THE NERVOUS SYSTEM. 207 
The Spinal Nerves.—There are forty pairs of nerves 
connected with the spinal cord. They issue from the 
vertebral canal through the intervertebral foramina. Each — 
nerve is connected to the cord by a ventral and dorsal root 
(Figs. 93 and 102). The former is also known as the 
motor root, since its fibers are almost entirely motor, while 
the latter is the sensory root, as it is composed of fibers 
transmitting’ impulses to the central nervous system. A 
ganglion about the size of a pinhead is located on the dorsal 
root immediately proximad of its junction with the ventral 
root, within the intervertebral foramen. This anatomy can 
be displayed by cutting away the dorsal muscles on either 
side of the column, and then, with the bone-cutters, severing 
the laminze of several of the arches of the vertebrz, so that 
the roof may be removed from the vertebral canal (Fig. 
22). 
Immediately beyond the intervertebral foramen each 
nerve gives off a dorsal branch to the muscles of the back, 
and a small connecting twig to the sympathetic system. 
The main nerve is then spoken of as the ventral branch. 
These main nerves or ventral branches, in various regions 
of the trunk, anastomose with each other, forming plexuses. 
In the region of the neck there is formed the cervical plexus ; 
in the region of the shoulder, the brachial plexus; in the 
region of the loins, the lumbar plexus; and in the region of 
the sacrum, the sacral plexus (Figs. 105 and 106). 
There are eight cervical nerves, the first of which does 
not make its exit through the intervertebral foramen, as do 
all the other spinal nerves, but traverses a foramen in the 
atlas. The first five cervical nerves, the three posterior 
cranial nerves, and branches from the sympathetic trunk, 
form the cervical plexus. These five cervical nerves 
supply mainly the structures of the neck. 
There are thirteen pairs of thoracic nerves. The ventral 
