerve is, then, formed of—1. A posterior root, the 
ganglion of which is stationary in position, and connected with 
splanchnic and somatic nerves, both of which are afferent. 2. 
An anterior root, the ganglion of which is vagrant, and con- 
nected with the efferent small-fibered splanchnic nerves. 
Among the lower vertebrates both anterior and posterior 
roots pass into the same stationary ganglion. Such is also the 
case in the first two cervical nerves of the dog. 
